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Community and Q&A

Moisture and dampness under roof underlayment

derekr | Posted in General Questions on

I’m in the process of building and the underlayment was just put on the roof last week, I notice if I slip my hand under the underlayment there’s a decent amount of moisture

will this moisture be trapped forever if they put the shingles over it in the next few days?

the plumber is also putting up the exhaust pipe before the roofers come which he will have to cut the underlayment, if it rains between the time he puts up the exhaust and the roofers come will a lot of rain get underneath the underlayment where he cuts it?

im just worried about water being trapped under there before the shingles comes, will it eventually dry out even after the shingles are put on?

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  1. Expert Member
    Akos | | #1

    Most underlayments are not 100% water tight, so you will get some water through it. The lower the slope the worse this is. I generally prefer to use permeable underlayment as it allows for faster drying when the sun hits it. An impermeable synthetic underlayment would limit drying to the outside, but you will still get a lot of drying to the interior once the house is under roof. With a vented attic, it will continue to dry, but if you are doing any spray foam, make sure to check that the roof deck is dry enough before install.

    As for big holes like a stack, it is best to at least tape a bit around it with sheathing tape. It won't keep all the water out, but will keep out most of it.

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