Tag Archives: roof sheathing

Endwall Trusses, Valley Flashing, and Roof Sheathing

This week’s Ask The Guru addresses how a single truss is used at endwalls instead of a double truss, use of valley flashing on roof applications, and the need for sheathing on a roof.

DEAR POLE BARN GURU: On a double truss system with 12′ o/c how do you do the gable ends? MICHAEL in KALAMAZOO

DEAR MICHAEL: A single truss is placed on each endwall. If building has end overhangs, they are supported by purlins running over top of end truss (end truss is lowered by height of purlins adjusted for slope of roof).

 

DEAR POLE BARN GURU: Roof only pole building, does flashing go in the roof valley on the dormer? STACY in CARRIERE

DEAR STACY: Regardless of whether a building is a roof only, or fully enclosed – if it has steel roofing, then valley flashing must be used to properly seal any valley. For extended reading: https://www.hansenpolebuildings.com/2021/01/dont-let-valleys-get-you-down/

 

DEAR POLE BARN GURU: Do I need to use sheathing under my pole barn garage roof? JOHN in BRIGHTON

DEAR JOHN: If using anything other than structurally rated, through-screwed steel roofing, then your answer is most likely yes. With through screwed steel roofing, provided you have utilized appropriately sized fasteners, at correct spacing, then sheathing would only be a requirement if shear loads are greater than what your roofing’s shear load capacity is. In most instances fasteners at top and bottom of panels should be something such as Leland Industries ‘diaphragm screws’ on both sides of each high rib. For extended reading on these screws, please read: https://www.hansenpolebuildings.com/2024/03/thru-screwed-steel-screws/

Sheathing Under Portion of Steel Roof

Sheathing Under Portion of Steel Roof

Loyal and very kind reader JOSEPH in LaPORTE writes:

“Good afternoon, let me start off by saying that I’m extremely appreciative of your content. The reason for my email is that I’ve been contracted to build several 30x180x9 post frame buildings and my plans are calling for 5/8” OSB for the center 60’ of the buildings. Do you have a diagram that shows how to “lower” the purlins 5/8’s to reduce the transition? Thank you in advance.”

Thank you very much for your kind words.

Cases such as yours should always (but rarely are) specified by your building’s Engineer of Record on sealed building plans, but somehow rarely are. Our third-party engineers take it upon themselves to actually account for situations such as these and don’t just leave it up to contractor’s whims to resolve.

Your solution is actually fairly simple – in areas with OSB order trusses with standard 1/4″ heel cuts (vertical ‘butt cut’ on end of bottom chords). For areas without OSB have heel cuts increased by thickness of OSB (in this instance heel cut would be 7/8″). This will allow for all truss carriers to be set to same height and result in a smooth roof plane.

As you have trusses on top of truss carriers, where your transition in heel heights occurs, attach a 2×4 ledger alongside top chord of first 7/8″ heel cut truss (at extreme edges of OSB sheathed area) to support ends of 2×4 purlins laid flatwise.

Somewhat unrelated to your question – I can only assume OSB has been added due to buildings being very long and narrow (aspect ratio is 6:1). If this is indeed your situation, then steel roofing by itself is unlikely to be able to carry applied shear loads. However, greatest shear is at each endwall, not at center of roof. I would have expected to see a need for OSB closest to each end, rather than in center 1/3rd. You might want to point this out to buildings’ engineer as (given information at hand) it could prove to be a serious design flaw and could potentially result in a failure situation under extreme wind loads.

Purlin Spacing, Wall Insulation, and Roof Sheathing

This week the Pole Barn Guru answers reader questions about purlin spacing for 2×8 that span 17 feet, wall insulation recommendations, and if roof sheathing is needed for a new shop.

DEAR POLE BARN GURU: I need to span 17 feet with 2×8 purlins. Do I need to go 12 inch on center, or can I double 2x8s 24″ on center? Will deck with 7/16 OSB and install standing seam metal roof. BRAD in SPARTA

DEAR BRAD: IRC Rafter span tables are available online here: https://codes.iccsafe.org/content/IRC2021P2/chapter-8-roof-ceiling-construction

Scroll down to Table 802.4.1(1) (this would be for a minimal snow load).

Assuming 2×8 #2 Southern Pine 24″ on center will span only 13’1″ (even 2×10 #2 will only span 16’6″).

Going to a spacing of 16″ on center 2×8 #2 Southern Pine will span 17’1″.

 

DEAR POLE BARN GURU: Hello, I am looking to insulate the walls of my pole barn and plan on putting up vapor barrier. I insulated the ceiling last year and used Tyvek® basically to hold up the insulation as I can’t afford the steel yet. My question is should I also put up a vapor barrier on the ceiling? Thanks in advance, I appreciate the guidance as this is not my expertise. CHAD in GENESEO

DEAR CHAD: Henry County is Climate Zone 5A.

For your walls, I would recommend unfaced Rockwool either R-30 with a well-sealed interior vapor barrier or R-20 with R-5 well sealed continuous insulation boards on the interior (Comfortboard® 80 or EPS).

As you have under 8000 heating degree days, a ceiling vapor barrier would not be required. Hopefully your attic insulation is Rockwool as fiberglass is affected by moisture and loses R value when exterior temperatures drop. Make sure your attic space is properly ventilated (https://www.hansenpolebuildings.com/2018/03/adequate-eave-ridge-ventilation/).

 

DEAR POLE BARN GURU: Hello, I am building a 36x50x12 shop with 5:12 roof pitch, that will be conditioned space. The plan is to build out the interior with 2 small bedrooms and a bath then the rest open floor. I am putting in a ceiling at 12′ and will insulate the walls and ceiling leaving the roof uninsulated. Building is located in South Central Texas near La Grange. Does the roof need sheathing and felt or can I just lay the metal roof directly on the purlins? What about adding house wrap to entire building, walls and roof? Being in Texas, heat is biggest factor and with the a/c pumping inside I want to keep attic space vented correctly to keep moisture from building up. I plan to have large soffits to allow for soffit venting then ridge vent on roof. One of my biggest concerns is keeping out all the elements, including creep crawly bugs and concerned the garage doors will be source intrusions. It is my understanding that a sectional garage door is best for keeping out the elements vs roll up type. Can a garage door be truly sealed from all elements including bugs from crawling in? STEVEN in SUGAR LAND

DEAR STEVEN: Roof probably does not need to be sheathed unless your design wind speed is 140 mph or greater (Code requirement) or your engineer specified it. Do NOT housewrap roof, order roof steel with an Integral Condensation Control factory applied. Order raised heel trusses so you can have full attic insulation depth from wall-to-wall. While a sectional garage door is best, there is never a 100% guarantee of keeping crawling critters out – because you are going to open door at some point.

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.