Tag Archives: roof sheathing

Truss Spacing and Design

Truss Spacing and Design for Sheathed Post Frame Roofs

In most instances, there is not a structural or Code requirement for solid roof sheathing (plywood or OSB – Oriented Strand Board) to be placed below through screwed roof steel for post frame buildings. In some cases, clients look upon this as being an easier installation when doing a DIY build. For others, it is about providing a thermal break to eliminate underside of roof steel condensation. And a few look towards minimization of potential hail damage.

Reader CARROLL in PORTER writes:

“ Wanting to build Pole Barn that is about 35’x80’x12′ My question is, if I want to install 1/2″ decking plywood or OSB decking with underlayment and metal panels how far apart will I need the trusses to be center to center or what kind of truss design will I need? I guess it could be a 4/12 or 5/12 pitch if that helps any.”

Provided you have adequate available space, you may want to tweak your footprint dimensions in order to optimize your return for your investment. As steel comes in three foot widths and lumber in two foot lengths, your most cost effective dimensions of length and width will be multiples of six feet. In your instance, I would recommend 36 feet wide and 84 feet long.

With this said, I would place a single truss on each endwall and a two ply truss every 12 feet to align with your sidewall columns. Purlins can be placed on edge, using engineered steel joist hangers, between each set of trusses and spaced every two feet to support your sheathing. Whether plywood or OSB, panels are best installed running up roof from eave to ridge (perpendicular to purlins, parallel with truss spans). If not using synthetic underlayment, you should use 30# asphalt impregnated paper (roofing felt). With Hansen Pole Buildings, we purposefully design all trusses spanning 40 feet or less with a greater than minimum requirement top chord dead load – in order to accommodate those who want to install solid sheathing.


Steel Roofing Over Living Areas

 Steel Roofing Over Living Areas Requires Solid Decking?

Barndominiums, shouses and post frame homes have become a recent and trendy rage. Seemingly everyone wants one, at least as gauged by hundreds of weekly requests received by Hansen Pole Buildings would attest to.

Reader STEVEN in BOONE writes:

“I visited with the building inspector with your planning guide and asked if there were any metal roof over living area requirements. IE attach to purlins or deck required. I received an email response that states per IBC 2012 the living space requires a deck first. This seems to defeat the cost savings of using steel and purlins.  Is this correct and if so what materials would be used? Would it be regular roof sheathing? (OSB or plywood be it?) How do pole builders handle the height difference incurred by adding the sheathing. Does this required design change to make trusses closer together in the living area?”

Building inspectors have to deal with not only building codes themselves, but also literally hundreds of referenced titles mentioned within these codes. Thorough knowledge of the contents of this many documents proves to be an impossible task. Your inspector most probably deals with very few residential steel roofs.

From International Residential Code (“R” subsections) and International Building Code (IBC):

R905.10 or IBC 1507.4 Metal roof panels. “The installation of metal roof panels shall comply with the provisions of this section.’

R905.10.1 Deck Requirements. “Metal roof panel roof coverings shall be applied to solid or spaced sheathing, except where the roof covering is specifically designed to be applied to spaced supports.”

IBC 1507.4.1 Deck requirements. “Metal roof panel coverings shall be applied to a solid or closely fitted deck, except where the roof covering is specifically designed to be applied to spaced supports.”

Roof purlins qualify as spaced supports and through screwed steel roofing is designed specifically to be so applied under most wind and snow loads (an exception being hurricane areas of Florida, where a solid deck is required). Properly engineered to support extra dead loads being induced, one could install either plywood or OSB (Oriented Strand Board) sheathing between purlins and steel roofing, using 30# asphalt impregnated paper (felt) or a synthetic ice and water shield. Post frame builders deal with this extra roof thickness by adjusting building eave height downward by sheathing thickness adjusted for slope. Roof truss spacing would not need to be adjusted for sheathing, as purlins will be supporting any underlying sheathing, just as they support your roof steel.

The Ultimate Post Frame Building Experience

Hansen Pole Buildings is on a mission to provide “The Ultimate Post Frame Building Experience™”. (Read about “The Ultimate Post Frame Building Experience™” here: https://www.hansenpolebuildings.com/2016/06/ultimate-post-frame-experience/) In doing so, we often make what I will refer to as ‘tweaks’ to make not only our clients’ experiences better, but also their new post frame buildings better.

About Hansen BuildingsWe look for trends in questions asked by owners of existing pole barns – usually not even those we provided! There are a couple of these our team has decided to address and we have so far done a very poor job of letting our clients know we have done so.

Lesser of these items are folks who decide, for whatever reason, they would like to add either plywood or OSB between their new post frame building’s roof purlins and roof steel (https://www.hansenpolebuildings.com/2017/03/osb-steel-roofing-pole-buildings/).

Near universally pole barn builders and kit suppliers (as well as most truss manufacturers) have designed trusses with barely enough load capacity to meet minimums. In most instances, actual weight of materials (dead load) of roof truss top chords is around 2.5 psf (pounds per square foot). This is enough to account for truss weight, roof purlins, some sort of reflective radiant barrier or other minimal condensation control, as well as light gauge steel roofing. We have been using 3.3 psf just to give a little extra cushion (roughly 1/3rd more capacity).

½-inch plywood and 7/16-inch OSB both weigh 46 to 48 pounds per four foot by eight foot sheet or 1.5 psf. In order to account for possibilities of someone wanting to add one of these sheathings during building assembly, Hansen Pole Buildings has opted to increase our design top chord dead load to five psf for clearspan trusses up to and including 40 feet. This is DOUBLE minimum requirements.

Tomorrow, I will share with you a solution to an all too frequent challenge.

Stay tuned……

OSB versus Plywood

Plywood or OSB; OSB or Plywood?

Oriented strand board (OSB) long ago became the market leader, in relationship to plywood. As much as 75% of all sheathing is now OSB, thanks mostly to cost conscious buyers.

OSB vs. PlywoodOSB versus Plywood Prices

Prices for commodities like structural panels are notoriously volatile, and plywood can often be nearly double the price of comparable OSB sheathing. This can result in a savings of hundreds, if not thousands of dollars on an average OSB sheathed post frame building.

OSB versus Plywood Structural Differences

APA – The Engineered Wood Association (the main trade group representing panel manufacturers) says there is no real difference between the two panels. The structural characteristics are equivalent, and they can be used interchangeably. Both are rated Exposure 1 for temporary vulnerability to the weather; they have equivalent nail withdrawal resistance; and they’re installed using the same methods and construction details. However, there are differences.

OSB has more going for it than just cost. “Green” folks appreciate it can be made from small, fast growing trees, many of which come from tree farms rather than forests. OSB boasts a more consistent density. While a sheet of plywood might be 5 to 7 plies thick, a sheet of OSB is made from as many as 50 strand layers packed and compressed into the same thickness. There’s no equivalent of the weak spots which can be left in plywood when knotholes in adjacent plies overlap.

OSB versus Plywood Moisture Reaction

The biggest difference between the two panels is how they react when exposed to large amounts of moisture over extended time periods. With the exception of projects in very arid regions, sheathing and flooring panels are routinely covered with rain, snow, and ice during construction delays. This is where plywood has the edge.

When plywood gets wet, it tends to swell consistently across the sheet, and then returns to its normal dimensions as it dries out. It dries out relatively quickly, and the swelling is usually not enough to affect floor or roof finishes.

OSB takes longer to get wet than plywood but also takes longer to dry out. When used as roof sheathing, this tendency to hold moisture means it can degrade faster than plywood when exposed to chronic leaks.

When OSB does get wet it also tends to swell along the edges, and those edges stay swollen even after the material has dried out. Swollen edges have been known to telegraph visible ridges called “ghost lines” through asphalt roof shingles (just another reason to use steel roofing over purlins for pole barns).

Manufacturers insist OSB’s moisture problems have been corrected, thanks to the development of water-resistant edge seals. But of course that edge seal is lost when panels get cut on site, as they often do.

Screw Adherence in OSB versus Plywood

For post frame applications where screws will be placed into sheathing only (rather than into roof purlins or wall girts), steel roofing manufacturers specify the use of plywood, rather than OSB, as screws have a greater propensity to pull out of OSB under a wind load.

There you are…it’s 27 of one and 14 of another, but when it comes down to it…you need to weigh the pros and cons of how the osb/plywood will be used to make a conscientious choice.