Strapping necessary under metal roof?
Ry_Buc
| Posted in General Questions on
Hi folks,
I’m building a 7:12 pitch gable roof in Nova Scotia, Canada. Planning on either 1/2″ or 5/8″ CDX sheathing over the trusses and will cover with Mento 3000 as our underlayment. The metal roofing we’re using (see attached) is not standing seam, but does have ridges that (maybe?) would allow some drying between the metal and the the underlayment. Given this assembly, how necessary do you think it is to add horizontal (or diagonal) 1X strapping between the metal and the underlayment? Should I think of this drainage plain like a wall rainscreen, or is it different in kind?
Thank you!
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Replies
Looks very similar to the roofing I am putting on which is from a company called everlast. Anyway, I am installing on strapping, but when I went to order it I noticed the install instruction were on solid sheathing. I had to ask if strapping was OK which it is. The point is you should probably talk to the manufacturer see what they recommend.
Thanks squarpeg. No sheathing for you at all? The manuf wants EITHER 5/8" sheathing, or strapping. Though my brain has trouble with materials that could potentially get wet being sandwiched together. But maybe roofs are different and I should relax.
Sheathed, vapor barrier, then rigid foam, then strapped. The first house I built 40 years ago I constructed the same way except I used fiberglass batts, a cathedral ceiling, rafters exposed, sheathed with ship lap, then duplicated the rafters on top of that 2x10's laid a vapor barrier over it filled it with fiberglass batts strapped it, hung metal. 40 years later my son bought the house and decided to put a standing seam on it. They pulled the metal off and it looked like the day I put it up there. The strapping part didn't seem to be an issue, way to steep to walk on so that was never going to be an issue.
Ry_buc,
I'm not a fan of strapping under metal roofs.The only circumstance where I think it makes sense is on retrofits to provide horizontal airflow over vertical air gaps that weren't adequately vented.
- If you need a ventilation channel to help the sheathing to dry, or to cool the roof, the place for that is underneath.
- Adding a gap provides the space for moist air to enter and condense. No gap, no moisture to deal with. The incidental condensation that may may occur between the sheathing and the underlayment is easily dealt with by the profiles of the metal.
- On its own providing a gap doesn't help. If it's for ventilation, it has to be continuous, deep enough, and have both an protected entry and exit to be effective. If it's for drainage, it needs a continuous drainage plane to the eaves, which has to somehow be detailed to allow that water to escape at that vulnerable point where the panels and drip-edge meet. Whenever I see this on sections, I notice they get a bit vague about what actually happens there.
- Adding strapping moves the load-bearing from the sheathing to the panels, meaning they are more prone to damage from workers, or the impact of debris.
- Strapping alone (no sheathing) reduces the strength not only of the roof diaphragm, but of the entire structure.
Thanks Malcolm. Those are some solid points in favor of no strapping.
Ry_Buc,
If you have a vented, trussed roof, where the sheathing can dry to the underside, I don't think a permeable underlayment like Mento 3000 adds anything either.
A vented, trussed roof is what we have. Would you recommend an impermeable underlayment instead?
Ry_Buc,
I would. When building science research firm RDH looked at that they concluded that in wet marine climates the increased protection of an impermeable underlayment outweighed the benefits the small amount of topside drying you might get.
I've often wondered if direct contact with the sheathing provides a thermal coupling of significance to keep the temperature of the metal higher in a night sky radiation situation. Metal, being as thin as it is, seems prone to night sky cooling at a faster rate than sheathing. Not sure if this would be significant though.
maine_tyler,
It could well, but I wonder whether the cooling of the metal matters much if the lower temperature doesn't affect anything? With a gap, you might get a bit more dew on the top side, but I think what really matters below is limiting the amount of moist air that is in contact with the panels.
Well if you put the metal in contact with the sheathing you're reducing the air space and (in theory) increasing temperature in a night sky cooling scenario-- i.e. your doing both at the same time, so cannot isolate the variables. But you're probably right that the reduced airspace matters more, or perhaps only.