reason for felt-pad roof underlayments?

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what are the reasons why felt padding (and its many successor materials) are used in constructing a roof?
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On Mar 8, 6:34 am, dances_with snipped-for-privacy@yahoo.com wrote:

The primary reason is to prevent shingles from adhereing to the plywood. This was driven home to me once when I inspected a roof that had been shingled without felt. The seams between the plywood sheets telegraphed through to the shingles and they tore. Whole roof had to be replaced even though it wasn't that old.
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marson wrote:

How did the plywood seams tear the shingles?
Matt
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wrote:

Rafters move around, especially on a stick-framed roof. Plywood expands and contracts in the sun at a different rate than the shingles. If the shingles are completely stuck to plywood accross a crack, and the crack gets bigger, the shingles can tear. If the rafters are 24" OC, and the roof is decked with the current cheap 7/16, if someone heavy steps on a horizontal crack, one sheet can flex where the one above or below doesn't, and again you get tearing. I'm pretty big, and my roof is flimsier than I would like. Up there, I only wear clean tennies, and walk as gently as possible, even though the roof is less than a year old. (I got spoiled as a kid- on the houses my father designed and built, the roofs were overbuilt by modern standards, and felt as solid as any of the floor decks. And he used 2x10s for those, not 2x8s like the cheap builders.)
aem sends....
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It's purpose is to protect the roof deck from rain, snow and ice.
On low-slope roofs (2:12 - 4:12) rain water drains slowly and creates the potential for water to back up under the shingles. Also, wind-drive rain can force water under the shingles,wetting the deck. Because of this, two layers of underlayment are required to ensure the deck remain weathertight.
On roofs over 4:12, a single layer of 15 lb underlayment has been found to be sufficient to protect the deck.
In areas where the average daily temperature is 25 deg or less, there's also the additional possibility for ice damming at the eaves. So the codes require additional requirements of having two layers of 15 lb underlayment, cemented together to a point 24 inches up from the exterior wall. (Could also use one layer of 30 lb, self-adhering material in place of two layers of 15 lb felt.)

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Never heard of two layers of 15#. Shingle wrappers still talk about two layers of 30# mopped with tar on 2/12 or less and on eaves, but noone does that any more--ice and water shield is used in those cases where water can get under the shingles. 15# is very prone to tearing and is a poor choice for protection. Again, 15# is used to prevent shingles from bonding to the plywood.
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We've got a house (in southern CA) that has NO felt at all -- and concrete tiles for the roof material.. We've got quite a few cracked tiles now (after 15 years) and have leaks in quite a few places.. Needless to say it's my opinion that the roof was installed incorrectly (no plywood sheathing is down either -- just well spaced slats) and so the rain just comes into the attic. If there was a layer of felt and plywood sheathing, the rain would stay out.. Oh well.. I wasn't around when the current roof was installed or I would have caught it.. Needless to say, it's going to be ripped off soon and done properly.
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There are plenty of instances where a felted and plywood-sheathed roof leaks. A poor installation is a poor installation. The skip sheathing (spaced slats) was the standard way to do roofing for centuries. Roof felt is not waterproofing and should not be relied on to stop a roof from leaking.
R
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Yep. I don't know where the idea that the felt is the primary waterproofing came from. Anyone who has ripped off an old roof knows that the heat over the years pretty much destroys any waterproofing the felt had at installation. If water is getting as far as the felt, the roof has problems.
Harry K
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Roofing felt works well as a vapor barrier and protection from leaking shingles. I had a place in my roof, where an addition was to be added, where I had nothing but roofing felt for six months. The sun's UV rays destroy everything, and roofing felt is more prone to this wear and tear than asphalt shingles, but if the felt is covered by shingles, it does provide an additional layer of protection. I like 30lb felt--felt is cheap protection--but two layers of 15lb is good too. On a low slope roof, I recommend the peal and stick Grace underlayment OVER 30lb or 2 layers 15lb. Future demolition and removal of the roofing materials without destroying the roof deck is an important consideration.

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That's just not so, and even if it were, why would you want one under your shingles?
--


MichaelB
www.michaelbulatovich.ca
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Michael Bulatovich wrote:

Actually, it is so.
Matt
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With continuous open joints every 36 inches? Even if you caulked it, the material itself is at least 200 times more permeable than a poly VB *when dry*, and 1000 times when wet. See the link, half way down under "Sheet Good Building Products"
http://www.buildingscience.com/bsc/designsthatwork/buildingmaterials.htm
The question remains, even if it were, why would you want one under your shingles?
--


MichaelB
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Michael Bulatovich wrote:

I was referring to protection from leaking shingles. I wouldn't want a moisture barrier under my shingles, but something that helps with small leaks isn't a bad deal. And why would you leave open joints? The felt should be overlapped at least a couple of inches.
Matt
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It's been my observation that any water leaking past the shingles will also get thru the felt. If we consider that felt is made and sold by the companies what make and sell shingles--
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And considering all the holes punched through it from the nails/ staples. To say nothing of the incidental little rips and tears from working on it while shingling.
Harry K
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Chas Hurst wrote:

My roof had only felt for 5 weeks prior to being shingled and it was quite waterproof until it was torn off by the wind.
Matt
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I meant for houses built in the real world.
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Chas Hurst wrote:

How many houses have you built?
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Exactly... The last thing you want under a roof deck is moisture buildup. The decking and trusses wouldn't last more than a couple of years.
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