roof dead load Archives - Hansen Buildings

Today the Pole Barn Guru answers reader questions about adding sheeting and tar paper to a metal roof, if we could supply blueprints for project 08-0602, and condensation control for a tight structure.

DEAR POLE BARN GURU: Can most pole building metal roofs support the extra weight of sheeting and tar paper (underlayment)? TAMI in SCOTTSDALE

DEAR TAMI: Most pole building roof systems are designed to support a bare minimum roof dead load – usually 2.5 to 3.3 psf (pounds per square foot). If this is your case, then your structure would not be able to support this added weight. We made a decision a year ago to use a minimum top chord (roof) dead load on all our roof trusses spanning 40 feet or less of five psf – just in case our clients decided to sheet their roofs with OSB or plywood with an underlayment and didn’t tell us about it in advance.

DEAR POLE BARN GURU: We want to build a pole barn 48×60 feet, like the Hansen building: Project# 08-0602 in Decatur, AL. Is there an existing blueprint we could use to built the structure and see all the bearing poles with all the dimension? We will have a designer to create the inside of the building. Are there some existing construction plans with detailed planning?
Nive Regards, PETRA in GREENBRIER

DEAR PETRA: Thank you for your interest in a new custom designed, fully engineered Hansen Pole Building. While we, of course, have in our records engineered plans for this structure. Due to differences in Building Code versions (referenced building was under 2006 Codes), and variations in design snow and wind loads, we would furnish an updated structural set of blueprints with your investment in your new building. These plans detail out locations and connections for every structural component of your building.

DEAR POLE BARN GURU: I have a 24×40 barn (steel sides, shingled/sheathed roof, concrete floor) that I’ll be heating. Walls and ceiling will have fiberglass batts insulation along with a vapor barrier on walls under OSB sheathing. OSB on ceiling as well. A buddy of mine caulked all his OSB joints then painted (latex) walls and ceiling in an attempt to further improve heating situation. In other blogs/questions you’ve answered with similar setups you talk about the importance of vapor and/or heated air needing to escape through ceiling into attic to help get rid of moisture. Will caulking and painting walls and ceiling limit the proper venting of moisture vapor from escaping my heated barn? JOHN in RICHLAND

DEAR JOHN: When buildings get so tight, it becomes necessary to use mechanical dehumidifying in order to prevent condensation challenges. For a barn, this might prove to be a greater investment than you wish to undertake.

Provided you have either no vapor barrier, or a Weather Resistant Barrier directly beneath your steel siding, if you caulk and paint interior walls, no vapor barrier and unfaced insulation will probably be more than an adequate design solution. You may want to consider rock wool batts as you are likely to have some condensation on inside of wall steel and fiberglass loses performance and can be prone to mold once it has gotten wet.

Collateral loads are additional dead loads hung from the building, such as sprinkler systems; drop ceilings, HVAC equipment, lighting etc. These are the loads which are most often overlooked or neglected in the design of a new pole building. Many times, it is just no one having made the client aware. Most building sales people and even designers and engineers, do not go beyond present day use to discuss what the building may be used for in the future.

Generally, the actually dead load weight of roof truss bottom chords and the minimal lateral bracing needed to restrain them from movement is about one-half of a pound, per square foot (psf). Most pole building trusses are designed using a one psf load, which will allow for the weight of standard electrical wiring and lights. As an example, for a 30’ x 40’ building approximately 600 pounds of wiring and lighting could be evenly distributed across the roof truss bottom chords, without the need to increase the load carrying capacity.

The Metal Building Manufacturers Association (MBMA) Low Rise Systems Manual recommends the following collateral loads, per square foot:
Sprinkler system – dry 1.5 wet 3
Ceilings – Suspended Acoustical Fiber 1
Gypsum Board – 1/2″ 2 – 5/8″ 3
Lighting 0.1 to 1
HVAC Ducts 1

2×6 ceiling joists at two foot on center will add about ¾ psf.

These loads are cumulative. If you have more than one, you add them together. In most cases, a ceiling load of 5 psf will be adequate to handle a layer of 5/8″ drywall, ceiling joists and blown in insulation.

Many of us are packrats. We accumulate “stuff” and the space in the trusses looks like a good place to put it. When I first visited the home of my now bride of 12 years, I took one look “up” in her garage and quickly ran for cover! The trusses just weren’t designed for the “stuff” she had stored up there. Her defiant assertion of “if it ain’t broke…” finally gave way to “ok, if you insist” and we cleaned it all out…ten years later!

The International Building Codes have a provision to add 20 psf of live load onto the bottom chord of trusses where a box 42” tall and 24” in width would fit between the top and bottom chords of prefabricated roof trusses, in areas where a truss web would not interfere. This same provision mandates an increase in the bottom chord dead load to 10 psf.

The roof system of your new building is not a place to become penny wise and pound foolish. Overloading a roof system, beyond its design capacity, can result in catastrophic failure.

Planning a new building? Discuss not only your future needs, but also what other owners of your building might do with the building, with your Building Designer.

Some folks are pushing budget just to get the basic building but have plans for future interior build out. From a liability position, the building owner bears the risk if ordering a building which assumes no collateral loads, then later turns it into living space with joists, drywall and insulation.

Always look towards solving future needs. It is fairly inexpensive to add ceiling loading to trusses, and saves a world of potential grief later.

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