From: "aemeijers" 
Newsgroups: alt.building.engineering alt.building.construction misc.consumers.house
Subject: Re: reason for felt-pad roof underlayments?
Date: Fri, 09 Mar 2007 00:52:26 GMT


"Matt Whiting"  wrote in message 
news:wI0Ih.3044$Oc.170996@news1.epix.net...
> marson wrote:
>> On Mar 8, 6:34 am, dances_with_barka...@yahoo.com wrote:
>>> what are the reasons why felt padding (and its many successor
>>> materials)  are used in constructing a roof?
>>
>> 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.
>
> How did the plywood seams tear the shingles?
>
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....