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.
Insulating a Steel Truss Building
Reader JONATHAN in MISSISSIPPI has been planning a building using steel trusses and has insulating questions. He writes:
“I have recently found your blog and I have to say I am on good information overload. I’ve read your posts on insulation and air barrier more than twice maybe more. I live in Mississippi so hot and so humid.
My plan is to build a 32×60 using steel trusses 10′ on center and 2×6 purlins and at the 28′ mark I am wanting to put up a wall to cut the space in two, half wood shop half living area. My biggest question is about insulating the roof for both areas the same, which would be a closed/unvented roof (no attic). I am going to put sheeting over the whole building (walls and roof) and use closed cell spray foam for insulation on the roof, filling the entire cavity of the 2×6’s. On the underside of the 2×6’s I am going to install some seasoned metal for the ceiling.
My question is, what if anything do I need to install between the metal roofing and the sheeting? Tyvek? 30# roof felt? or would this work https://www.lowes.com/pd/48-in-x-250-ft-1000-sq-ft-Synthetic-Roof-Underlayment/3151833? Does a unvented/closed roof need to breathe any? Because if it doesn’t I really like the synthetic roof underlayment. Or do you have any suggestions?
On the walls I am going to stud vertically between the posts with 2×6’s with sheeting on the outside, cover it with Tyvek, and metal over that. What suggestions do you have on insulating the walls? Do I need an additional vapor barrier on the inside of the walls? I was thinking maybe a thin layer of closed cell foam on the inside and going with mineral wool insulation batts between the studs.
I had a lot more questions than I thought I did, whew! I just want to make sure I am doing it right, without any problems down the road and I am ok with a little overkill and cost to do it. Just wish I could afford/justify SIP panels for the roof.
Any and all information and guidance is appreciated.”
Mike the Pole Barn Guru writes:
I will first express my concern for your desire to use steel trusses. Unless your provider can furnish engineer sealed drawings showing adequate load carrying capacity for your particular circumstances (you have added dead loads beyond what they are typically designed for, as well as an appropriate wind load) I’d be running away from them. They also should be designed to minimize deflection. I’d want some written proof of these trusses having been third party inspected for quality as well. You are going to be making a significant investment into your new building – no reason to have it fall down around you.
Moving forward. Between roof sheathing and steel roofing you do need to have something. A minimum of 30# felt should be used, although synthetic underlayment would be just fine. You may want to investigate a system including a ventilated roof mat, as it will reduce thermal heat transmission. A weather resistant barrier such as Tyvek would be an absolute wrong product.
For walls, you should create a thermal break between studs and interior. I’d glue two inch closed cell foam boards to stud inside face and then glue 5/8″ gypsum wallboard to foam board inside face. I’d probably fill wall cavity with BIBs insulation rather than closed cell foam and mineral wool batts. This will more fully fill cavity without creating voids.
I have yet to see SIPs as being economically practical. They appear to be expensive enough so as to preclude ever being able to recoup investment costs.