Tag Archives: code requirements

Code Requirements for Residential Roof Trusses

Code Requirements for Residential Roof Trusses

Reprinted from a March 2019 article in Structure Magazine authored by Brent Maxfield, P.E.

Part 1 of 3:

There are many roles played in the design and delivery of residential wood roof trusses. Engineers can play various roles in this process, and it is essential to understand which role you play. This article discusses the scope of work required of the various roles as defined by the various codes and standards for residential roof truss. If a building falls within the IRC, all roles can be played by non-engineers, unless the jurisdiction requires the construction documents to be prepared by a Registered Design Professional.

Code Requirements

The International Residential Code (IRC) is the governing code for one- and two-family dwellings. It is a prescriptive code. For those elements that fall outside of the prescriptive criteria, engineering design (i.e., using the IBC) is required (See IRC R301.1.3). The IRC does not have prescriptive provisions for the design and installation of prefabricated wood trusses, but they are allowed per Section R801.10. The applicability limits for trusses are found in Section R802.10.2.1. These must be followed in order to stay within the purview of the IRC. The limits that apply when snow loads control the design are:

· Building width not greater than 60 feet perpendicular to the truss span

· Truss span not greater than 36 feet

· Minimum roof slope of 3:12

· Maximum roof slope of 12:12

· Maximum design wind speed of 140 miles per hour (63 m/s), Exposure B or C

· Maximum ground snow load of 70 psf (3352 Pa), with roof snow load, computed as 0.7pg

 

The IBC becomes the governing code for the truss design and associated load paths if the structure falls outside of these limits (See IRC R301.1.3).

Come back Thursday August 29th for part two

All Graded Lumber is Not the Same

Standard & Btr and #2 Graded Lumber Are Not the Same

Although I had a couple of years’ experience in the prefabricated metal connector plated truss industry, my first opportunity to truly “fly solo” was when I was hired, at the end of August 1979 to manage the truss plant at what was then Lucas Plywood and Lumber, in Salem, Oregon.

My predecessor, Ken Chowning, had managed it for several years and was going into retirement on the Oregon Coast. I was assured he would remain around for a month or so, to make sure there was a smooth transition. He did remain around – until noon of my first day, and before he left, I was given a tour of the facility.

Once I got past being traumatized by trusses laid out on steel tables having their truss plates savagely beaten in by hammers with flat steel plates welded onto their faces, the next shocker was – they were using all green lumber! https://www.hansenpolebuildings.com/blog/2011/09/499green-lumber-vs-dry-lumber/

Reorganizing my psyche, and beginning to wonder what world I had put myself into, Ken proudly showed me their completed trusses. Built from 2x4s (no issues there). He happily explained how using #2 graded 2×4 was making them a whopping pile of money.

grade-stampThe problem was….all of the “#2” 2x4s were graded as “Std&btr”.

How he had gotten sold the farm on this one and never gotten snagged by a Building Inspector or during a random third party truss inspection was totally beyond my comprehension!

“Standard” graded lumber is classified as being “Light Framing”. #2 lumber fits into the category of “Structural Light Framing”. The difference in permitted characteristics between the two grades being slope of grain, where standard can be up to 1 in 4 and #2 is 1 in 8. Basically the #2 grade is closer to being vertically grained.

Whilst this one characteristic may seem minimal, it plays a huge part in the strength of the material. #2 grade is roughly 235% stronger in bending, 240% stronger in tension, 125% stronger in compression parallel to the grain and 123% greater in modulus of elasticity. For bending and tension, a single #2 2×4 is stronger than two standard grade pieces!

Most lumberyards do not carry #2 grade 2×4, which leads to many pole buildings and trusses every four feet on top of truss carriers atop posts placed every eight feet (what we refer to as a four and eight building) ending up being built to less than Code requirements (read more about girts and bending at: https://www.hansenpolebuildings.com/blog/2012/03/girts/).

Considering a four and eight pole building of your own? Take a cruise around the lumberyard first, if all you can find are “Std”, “Standard” or “Std&btr” graded 2×4, the supplier isn’t prepared to provide a building which will adequately support climactic loads!

Materials List: Scary Disclaimer

From time-to-time our potential clients will send us a quote from another provider to compare.  Often it includes a materials list. Typically this happens when Brand X is “thousands” of dollars less expensive for what is supposed to be (but rarely is) the same building size, quality and features.

One such company, provides only a list of pieces, and nowhere on the materials list does it state the dimensions of the building, or the load carrying capabilities (wind, snow, seismic).

At the bottom of these lists is the following disclaimer:

You may buy all the materials or any part at low cash and carry prices. Because of the wide variation in codes, Xxxxxxx cannot guarantee the material list will meet your code requirements. These post frame buildings are suggested designs and materials list only. Some items may vary from those pictured. We do not guarantee the completeness or prices of these buildings. Labor, concrete flooring, some finish materials and delivery are not included. Some special order truss sizes may be jobsite delivered. Delivery is extra. This post frame may have been altered from the plan’s original design.”

Personally, I find this statement to be seriously disturbing, if not totally misleading.

Provided the potential client has verified the loading requirements with their permit issuing jurisdiction, any competent provider should be able to guarantee to provide a building which will indeed meet the given loading requirements as well as clearly stating on the quote, what those loads are.

Amazingly this particular company runs advertisements for buildings, and includes prices. How is it then they can say in good conscience, “We do not guarantee the completeness or prices of these buildings”?

Regardless of who might be quoting a given building, it would be my strong encouragement to run, don’t walk away from any prospective provider who will not clearly state on their quote the dimensions, full code and load information.  There also should be an explicit guarantee to fully provide the required materials to complete the building, per the plans. Any not included items should be clearly stated and all price quotes should include jobsite delivery.

Don’t become one of those horror stories where thousands of dollars of materials had to be added because they were left out of the original materials list.  Carefully read every word on the quote you are given, and if you don’t understand it, ask questions.  The one question you didn’t ask may well be the one which cost you thousands of extra dollars “out of budget”.