# roofing question...rain issue



## green2man0 (Jul 23, 2015)

I'm doing an add on to my garage. I'm finishing up the osb sheeting on the trusses and am wondering what the better option is. Getting rain on the osb or putting tar paper on what's up so far and hoping it doesn't bubble? It's foretasted for light rain most of the end of the week.


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## joecaption (Nov 30, 2011)

Get the paper on asap!
Paper is cheap and easy to replace.


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## Windows on Wash (Aug 30, 2011)

Tar paper or synthetic. If they tar paper gets wet enough, it will wrinkle and you just need to replace it before shingling.


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## SPG43 (Jul 17, 2016)

I had synthetic (GAF Feltbuster?) on mine and got hit by an unexpected downpour overnight. No problem. I took a peek underneath and it was completely dry. The synthetic didn't wrinkle or move at all. Gotta use those button cap nails though...


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## Nehmo (Oct 25, 2016)

green2man0 said:


> I'm doing an add on to my garage. I'm finishing up the osb sheeting on the trusses and am wondering what the better option is. Getting rain on the osb or putting tar paper on what's up so far and hoping it doesn't bubble? It's foretasted for light rain most of the end of the week.


Just for your own education, ask (perhaps in a different forum) what the purpose of "tar paper" is. If the group has a decent following, you'll get all kinds of answers. 

Once the shingles are on, the shingles are supposed to be the water barrier. Felt paper (underlayment) is just something to cover the roof deck *while* you put the shingles on. If the shingles are on, the paper doesn't do anything. If you happen to lose a shingle, then the paper may protect the roof, but that's about it.


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## SPG43 (Jul 17, 2016)

Nehmo said:


> Felt paper (underlayment) is just something to cover the roof deck *while* you put the shingles on. If the shingles are on, the paper doesn't do anything.


Not quite. The underlayment of either tar paper or synthetic is there for a reason. Or actually several reasons. 
-Vapor barrier. Warm air will travel through the ceiling, attic, and then needs to escape instead of sitting on the plywood sheathing causing rot. The tar paper allows the moisture through as vapor where it forms condensation on the back of the shingles instead of in contact with the plywood. Tar paper/synthetic underlayment blocks water drops but allows movement of vapor.
-Separation layer. Wood sap in plywood can damage the shingles and tar on the shingles can cause problems for plywood. Tar paper keeps them apart so they all last longer. 
-Release layer. The tar paper acts as a release to allow a little movement in the shingle so that they can expand and contract with the temperature without bunching up.
-Oh crap it's raining and I'm not done shingling protection. Besides that scenario, it also can be a bit of a protector from wind driven rain or other small amounts of moisture that find their way behind a shingle to be directed out. 

Tar paper and synthetic underlayments do have a very important function and if they didn't do you really think that every single asphalt shingle roof would still have them?


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## Yodaman (Mar 9, 2015)

I know this is a couple days old, and my comment may be pointless.
You didn't mention the size of your project, but I often tarp exposed roof work that needs temp. protection. A med duty tarp laid down with vertical furring strips will get you the time needed to complete.
Don't wast your money on the thin blue tarps. You need the brown or gray.


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## Nehmo (Oct 25, 2016)

SPG43 said:


> Not quite. The underlayment of either tar paper or synthetic is there for a reason. Or actually several reasons...


Since all of your points (except the one I already mentioned) were borderline ridiculous, I'm wondering if you just meant to be sarcastic or something. That being the case, I applaud the joke, and, of course, I won't be suckered into making a rebuttal of those points.

Your next-to-last point (precautionary protection of the roof while doing the shingling) was the one I already made, and that's the only valid reason other than having a sort-of second layer in case a shingle gets blown off.


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## SPG43 (Jul 17, 2016)

Nehmo said:


> Since all of your points (except the one I already mentioned) were borderline ridiculous, I'm wondering if you just meant to be sarcastic or something.


So sorry to be so completely wrong about how every single roofer has put on every single roof for the last 50 years. I guess you know better. <---Now that's sarcasm.


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## Windows on Wash (Aug 30, 2011)

I have seen it mentioned that underlayment is somehow related to "vapor barrier" above and that is incorrect. 

There is virtually not change in vapor permeance when you consider that the plywood is already a vapor retarder. The primary reason is the second mentioned of it being a release layer. 

Helps to protect the roof deck between removal and put back and is a release layer. Also just a good additional protection against blow offs and mechanical damage to the roof.


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## roofermann (Nov 18, 2013)

Windows on Wash said:


> I have seen it mentioned that underlayment is somehow related to "vapor barrier" above and that is incorrect.
> 
> There is virtually not change in vapor permeance when you consider that the plywood is already a vapor retarder. The primary reason is the second mentioned of it being a release layer.
> 
> Helps to protect the roof deck between removal and put back and is a release layer. Also just a good additional protection against blow offs and mechanical damage to the roof.


Underlayment is also required to meet the manufacturers specs (read the shingle wrapper) How all these guys in Canada are getting away with nailing shingles to a naked deck is beyond me.:vs_worry:


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## PatChap (Nov 17, 2012)

roofermann said:


> Underlayment is also required to meet the manufacturers specs (read the shingle wrapper) How all these guys in Canada are getting away with nailing shingles to a naked deck is beyond me.:vs_worry:


We get different wrappers than you. Because its not code required they can only recomend its use.


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## Windows on Wash (Aug 30, 2011)

Must pulling the roof for replacement a SOB to remove.


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## roofermann (Nov 18, 2013)

PatChap said:


> We get different wrappers than you. Because its not code required they can only recomend its use.


Remind me not to buy a house in Canada then.


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## PatChap (Nov 17, 2012)

Nah they usually strip fine. I dont really notice any difference on felted decks vs bare. 
Once every couple years we run into old sappy boards and wind up scrapeing. Very rare, its like scraping off ice and water when it happens though.


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## SPS-1 (Oct 21, 2008)

roofermann said:


> How all these guys in Canada are getting away with nailing shingles to a naked deck is beyond me.


 There are some new townhouses near my place ---- no underlayment under the shingles. And every time there is a heavy rain, next day there is a blue tarp over one (or more) of them.


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## PatChap (Nov 17, 2012)

Underlay has nothing to do with keeping the house sry once the shingles are installed. What they have there are sone poorly installed roofs


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## Windows on Wash (Aug 30, 2011)

As stated earlier, more about keeping the roof dry before getting the new shingles down.

That being said, when you talk about synthetic underlayments, they are cheap insurance and an additional layer of protection with a bunch fewer seams in them.


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## Nehmo (Oct 25, 2016)

SPG43 said:


> So sorry to be so completely wrong about how every single roofer has put on every single roof for the last 50 years. I guess you know better. <---Now that's sarcasm.


Ho ho, lol, *☼Doubble sarcasm!☼* You are too advanced for me. 
We must assume you have the common sense to know that we know you couldn't possibly have known about _all_ those roofers. So, you must be making a joke on a joke! Very clever!
And I thank you for the compliment, but it wasn't necessary. The comedy you provided was enough.


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