Slate roof: Best underlayment?
jcrittenden
| Posted in General Questions on
My contractor wants to put ice & water shield on the whole roof under the slates. It is of course more expensive, but I like the idea of it being “more waterproof” than just felt or synthetic. This is in southern Oklahoma in a humid-“ish” climate, with severe weather and wind driven rain. The roof has a gable vented attic. Is ice & water shield okay? Will it trap moisture and rot the sheathing? Should we go with a more breathable underlayment? What is the best long term option for this potentially 100 year roof?
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Replies
J.,
Regardless of what type of underlayment you settle on, I hope that your slates will be installed on skip sheathing above the roofing underlayment. For example, if your slates are being installed with an exposure of 6 inches, and you are installing 1x4 skip sheathing (with a width of 3.5 inches), then you'll have a gap of 2.5 inches between each course of sheathing.
Skip sheathing ensures that there will be some air under the slates, facilitating drying after a rain storm.
Properly installed, your slates won't leak. A slate roof demands top-notch valley flashing, drip edge flashing, and ridge flashing -- generally 16 ounce copper flashing, so that the longevity of the flashing approximates the longevity of the slates. You also need to insist on either copper nails or stainless-steel nails.
A traditional asphalt felt roofing underlayment should work fine, but if your contractor insists on Ice & Water Shield, it won't cause any harm -- as long as (a) you include the skip sheathing above the underlayment, and (b) the continuous sheathing under the Ice & Water Shield can dry inward.
My guess is that an underlayment like Tyvek Protec 200 is more likely to be working a century from now than anything containing asphalt.
I think the key to your situation is the vented attic. You will get sufficient drying of your roof system (rafters and sheathing) drying down into y0ur attic. Gable to gable venting only gives you air flow with enough wind to drive air in one gable and out the other but you also get diffusive drying to the attic space when the attic space moisture content is less than that of your sheathing and framing.
Better if your attic venting was soffit-to-ridge but what you have is good enough.
Peter