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Old 05-23-2011, 02:39 PM   #1
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Can 2x12 tongue and groove expand & tear 15-lb tar paper?


I have an old roof deck made of tongue and groove 2x12s. It's currently covered with tar & gravel and want to change to Asphalt shingles. Due to it's low pitch I planned to use standard 15lb tar paper with a 17" overlap, Another contractor mentioned that the expanding & contracting of the tongue & groove could rip the
15-lb paper. He says I should use 30-lb, or even better yet go over the whole roof with plywood 1st. What's your thoughts?
dbl layer 15#, single layer 30#, or plywood overlay 1st?
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Old 05-23-2011, 04:22 PM   #2
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Can 2x12 tongue and groove expand & tear 15-lb tar paper?


2/12 is as good as flat imo. Shingles are not to be used below 3/12. Well some manufacturers may let you but I have only done it a handfull of times at the demand of the customer who didn't want a low slope membrane.

Last spec I read for this application called for a minimum of a double layer of 15# felt or full coverage of ice shield. I'm on the fence about full I&W as I have heard it lead to condensation problems, but maybe that's because the ventilation wasn't proper. I did have this problem once with a full coverage of ice shield, but though the ventilation was proper, there was a heater in a connecting attic space which put a huge ammount of moisture into the attic. We blocked that air flow and installed additional ventilation, and haven't had any problems since. So I gues the moral of this story is make sure the ventilation is better than perfect.

Then again you said it's a flat roof with T&G, which in my kneck of the woods means the ceiling is exposed roof deck, which means there is not going to be ventilation, which means you need to build up a false roof deck, which means the cost of the job will double, which means Just install single ply.


FWIW I wouldn't use 15#, regardless of slope. I use 30# or fiberglass reinforced felt only.
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The information found in this post is not to be considered legal advice. All information should be considered relative, not specific. Never attempt any repairs you are not comfortable with. Always maintain safety! The author of this post takes no responsibility for any losses that occur. Use at your own risk.
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