Metal Ag roof over doubled up foam and ice and water shield
user-883383
| Posted in Energy Efficiency and Durability on
I would like people’s opinion as to whether an ag style metal roof (exposed fasteners w/ neoprene washers) would be sufficiently leak proof if installed on strapping over two layers of foam and ice and water shield as detailed on the site here.
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Replies
Joel,
A properly detailed metal roof should be leakproof, even without the Ice & Water Shield.
To be sure that a roof is leakproof, it has to (a) be designed according to good roofing principles, without any stupidities like dead-end valleys or chimneys located in a valley, (b) include good flashing, and (c) be installed conscientiously. If those principles are followed, the roof won't leak.
There are sometimes issues with water vapor condensing underneath ribbed roofing then getting into buildings. If it is going flat on plywood I think some manufacturers recommend a speciam foam strip to keep cold air from getting in at the bottom. There was a discussion once in FHB about how to run two layers of strapping perpindicular to each other, to facilitate draining this condensate.
Martin, So standing seam metal roofing is not required to get a leak-proof roof? I was under the impression that exposed fasteners and the expanding/contracting nature of metal could have potential to leak at the fasteners. I have also seen leaks on relatively new metal barn roofs (less than 10 years old) but maybe this was due to other factors such as poor installation.
Joel,
I have a barn that has exposed fastener steel roofing over 2x4 strapping and there is one leak in the field of the roof.
Turns out it is an over-driven screw that is the cause - the rib is dented under the screw cap.
But it doesn't leak when it rains, only when there is a pile of melting snow on top of it.
I have also put the same steel roofing panels on the house I'm building though the assembly is different.
There are no leaks into the attic.
I have heard that expansion/contraction can be an issue with very long panels, especially if painted in a dark colour.
Best to contact the panel manufacturer about these types of design issues...
They have engineers that can answer specific questions for different products.
I know several contractors who are uncomfortable with 5V 'farm tin' roofs on anything but skip sheathing over a vented accessible attic, so that if there is any shortcoming in any of Martin's three requirements the location of the leak can be easily traced and fixed. It's in the nature of the material that water can travel a distance from the point of origin before showing up in interior finishes and this can be an especial nightmare with cathedral ceilings.
FWIW I've seen far fewer problems with the master-rib profile than with 5V. Plus I think it's a better-looking product without 5V's propensity for oil-canning.