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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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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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