Is two layers of roof felt OK?

I am about to start to re-roof my house using the same fiberglass standard three tab shingles. If I am able to remove just the shingles and leave the 23 year old felt paper on the roof should I. Assuming it is laying flat would adding a new layer of 15 or 30 lbs. (?). over it plus the new shingles possibly cause any problems? I.E. moisture / condensation or heaving? My thinking is during the project that it might protect the interior from any rain even if I temporarily have the construction area tarpped. Also even though it has nail holes it would add a bit more waterproof coverage over the sheathing after the job is finished not to mention less to go into the landfill.
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My guess is that not much of the original roofing felt will be left after the shingles are removed. As long as the nails are driven flush with the deck, I can't think of a problem that would result from leaving what's left. A lot of folks who should know don't think the roofing felt adds much to the water resistance of the roof system. (Ref Joe Barta & Building Science Corporation) TB
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snipped-for-privacy@bellsouth.net wrote:

Unless of course the roof covering is tile, in which case the underlayment IS the primary water barrier.
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Travis Jordan wrote:

And, I should add -- one very good reason for having a semi-permeable or impermeable moisture barrier between the roof covering (ie: shingles) and the sheathing is to prevent any condensation that forms on the back of the shingles from dripping onto the sheathing. And I think all current shingle manufacturers require an underlayment layer, as do many state and local codes.
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Folks around here use a lot of "wrinkled tin" as roofing. I am now suggesting they use one of the "rubber" sheets. TB
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i think the primary function of felt under three tabs is to prevent the shingles from adhereing to the deck. i once inspected a roof that didn't have felt. the shingles were cracked along all the plywood edges.
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tar paper or a much better choice rubber impermable membrane is a essential part of the keep dry system.
blowing rain can get under the shingles and cause a leak. this happened to my travel agent her home builders kids crew didnt remember the tar paper, after numerous roof leaks and insurance dis owning the water damage it finally got fixed had to replace roof.
other things to do when replacing roof, REPLACE flashing everywhere, covers around serer vents, how about ridge vent upgrade, drip edge and inspect all chimneys and chimney caps for cracking which can cause CO2 poisioning and death... lighter color roofs last longer and help keep home cooler in summer, dark colors absorb heat.
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snipped-for-privacy@aol.com wrote:

lighter color roofs do last longer and they keep the attic cooler in the summer. However, studies at the University of Florida have shown that if you have adequate attic insulation (R-30+) then the attic temperature has no discernable impact on the house's heat load.
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lighter color roofs do last longer and they keep the attic cooler in the summer. However, studies at the University of Florida have shown that if you have adequate attic insulation (R-30+) then the attic temperature has no discernable impact on the house's heat load.
Thats very likely BUT lighter colored roofs reflect more heat thus the attics are cooler so the shingles last longer. I recently sold a home with gable end and ridge vent. The home inspector wrote it up because the attic was 20 degress warmer than the outside on a hot july day. He recommended adding power vents to the roof.
Light colored roofs just make everything easier
you want a cool attic if your like many and use it for storage.
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snipped-for-privacy@aol.com wrote:

That inspector was uninformed. Power vents might be an OK idea if they are solar powered, but the University of Illinois did a study in 1990 that showed that the cost of operating power vents exceeded the energy savings in a properly insulated attic.
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the home inspectors were a joke, had 2 home inspections on the same home within a month of one another. the 2 inspections looked like 2 different homes.
the first inspection scared off the first buyer, sad because it was a nice solid home
first home inspector wrote up no GFCI on sump pump, 2nd inspector wrote up it was wrong to GFCI a sump pump. all sirts of stuff like this
I just couldnt win
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snipped-for-privacy@aol.com wrote:

That is generally my experience, too. Only once did I have an inspector make a good call - he was concerned about a stairstep hairline crack in an outside structural wall, and recommended a structural engineer. The engineer took core samples from beneath the foundation and found a vacant area most likely caused by decomposed organic matter in the original fill dirt. Negotiations with the seller went much, much better after that.
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Last few times I tore off shingles, the tarpaper got torn up pretty badly and for the most part just pulled right off with the shingles anyway. What was left was so wrinkled, torn, and bunched up that it would have been a pain to try and flatten it back out. I just pulled the rest off and started clean with new tarpaper. Lets you inspect the deck anyway, which you should do while you have the shingles off.
-Kevin
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