On Mar 8, 6:34 am, dances_with firstname.lastname@example.org 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.
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.)
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.)
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.
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.
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.
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.
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.
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
The question remains, even if it were, why would you want one under your
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.
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