Steel roof: to sheathe or not to sheathe?
I’m considering a steel roof for our new house build (hip design, 7 or 8:12 pitch) and I’m wondering whether sheeting first with OSB or plywood and a moisture barrier makes sense? I’ve done a little reading and it seems there are some who are in favor of sheeting and others who see it as a waste of time/money.
The main concern seems to be with moisture condensing under the roof when cool night air drops below the dewpoint of air in the attic. To this I question:
1. Is there a significant difference between a roof with shingles and sheeting vs. a steel roof? Why wouldn’t the underside of a conventional shingle roof condense moisture the same way?
2. Does adding a layer of sheeting under a steel roof change the physics of the condensation process? Does the added R value of the sheeting + air gap (maybe R1?) make a significant difference to the air in the attic?
3. Is there something else going on here? Do steel roofs naturally leak (at fasteners and panel joints) and require the sheeting + moisture barrier to keep attic insulation dry?
My house is planned to have a blown cellulose attic between R60-80 with meticulous air sealing and absolutely no penetrations (if I can help it), so moisture escaping from inside the building envelop should not be an issue. The roof would have 4′ deep eaves for most of the perimeter and can have generous venting if required, along with raised heel trusses to keep wind currents from disturbing the insulation.
Personally I don’t buy that sheeting will make my roof significantly quieter with the considerable insulation planned, but real world experience based feedback would be nice here too.
I appreciate any and all thoughts from this forum’s considerable wealth of Building Science experts.
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Replies
Forgot to mention, location is Ottawa Ontario, equivalent to 6A/7A US climate zone.
Lance,
Install the sheathing.
Steel roofing is conductive and thin, so on cold, clear nights, it quickly gets colder than the outdoor air (due to nighttime radiation cooling -- bascially the roofing's heat radiates rapidly into outer space).
Asphalt roofing is less conductive and thicker, and is always installed over sheathing. So the shingle + underlayment + sheathing sandwich has more thermal mass, and doesn't cool as quickly on clear nights.
Installed without sheathing, the steel roofing gets cold quickly. If the morning air is warm and humid, you'll get a lot of condensation on the back side of the steel roofing. It will drip.
The advantage of installing the sheathing is that it allows you to install roofing underlayment (which is code-mandated, by the way -- so skipping the underlayment is a code violation). The underlayment (for example, asphalt felt) catches the condensation and holds it until it can evaporate.
-- Martin Holladay
Martin, thanks for your thorough response.
I was in contact with a local steel roofing supplier, and he is recommending against sheething below his steel panels. He recommends installing an underlayment product directly over the trusses as the strapping goes on.
Other than the obvious issue of mis-stepping through the underlayment during construction, do you see an issue with this approach? He claims their 29ga sheet profiles have been installed this way for years with no issues.
I would assume an underlayment capable of spanning 24" truss spacing would be more expensive than good-old tar paper, which could offset some of the savings in sheething? Though an underlayment that wasn't held tightly to the strapping by the sheathing (drooping slightly) might allow for better drainage of any accumulated moisture? Just thinking out loud.
Thanks again!
Lance,
Do they sell a thicker panel? 29g a is pretty flimsy, especially when installed on strapping, and doesn't hold stitch screws well.
Malcolm, they do sell a 26ga product as well, but at a significant 40% cost increase.
Lance,
I considered a 26 gauge (25 year) galvalume roof for current home but decided to go with 24 gauge (45 year) panels instead. The difference in materials cost was less than 10 percent. May be you should check with a few other companies before finalizing your decision.
Lance,
Assuming that your roofer doesn't mind working on a roof without any sheathing, and is willing to drape the roofing underlayment over the trusses, that approach will work -- with one caveat: It will only work if your insulation is on the attic floor, not at the roof slope.
If you want to install insulation that follows the roof slope, you need a durable air barrier under your roofing. (That means either plywood sheathing or OSB sheathing with taped seams.)
If you have your insulation on the attic floor -- meaning that this is a vented, unconditioned attic -- your roofer's approach could work. Just remember that if you ever change your mind, and you want to created an unvented conditioned attic, the lack of an air barrier becomes a big hassle.
-- Martin Holladay
Lance,
Ouch - that's quite a bit more. I haven't seen 29 ga used around here in a decade, but maybe it's a regional thing. When you get to thicker panels the difference isn't that important - it is the finish that provides the longevity. But with 29 ga you are skirting the point where the material is so thin it is pretty susceptible to mechanical damage.
Edit: As to your third question. Metal roofs don't (or shouldn't) leak at all. Not when installed, or in the future, if detailed properly.