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What is a Roof Felt Underlayment?

A pitched roof underlayment
is typically a membrane that is laid between the roof deck, which usually
consists of plywood or OSB sheeting, and the roof product itself. It can also
be used in-between layers of roofing material if 
one decides to add a second
layer (not recommended).

A pitched roof underlayment
is typically a membrane that is laid between the roof deck, which usually
consists of plywood or OSB sheeting, and the roof product itself. It can also
be used in-between layers of roofing material if 
one decides to add a second
layer (not recommended). Roofing underlayment’s come in several weights (15#
& 30#) along with several types 
of materials choices
(asphalt saturated paper) or a (synthetic weave). Most manufactures of composition
shingles are generally fine with both types and weight of felt underlayment,
though there may be restrictions on brand preference regarding warranty.

A pitched roof underlayment
is extremely important when it is considered 
part of the actual roofing
system, for instance, a shake roof or a low slope composition roof. However,
when it comes to standard composition roof 
installations, most quality
felts with an ASTM certification will do just fine.