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A Wood Purlin Design Question

A Wood Purlin Design Question

Chances are good if you have to ask a structural design question, then you are in over your head.

Reader LARRY in DITTMER writes:

“Can you 2 by 4 flat on an 8 foot span Truss”

A few years ago, one of my neighbors bought a pole building kit from someone other than Hansen Pole Buildings. It was for a garage and sidewall columns and single roof trusses were placed every eight feet. Now I am relatively certain this building’s roof purlins were supposed to be 2×8 on edge between trusses – however for some obscure reason, they got installed flat wise! I am unsure as to how they were even able to get roofing installed without falling through.

This is just one of many reasons why post frame buildings should be designed by a Registered Professional Engineer.

When it comes to designing whether a roof purlin can achieve a given span, it takes a lot of calculations – both for live or snow loads, as well as wind loads. In high wind areas, wind will fail purlins (or their connections) rather than snow! I have condensed calculations down to just bending and deflection and will use minimum snow loads in this example:

ROOF PURLIN DESIGN – Main Building (Balanced snow load)

Assumptions:

Roof slope = 4:12 (18.435° roof angle)
Trusses spaced 8-ft. o.c.
Purlin span = 8-ft.
Purlin spacing = 24 in.
Purlin size 2″ x 4″ #2 Southern Pine
Roof steel dead load = 0.63 psf steel American Building Components catalogue
Roof lumber dead load = 0.587 psf
Total purlin dead load = 1.217 psf

Check for gravity loads

Bending Stresses

Fb: allowable bending pressure
Fb‘ = Fb * CD * CM * Ct * CL * CF * Cfu * Ci * Cr
CD: load duration factor
CD = 1.15 NDS 2.3.2
CM: wet service factor
CM = 1 because purlins are protected from moisture by roof
Ct: temperature factor
Ct = 1 NDS 2.3.3
CL: beam stability factor
CL = 1 NDS 4.4.1
CF: size factor
CF = 1 NDS Supplement table 4B
Cfu: flat use factor
Cfu = 1.1 NDS Supplement table 4B
Ci: incising factor
Ci = 1 NDS 4.3.8
Cr: repetitive member factor
Cr = 1.15 NDS 4.3.9
Fb = 1100 psi NDS Supplement Table 4B
Fb‘ = 1100 psi * 1.15 * 1 * 1 * 1 * 1 * 1.1 * 1 * 1.15
Fb‘ = 1600 psi

fb: bending stress from snow/dead loads
fb = (purlin_dead_load + S) * spacing / 12 * cos(θ) / 12 * (sf * 12 – 3)2 / 8 * 6 / b / d2 * cos(θ)
S = 21.217 psf using the appropriate load calculated above
fb = 21.217 psf * 24″ / 12 in./ft. * cos(18.435) / 12 in./ft. * (8′ * 12 in./ft.)2 / 8 * 6 / 3.5″ / 1.5″2 * cos(18.435)
fb = 2961.59 psi > 1600 psi; stressed to 185.1%

Deflection

Δallow: allowable deflection
Δallow = l / 180 IBC table 1604.3
l = 96″
Δallow = 96″ / 180
Δallow = 0.533″
Δmax: maximum deflection
Δmax = S * spacing * cos(θ * π / 180) * (sf * 12)4 / 185 / E / I from http://www.awc.org/pdf/DA6-BeamFormulas.pdf p.18
E: Modulus of Elasticity
E = 1400000 psi NDS Supplement
I: moment of inertia
I = b * d3 / 12
I = 3.5″ * 1.5″3 / 12
I = 0.984375 in.4
Δmax = 21.217 psf / 144 psi/psf * 24″ * cos(18.435° * 3.14159 / 180) * (8′ * 12 in./ft.)4 / 185 / 1400000 psi / 0.984375 in.4
Δmax = 1.118″ > 0.533″; 209.68% overstressed in deflection

These calculations are based upon purlins every 24 inches on center. If you were to reduce spacing to say 11 inches on center then flatwise 2×4 #2 Southern Pine with a 20 psf roof snow load would be adequate.

If you were able to somehow acquire 2850f Machine Stress Rated 2×4 with a E value of 2300000 psi (very high grade material used by some truss manufacturers) spacing could be 18 inches on center.

Again – remember these equations are just for checking for bending due to a minimal snow load, wind conditions may dictate. Please consult with a Registered Professional Engineer for actual designs.

This entry was posted in Pole Barn Planning, Pole Barn Structure, Professional Engineer, Lumber, Pole Barn Questions, Pole Barn Design, Roofing Materials, Constructing a Pole Building and tagged wind load, flat roof purlins, roof slope, snow load, deflection, live loads, wood bending calculations, roof purlin design on October 8, 2026 by Rick Ochs.

Permit Problem, OSB Wall Sheathing, and a Flat Roof Slope

This week the Pole Barn Guru answers reader questions about a permit problem for building the reader would now need a permit for, whether or not OSB wall sheathing is necessary for an addition, and if a 12′ peak to 10′ eave will appear flat.

DEAR POLE BARN GURU: I built a pole barn in 2020 without a permit. I sank the posts (6″ x 6″ red fir) in the ground 4′. However, I did not use concrete under the poles or around the poles. Since the ground is quite rocky here I just sunk the poles in the ground. Prior to inserting the poles in the ground I burned the poles with a torch and then painted 3 coats of asphalt paint on them for treatment.
I am now wanting the building to be a commercial building and need to go through the permit process with my county. I don’t think how I built it is to code and wondering if you have any ideas on how to make it right after the fact? Build a foundation under slab and tie the poles to it?
Any recommendations are welcome. Attached are my drawings. MICHAEL in EL RITO

DEAR MICHAEL: As you have realized, your immediate challenge is your columns, their lack of adequate treatment for structural in ground use, and a missing foundation system.

Your solution is going to involve hiring a Registered Professional Engineer, experienced in post frame construction and registered in New Mexico, to review your ‘as built’ situation and approve appropriate fixes throughout your structure. I am copying him with this response and will forward your drawings to him as well.

Most likely solution will be for your untreated building columns to be cut off an inch above any existing (or future) concrete slab. Concrete piers can be poured beneath each column (once remaining embedded column has been removed) adequate in dimensions to prevent uplift, overturning and settlement. Code Approved wet set brackets can then be placed in each pier and bolted to column.

DEAR POLE BARN GURU: Have a 32’wide by 30’long pole barn garage heated and insulated going to add on for storage only. It will have a concrete floor with vapor barrier and 2 inches of rigid foam. The walls and ceiling will be steel. There will be a 1 foot overhang all the way around to match the existing building, and one garage door at the rear of the building. The eves will be vented along with a rig vent. My question is that normal I would have used OSB for roof and sidewalls cost is an issue, what are your suggestions for the underside of the steel in both the walls and roof? ERIC in IRONS

DEAR ERIC: Provided your addition is properly engineered, there should be no structural reason to sheath your walls or roof with OSB or plywood. Order your roof steel with an Integral Condensation Control factory applied and between wall framing and siding place a Weather Resistant Barrier (Tyvek or similar).

DEAR POLE BARN GURU: I am wanting to build a single slope pole barn. The highest point would be 12 feet and the lowest 10 feet. Is that enough height difference to create a slope or will the roof appear to be flat? ZOE in LAREDO

DEAR ZOE: It will depend upon how wide your building will be. If 12 feet wide, it may appear okay, if wider, it is going to start to look flat. One thing to keep in mind, most steel paint warranties are void on roof slopes of less than 3/12. Side lap sealants are also required for steel roofing on slopes under 3/12, adding to investment and complexity.

This entry was posted in Professional Engineer, Pole Barn Design, Columns, Roofing Materials, Constructing a Pole Building, Concrete, Concrete Cookie, Footings, Rebuilding Structures, Pole Barn Questions and tagged treated columns, OSB, wet set brackets, permit, registered design professional, OSB wall sheathing, roof slope on July 6, 2022 by admin.

Making Everything Fit Under an Attached Lean-To

Reader GEOFF in MILFORD has an often found potential challenge, he writes:

“I am looking to build a 30×40 pole barn and want to also have a covered lean to for my camper that will run the length of the barn. In order to have the lean to tall enough for the camper how tall do I need to make the peak of the barn? I need at least 11 feet of height under the roof truss to make the camper fit. My thought is that I should be able to make the barn tall enough to just extend the roof out at the same pitch to cover the lean to. But at the same time I don’t want to make the barn taller than I need to make it.

Hope this makes sense

Thank you”

Mike the Pole Barn Guru responds:

Yours is a frequently seen dilemma – trying to fit tall things under an attached side shed (lean to). Even worse, when a future situation arises and a shed addition is needed and main building walls were just not tall enough to make everything work comfortably and not have a very low slope shed roof.

Rather than having to make your enclosed building portion significantly taller, I would recommend you approach this with an idea of it basically being a 40 foot square building, with one sidewall ‘pulled in’ 10′. If you went with say a 13 foot eave height, you could have 12 feet of interior clear height both inside, as well as under your roof only portion. This will allow for plenty of headroom both inside (where you could have a vehicle lift) and outside for your camper. With a 4:12 roof slope your overall building height would be 19’8″ under this scenario.

Most folks would take a design approach of trying to work this as a side shed. With a 13 foot eave height on the low side of a 10 foot width shed, at the same 4:12 roof slope, your eave height of enclosed portion would need to be 16’6” making overall building height 21’6”!

This entry was posted in Pole Barn Questions, Pole Barn Design, Pole Barn Planning, Sheds, RV Storage and tagged eave height, roof slope, lean-to shed, lean-to, side shed on March 12, 2021 by admin.

A Wood Purlin Design Question

Chances are good if you have to ask a structural design question, then you are in over your head.

Reader LARRY in DITTMER writes:

“Can you 2 by 4 flat on an 8 foot span Truss”

ROOF PURLIN DESIGN – Main Building (Balanced snow load)

Check for gravity loads

Bending Stresses

Deflection

Can We Do This?

Can we do this?

Engineered post frame building construction allows for nearly any situation a client can imagine to be achieved structurally. As some of you long-time loyal readers may have read – “You are only limited by your imagination, budget and available space”.

Hansen Pole Buildings’ Designer Doug has a client who contracted with a third-party to create floor plans and elevation drawings. Sadly, Doug’s client paid $900 for this work, when it might have been done for $695 or even free with this service: https://www.hansenpolebuildings.com/post-frame-floor-plans/

As drawn, this design would have a fairly low sloped ‘shed’ style roof spanning 20 feet from building face to outside with a trussed roof system. These two reverse gables would be framed in on top of shed roof purlins.

I can see some potential challenges occurring here.

Shed roof slope appears to be less than a 3:12 roof slope. This voids steel roofing paint warrantees provided by most roll formers. It also means every side lap has to have a butyl sealant between overlap and underlap per R905.10.2 of the International Residential Code:

“1. The minimum slope for lapped, nonsoldered-seam metal roofs without applied lap sealant shall be three units vertical in 12 units horizontal (25-percent slope).”

While I was not privy to distance along the wall length of this shed roof, it appears to be a great enough distance so a fairly significant structural header will need to be placed from column-to-column to support the low heel of shed trusses.

If this is in snow country, snow is going to build up between these two reverse gables and weight will need to be accounted for.

While this design is totally doable, it will entail additional investment in materials, plus more than a fair amount of time to assemble everything and maintain water tightness.

What would I have recommended?

Instead of a shed roof design, use a reverse gable porch with a single gabled truss spanning from corner column to corner column. Roof slope could match the main building, being steep enough to maintain warranty and leak free integrity. Plus – much easier to construct!

This entry was posted in Uncategorized and tagged Warranty, reverse gable, 3/12 roof slope, shed style roof, butyl sealant, roof slope, International Residential Code on August 7, 2020 by admin.

Things Roof Truss Manufacturers Should Ask

Things Roof Truss Manufacturers SHOULD Ask, But Don’t Always

I didn’t just fall off of a turnip truck yesterday, even though there are a few who may doubt my claim!

Prefabricated metal plate connected wood trusses and I became close friends back in April of 1977. Yes, we had electricity then and no, I did not watch space aliens build Egypt’s pyramids. Eventually I owned and operated two truss plants for 17 years. I know it may sound odd, but I did learn a couple of things.

Most of us do not know to ask (or tell) what we are not asked. When Hansen Pole Buildings’ Wizardress of Ordering Justine gives information to our trusted truss suppliers, all of these factors below have been incorporated into our order. This insures your trusses will be adequate to handle loads being placed upon them.

Chances are you (as well as most other post frame building kit suppliers and/or contractors) have not taken all of these into account. In failing to do so, your building may not do everything you want it to do not only today, but also years from now (or could even fall down and go boom).

Here we go:

Span from outside of wall to outside of wall. Eave overhangs are not considered as part of a span.

Desired slope(s) – with scissor trusses to achieve a vaulted ceiling or added center headroom, provide an interior or exterior ‘must have’ and other slope will be determined from ‘must have’ slope.

Spacing (ideally you will be using double trusses spaced directly on columns) rather than going into some lengthy dissertation on truss spacing, please read this article: https://www.hansenpolebuildings.com/2011/06/pole-barn-truss-spacing/.

Overhangs on eave sides (measured parallel to ground), as well as beyond endwalls. Why is beyond endwalls important? For sake of discussion assume single trusses placed every two feet, unless specified and designed otherwise and end truss in this scenario can only support a foot of overhang past an end. Single trusses placed every four feet can support a maximum two foot end overhang.

Roofing material and any solar or rooftop arrays. How many psf (pounds per square foot) must your trusses support? Steel roofing is fairly light weight. If using shingles, Code requires incorporation of enough load capacity for a reroofing down the road.

Is roofing over purlins, sheathing? Maybe sheathing AND purlins. Whichever is your case, these weights need to be accounted for as top chord dead loads.

Ceiling? If not now, ever? One of my most asked questions is in regards to adding ceilings in existing post frame buildings. At a bare minimum to support steel liner panels bottom chord dead load should be three psf, for gypsum wallboard (sheetrock) five psf. Last year Hansen Pole Buildings opted to increase bottom chord dead load for all trusses spanning 40 feet or less to five psf. We do not frankly do a very good job of promoting the benefits of this feature.

If attic is insulated, weight of ceiling material, ceiling joists and attic floor insulation.

Design wind speed and exposure. More buildings fail from roofs blowing off, than from snow. Depending upon Code year and version, wind speed may be expressed as Vult or Vasd. There is a difference and whoever is going to build trusses needs to know. If you do not fully understand differences between B and C wind exposure, learn quick: https://www.hansenpolebuildings.com/2012/03/wind-exposure-confusion/

Heated or Unheated? There is a factor relating to whether your building will be heated and it influences design snow load. Heated buildings help melt snow off.

Overall building dimensions: width, length, height above grade. These factors impact wind design.

Risk Category – How your building will be used impacts design snow and wind loads. Buildings with infrequent human occupancy have less risk and can be designed for a greater probability of failure in event of an extreme weather event.

Energy heels for full depth attic insulation? If ever insulation might be blown into a dead attic space for purposes of climate control, to be effective it should be full depth from outside of wall to outside of wall. For extended reading: https://www.hansenpolebuildings.com/2012/07/raised-heel-trusses/

Siding material (so proper gable truss recommendations can be made). If supporting other than roll formed steel siding, end truss probably will require vertical studs. Without a continuous wall beneath an end truss, it must be ordered as a structural gable.

Is the building fully enclosed, partially enclosed or open? This will again influence wind design.

Are you interested in parallel chords with a heel height to create a vaulted ceiling? You can get the same slope inside and out with this method.

Done right you would be offered options to increase load carrying capacities against either wind, snow or both. This is true value added design. Builders most always want bare minimums, while people who are doing DIY homes or barndominiums are most likely to increase capacities. If in doubt – own the last building standing!

When I was building trusses I had extremely high expectations of my truss staff, we always wanted to offer designs to exceed our client’s expectations.

Too many factors to juggle? You do not have to fret with an engineered Hansen Pole Building. Please call 1(866)200-9657 today.

This entry was posted in Shouse, Pole Barn Design, Roofing Materials, Pole Barn Structure, Steel Roofing & Siding, Trusses, Post Frame Home, Barndominium and tagged building span, overhangs on eave side, pounds per square foot truss support, truss spacing, top chord dead loads, wind speed and exposure, risk category, roof slope, attic insulation weight, scissor trusses on February 26, 2020 by admin.

Temporary Client Insanity – Truss Problems?

Long ago someone told me during the course of any construction project there comes a time when every client goes absolutely bat-pooh crazy. Personally, even knowing what I know, I am guilty of freaking out and having had a case of temporary client insanity during our own remodel and construction projects for our home.

For hyperventilation they have people breath from a brown paper bag, in my case – perhaps a plastic bag over my head and tied tightly about my neck would have been more appropriate.

Below I will share a client’s concerns. He remained much calmer (totally appreciated) during this process than I might have. He wrote to Justine (Hansen Pole Buildings’ Master of All Things Trusses):

“Justine, one more thing, the top chords of the trusses show 2×8 and the trusses were delivered with a 2×6 top chord, so all the bracing (purlins) will be hanging down. This roof is going to be insulated.

Also, the double trusses are not fastened together and I think I should have more than 1 set of scissored trusses.”

Our Technical Support response:

Building plans are drafted prior to receipt of truss drawings, so trusses as drawn on your plans are merely a depiction of what they may look like. Top and bottom chords as well as internal diagonal webs may be entirely different. The roof slopes will be accurate. Your building’s roof purlins certainly may hang below roof truss top chords, as this has no bearing upon your ability to insulate (please refer to Figure 9-5 of your Hansen Pole Buildings’ Construction Manual). As your roof has a Reflective Radiant Barrier, if you intend to use batt insulation between purlins, make sure to use unfaced insulation without a vapor barrier on underside, otherwise moisture can become trapped between two vapor barriers. This can lead to ineffective damp insulation as well as potential mold and mildew issues.

Per change order #3 your building is to have standard trusses in front 24 feet and a vaulted ceiling in rear 24 feet. With a pair of scissor trusses at 12 feet in front of rear endwall, this allows for the rear 24 feet to be vaulted and front 24 feet to have a level bottom chord.

Truss assembly people are not carpenters – and rarely do truss manufacturing facilities even have nail guns. It also avoids nail wounds from inexperienced or inappropriate use. As an example – back in 1979, I was shopping for a new employer designing and selling trusses. I interviewed with Tilden Truss, near Seattle. They used air guns firing a “T” staple to initially set steel truss plates. Their fabrication shop ceiling was covered with hundreds (if not thousands) of these “T” staples!

You will find it much easier to maneuver single trusses around your building site, than twice as heavy double trusses.

Please feel free to address any building assembly concerns to TechSupport@HansenPoleBuildings.com.

Another crisis averted.

This entry was posted in Pole Barn Structure, Trusses, Insulation, Pole Barn Design and tagged truss diagonal webs, batt insulation, roof slope, roof purlins, scissor trusses, Hansen Pole Buildings Construction Manual, reflective radiant barrier, unfaced insulation batts, truss top chords on June 25, 2019 by admin.

Free Post Frame Foundation Building Calculator

Free Post-Frame Building Foundation Engineering Calculator

No, such a thing as a free post-frame building foundation engineering calculator does not exist. However there always seems to be someone out there who is in search of “engineering for free”.

Reader KELLY writes:

“Guru,

Do you have a link to a pole foundation engineering calculator?

Looking for column depth / diameter for:

40x60x14

10 ft column spacing

35 PFS load

115 wind load.

No floor for constraint.

thanks.”

Mike the Pole Barn Guru responds:

There is no such thing as a “pole foundation engineering calculator” therefore, there is also no link to one. The design of post frame (pole) building foundations is one which is best left in the hands of RDPs (Registered Design Professionals – architects or engineers). When provided with all the pertinent information about your proposed building, they can design not only a structurally sound column embedment, but also your entire structure (which I whole heartedly recommend).

You’ve provided some of the information a RDP would require, but I will expand upon it:

Will the building have adequate sheathing (which could be roll formed steel roofing and siding) to transfer wind loads from roof to ground through endwalls? And will the sheathing be adequately fastened to underlying frame to take advantage of sheathing stiffness? If yes, diaphragm design can be utilized in your building design.

The difference in forces carried by sidewall columns with and without an adequate diaphragm is a factor of 4! If diaphragm design cannot be utilized, expect significantly larger columns, deeper holes and more concrete around columns.

What type of soil is at building site? Strength and stiffness of your soil will impact both depth and diameter of holes.

How are you measuring your 14′? It should be from bottom of pressure preservative treated splash plank, to underside of roofing at sidewalls. It does make a difference.

Does your building have overhangs?

What is the roof slope?

What is wind exposure at your site? The difference in force against columns between Exposure B and Exposure C is roughly 20%.

In the event you are not interested in procuring services of a RDP, the NFBA (National Frame Building Association) has available a Post-Frame Design Manual and you could attempt to do calculations yourself. For more information please see: https://www.hansenpolebuildings.com/2015/03/post-frame-building-3/.

Of course you could always invest in a fully engineered post frame building kit package. Besides engineer sealed blueprints and calculations, you would also get materials delivered to your site and a multi-hundred page Construction Manual to guide you through to a successful completion.

This entry was posted in Trusses, Pole Barn Questions, Pole Barn holes, Pole Barn Design, Constructing a Pole Building, Pole Barn Planning, Pole Barn Structure and tagged RDP, roof slope, NFBA, overhangs, post frame building kit package, sheathing, building site soil, wind exposure on February 22, 2019 by admin.

Lean To or Not to Lean To?

Lean To, Or Not to Lean To?

Over my post frame building career I have seen a plethora of buildings designed with an enclosed clearspan enclosed space plus a shed roof (or lean to) for one or both sides. Most often just a roof, a lean to provides shade and not much else for whatever it covers.

But, does just a roof provide a best design solution from practicality and economic standpoints?

I suppose I have been awaiting a reader to ask this question, as my radar has seen it coming! Thanks to DAN in OREGONIA who writes:

“I want a barn 40X64 enclosed with 14’ high overhead doors in the ends i also want a 15’ lean to down one 64’ side. I am trying to maintain RV height in the lean to. Would it be more cost effective to use a 55’ span truss designed to enclose 40’ and leave the 15’ open with a ceiling as the lean to, or make the enclosed area taller to continue the pitch to 14’ at the end of the 15’ overhang? I guess that would make the building walls 19’ tall.”

Before we dive into pool’s deep end, a couple of notes in regards to this building Dan ponders. In order to have a 14 foot tall overhead door, an eave height of 16 feet (and more probably 16’6”) will be necessary. More often than not, a roof slope of 4/12 (read about roof slope here: https://www.hansenpolebuildings.com/2018/09/roof-slope/) proves most cost effective. This means across his 15 foot overhang, there will be a five foot difference in slope.

As an alternative to a five foot elevation drop, a pitch break could be used between main clearspan and lean to roof. I usually try to avoid going steeper-to-flatter as it adds to construction complexity, adds to costs and provides a place for accumulation of debris (tree leaves and needles) as well as snow sliding off the enclosed portion.

Given a five foot difference and wanting to have RV height in the lean to, puts eave height of main portion of structure up to 21 feet or more! Certainly doable, but probably not affording many benefits unless one contemplates a mezzanine area. And while height increases are relatively affordable, they are not free by any means.

Most people view their new post frame buildings as ways to protect their valuables from not only elements, but also theft, vandalism and critters. Just a roof only partially accounts for only one of these.

In most cases, least expensive, easiest to construct and most practical design solution involves a clearspan of entire area and placing walls around perimeter. This keeps everything securely within a space of adequate headroom – without having to increase heights just to allow for clearance within the lean to.

Considering a barn or shop with an attached lean to? Give some thought to enclosing it all with a clearspan. Affordability might prove surprising and plus you gain practicality benefits.

This entry was posted in Sheds, Pole Barn Questions, Pole Barn Design, Pole Barn Planning and tagged eave height, roof slope, clearspan, shed roof only on October 12, 2018 by admin.

What Size Posts Should I Use?

What Size Posts Does My Building Need and How Deep Should They Be?

Reader ANONYMOUS in BENTON writes:

“1. If my building has 16 posts and posts are 12 feet apart do I need 4×6’s or 6×6’s?
2. If the plan shows 16 feet above grade how much do I need underground?
3. If the posts are set 12 feet apart will two 2x12s glued together support a truss system with a total length of 36 feet?”

Mike the Pole Barn Guru responds:

While I appreciate your questions, we as a company and me as an individual do not provide free engineering services. In answer to your questions:

1) Without knowing the full dimensions of your building, including roof slope and overhangs (if any), if the columns will be adequately tied into a concrete floor, as well as your site’s snow load, design wind speed and exposure, seismic zone as well as the dead loads which will be carried by your building there is no possible way for me or any RDP (Registered Design Professional – engineer or architect) to be able to answer this.

2) The depth of the columns into the ground should be shown on your engineered building plans (you do mention you have plans). At a minimum the holes should be no less than 40″ deep and must extend below the frost line. Ultimately the depth and diameter will need to be determined by the RDP in consideration of the factors listed in (1) above, as well as designing for the ability to adequately prevent uplift.

3) Since I would use double trusses which bear directly upon the columns, there would be no need to use any other type of dimensional lumber to provide headers for a truss system. Again, this is where your RDP can design to adequately provide an engineered system to support the trusses.

My best advice, since I am guessing you are somewhat floundering in this, is to invest in a fully engineered post frame building kit package which includes plans sealed by a RDP and designed specifically for your building, at your site. It just isn’t worth trying to avoid the small expense into a proper design – especially when the lives of the occupants depend upon it.

Here are some other articles which pertain directly to this subject and should be read: https://www.hansenpolebuildings.com/2014/12/free-pole-barn-plans/ and https://www.hansenpolebuildings.com/2017/11/dont-engineering-fool/

This entry was posted in Professional Engineer, Columns, Lumber, Pole Barn Questions, Pole Barn Design, Pole Barn Planning, Pole Barn Structure, Trusses and tagged roof slope, engineered building plans, Uplift, structural headers, columns depth, RDP, registered design professional on July 13, 2018 by admin.

Formula for Calculating Wall Girts

An Excel Formula for Calculating Wall Girts, Post Size and Hole Depth
John Minor and I have been friends for nearly 30 years – since his then father-in-law (and my business partner at the time) convinced me John could sell post frame buildings. Well Rod was correct, John could sell buildings – not only for me (twice), but also for some of the better known names in the post frame industry – Morton, Stockade, Cleary and FBi just to name a few.
John is also a cutting edge innovator and has gone from selling post frame buildings, to manufacturing and providing components to the post frame industry. Current he owns and operates Central Perma-column (https://www.heartlandpermacolumn.com/). John is smarter than the average bear and he has the thirst for knowledge which few non-RDPs (Registered Design Professionals – architects or engineers) in the post frame industry do.
December 13, I received this text message on my phone: “I need your help with something”. Although I did not recognize the number it came from, it turned out to be John.
The conversation went on like this – John, “No worries. I’m trying to find an Excel formula for calculating wall girts, post size and hole depth.”
Me, “You would have to write one and they will be very complex due to the tremendous number of variables involved. I’ve thought about having a Wall Girt calculator on our website for Building Officials to use and the reality is, it is a huge undertaking. For it alone, requires all of the building dimensions including roof slope, is building enclosed or partially enclosed? Which means one has to know if doors are wind rated, and where they are placed. Also makes a difference as to where girts are located on building as wind forces are greater at corners. If girts are barn style on outside of columns, do they span a single bay or multiple bays? Then one has to account for lumber species differences as well as visual vs. machine grading.

Best bet would be to layout all of the parameters and put it out for bid on Upwork. Should specify which versions of Code and NDS you want to cover as well as it must conform to the NFBA design manual. For columns, you are now talking 25-35 pages of calcs.
Fun, eh?”
John, “Damn!”
Those “simple” pole barns one drives by every day are in reality highly complex engineered structures (when properly designed). A full set of calculations for a small two car garage can generate nearly 200 pages of single spaced calculations to prove the adequacy of all structural members and connections, from the footings to the last fastener.
Don’t leave your new post frame building to chance – insist upon having only a building which has been designed specifically for you by a Registered Design Professional, on your site, with your dimensions and your doors!

This entry was posted in Lumber, Pole Barn Questions, Pole Barn Design, Pole Barn Structure, Columns, Pole Barn holes and tagged hole depth, central perma column, Heartland Perma-Column, wind forces, roof slope, pole barn hole depth, Post size, column size on February 15, 2018 by admin.

It Is Exactly the Same Building Part I

Well, maybe not exactly the same building.

In April of this year we had a client invest in a brand new 36 foot wide by 60 foot long post frame building kit package with a 16 foot eave height. Three months later, the building has been delivered, and one of the group which ordered the building sends us a quote on “exactly the same building” from a worthy competitor. And, of course, the competitor’s quote is way less expensive!

Now the competitor’s sales person advised the client the quotes were exactly the same, other than he had quoted a 25 psf (pounds per square foot) roof snow load, whereas we provided a 40 psf load, which is 60% more snow carrying capacity!

Turns out there were maybe a couple of other differences as well……

Things we have and they do not:

4/12 roof slope vs. 3/12 The steeper roof slope will look less industrial as well as more readily will shed snow.

C wind exposure vs. B wind exposure (for a detailed explanation of wind exposure please read here: https://www.hansenpolebuildings.com/2012/03/wind-exposure-confusion/). The benefit of an Exposure C wind load is it makes the building roughly 20% stronger in resisting wind forces, than the B exposure.

12″ enclosed overhangs vs. 18″ open overhangs. Not only are enclosed overhangs far more attractive, they provide ventilation and eliminate the wonderful nesting locations for flying critters which are provided by open overhangs.

12’x14′ residential overhead door vs. 14’x12′ commercial overhead door. If the client wants to get something taller than 12 foot through the other guy’s door, it just isn’t going to fit no matter how big a run one gets at it. Residential overhead doors come with “dog eared” openings and a far more attractive in a residential setting. Here I discuss why 14 foot wide doors are not what they are cracked up to be: https://www.hansenpolebuildings.com/2016/05/14-foot-wide-doors/.

One more entry door. Insulated commercial steel entry doors with steel jambs do not come cheap, especially when they are four foot wide!

Integrated J Channel on windows. So much easier to install than having to cut four pieces of steel trim to fit around a window and have them not leak!

The reflective radiant barrier with pull strip attached adhesive tab on one side vs. Metal Building Insulation (MBI) under the roof steel to minimize condensation challenges. My personal horrors of installing MBI can be visited here: https://www.hansenpolebuildings.com/2011/11/metal-building-insulation-in-pole-buildings-part-i/.

Lifetime paint warranty on steel vs. 40 year pro-rated. Your post frame building is going to be around for a long time, might as well have the best paint warranty available to minimize the effects of fade and chalk.

Base trim – keeps those creepy crawling critters from entering the building through the high ribs of the wall steel.

Top of wall trims – Even though roll formed steel siding lengths are controlled by a computer, they do vary slightly from panel to panel. The bottom of the panels should be kept at the same height as “stair steps” at the base of the walls is quite noticeable. Easiest way to hide any variants is to place the top edge into a piece of trim which covers any fluctuations.

Jamb trim on Overhead Door– exposed wood overhead door jambs are very popular in some parts of the country, however they do turn grey and then eventually black if not kept painted. The idea of a steel covered post frame building is to minimize future maintenance. Having to paint raw exposed wood does not meet with this criteria.

Heard enough? No? Then come back tomorrow for Part II. You won’t be disappointed!

This entry was posted in Pole Building Doors, Ventilation, Pole Barn Design, Pole Building Comparisons, Building Overhangs, Pole Barn Planning, Pole Barn Structure and tagged wind exposure, enclosed overhangs, roof slope, commercial entry door, base trim, integrated J channel, lifetime paint warranty, reflective radiant barrier on July 26, 2017 by admin.

Single Slope Roof

As a pole building “newbie” the first pole building I constructed back in the Spring of 1981 happened to be a 20 foot by 36 foot three sided loafing shed – with a single slope roof. As a builder, I didn’t make much money on it, as basically my head was still caught up in a stick framer’s mentality (studs and rafters go up and down, girts and purlins go left and right). Dealing with several feet of grade change was also something I was unaccustomed to, it was much different than having the level top of a concrete foundation to work from.

My second building was a 24 foot by 96 foot single slope roof, which was built over existing cattle pens! The pens were a challenge to work over, but at least the site was level.

Neither one of these experiences caused me to see single slope roofs as the less than ideal solution to most pole building needs.

In my humble opinion, in most cases, single slope roofs are neither aesthetically pleasing, nor the most cost effective solution. However, for small (say 12 foot in width by any multiple of 12 feet in length) run in sheds for horses or cattle, they may prove to be economically the best answer.

What are the most common reasons people select a single slope roof?

The top reason – the mistaken belief they will somehow be less expensive.

Most steel roofing manufacturer warrantees are void on roof slopes of less than 3/12. In order to keep the warranty in effect, this can quickly cause the “high” side of the building to become very tall.

Using the most common roof slope (4/12) a 24 foot wide building will be EIGHT FEET taller on one side than the other! From a structural aspect, building columns act as beams to resist wind loads. The formula for a simple span beam includes the square of the span – think of a 16 foot height having to carry FOUR TIMES the load of eight foot! The resultant is some potentially very large columns.

The resultant is paying to enclose a large area, in cubic feet, which cannot be practically or easily utilized.

The second most common reason for single slope roof, is so the weather (rain or snow) runoff will all travel to one eave side. This becomes more commonplace in snow country, where snow removal in front of sidewall doors can become a constant work in progress.

Rather than single sloping, it would be far less costly to use a pitched gable truss, with snow brakes to actually keep the snow on the roof and from sliding off in piles in front of doors. Having all of the snow slide in one direction could result in a crushing weight of snow piling up against the low side eave wall – buckling the siding, if not causing a catastrophic collapse.

Some single slope roof considerations:

Roof slope should be no less than 3/12 to maintain steel warrantees.

If desiring a clearspan, the overall height of the roof trusses should be no more than 12 feet. Many roof truss fabricators are limited to the ability to build trusses 12 feet in height, and for those who can, shipping becomes an issue as special permits must be acquired as well as pilot vehicles – adding to the expense.

In the event the single roof slope is fairly flat, and the clearspan width is greater than what can be supported by dimensional lumber or LVL (Laminated Veneer Lumber https://www.hansenpolebuildings.com/blog/2013/01/lvl/) rafters, prefabricated trusses are going to require added heel height (truss depth) at the low sidewall. This can result in a decreased ability to utilize all interior aspects of the building, as truss members will be taking up several feet of space below the low eave height.

Interior columns may need to be added to reduce truss costs, or get spans down to dimensional lumber manageable distances. In many cases, interior columns can get in the way, especially if the building is ever re-purposed.

Lofts – while the numerous feet of added height along one eave might prove to be a wonderful space for a second floor, consider the loss of headroom as the tall wall is moved away from. In most cases, those single sloping trusses will not lend themselves well to supporting a usable attic space within the truss. If so, then interior columns will need to be added to support the loft floor.

Doors – be wary of placing doors on the tall sidewall which are equal to or greater than the height of the low sidewall. While tall things might be able to enter the doors, those very same tall things will eventually run into the roof. With prefabricated trussed roofs, the lateral bracing between bottom chords and webs will be in the way.

Considering a single slope roof? Carefully weigh all of the options to insure it is the correct design solution for your circumstances!

This entry was posted in Uncategorized and tagged horse barn design, roof slope, pole building roof slope, cattle barn, loafing shed, shed roof slope on November 7, 2014 by admin.

What is the Best Solar Panel Roof Slope?

Hansen Pole Buildings’ Designer Rachel asked me today what the best solar panel roof slope?

There are three main factors which go in to the calculation of how much sun a roof receives. Roof angle or pitch, roof orientation (how south-facing a roof is) and location. Since Rachel asked about roof slope, I will focus on this one aspect.

There is an optimum angle for solar panels on a roof. This is the angle where, over the course of the whole year, the panels get the most amount of direct sunlight which produces the greatest amount of electricity.

The question is: what is the perfect angle, and if the roof is not at exactly this angle how much energy will be missed out on? I’ve arbitrarily picked a location of 49 degrees north (generally the United States – Canadian border in the western U.S.) and a south-facing roof for this example.

So for this roof the optimum angle is 35° (a bit over an 8/12 roof slope) and at this angle the roof generates 1,265kWh/m2/day. The energy reduces as we move away from this optimum angle. At 30° the energy drops to 1,260kWh/m2/year – a reduction of just 0.4%. At 20° (15° away from the optimum angle) the energy drops to 1,229kWh/m2/day – a reduction of 2.6%. It’s only with very steep roofs – say over 60° – when the solar energy really drops off.

It is obviously preferable to have a roof at the perfect angle. While I am not a fan of government subsidies, they are very generous when it comes to solar energy. Including them in the calculation, it absolutely makes sense to install on roofs which are not at the perfect angle. You’ll lose a few percent of solar energy but it should still make financial sense.

This entry was posted in Pole Barn Design and tagged roof slope, pole building roof slope, solar power on May 3, 2013 by admin.

Mike’s Roof Rules

Nothing is worse than a roof gone wrong. Leaks frustrate everyone involved, and are usually avoidable. Poor design and poor installation are equal factors in the roofs which just are not happy.

Avoid Valleys

Designing the roof of a pole barn? Then try to design a roof without any valleys. Valleys concentrate water and often clog with ice. It’s far more common to have leaks or ice dam problems near valleys than in the middle of a simple gabled roof. Many valleys exist because of a designer’s conceit rather than necessity. Often, these valleys trace back to the mistaken belief a chopped-up, complicated, multi-plane roof looks better than a simple gable. It doesn’t. And more complicated roofs are more expensive.

Just say no to Dormers and Skylights

No good reason exists for a new pole building to have a dormer. When I see a dormer, I conclude the designer or the architect made a mistake. They didn’t include enough interior space, and the building owner was forced to cut a hole in the roof because the ceiling was too low to stand up. Want to build a multi-story pole building – no problem. Want two floors, build two floors. Want three floors, build three floors. Then build a roof over the top floor. This roof should not have any deliberate holes in it. The “no holes” rule covers both dormers and skylights. Skylights are an invitation to leak – no matter how great the flashing kit is, pretty well plan upon them leaking, if they don’t it is a surprise bonus.

In most cases Single Slope Roofs are not less expensive

They also create a very tall wall on one side of the building, which has to be engineered for. Lots of “dead air” space ends up being paid for. There is not a ridge which can be easily vented. Worst of all – most people find them aesthetically unattractive.

An Unconditioned Vented Attic is better than an Insulated Roof

It makes more sense to put insulation on a flat ceiling than to try to insulate a sloped roof, for several reasons. Purlins usually aren’t deep enough to hold a thick layer of insulation; on the other hand, it’s usually easy to add a deep layer of insulation to the attic floor. Insulating the attic floor is also cheaper. Leave the space between purlins uninsulated, it will be easier to locate roof leaks. It is easier to air seal a flat ceiling, rather than a vaulted ceiling. If roof sheathing is used, damp roof sheathing will dry out quicker if it faces an attic than if it is part of a cathedral ceiling.

The Best Roof Shape is a simple Gable Roof

In a cold climate, the ideal design is a simple gable. Since gables don’t have any valleys or hips, they are easy to vent. It’s a straight shot from the soffits to the ridge. Chopped-up roofs with a variety of intersecting planes are hard to frame, hard to keep watertight, and hard to vent. Every nook and cranny creates somewhere for tree needles and ice to accumulate.

In a hot climate, a hipped roof can make more sense than a gable, because a hipped roof makes it easier to provide shade on all four sides of the pole barn. In a hot climate, shade is good. Fortunately, people in hot climates rarely have to worry about ice dams.

In all climates, make overhangs generous. If building a gable roof, don’t forget the rake overhangs; most rake overhangs are too stingy. Frame the rake overhang with overhanging purlins.

Don’t reduce the Roof Slope between the Ridge and the Eave

Monitor Pole Building A good roof plane has a consistent slope from the ridge to the eave. A roof which changes slope at midpoint is disturbing. Especially disturbing is a steep roof which suddenly switches to a shallow pitch (for example, when a porch with a shallow-pitched roof is affixed to a pole building with a steep roof). Shallow slopes hold snow and are susceptible to leaks. Most steel roofing warranties are void on slopes of less than 3/12.