- Q.What is the purpose of
tar-papering a roof other than to keep water out until
the shingles are laid down? It seems that if water does
get past the shingles that the tar paper, having all of
those staple and nail holes, would do little to keep
the water out.
A.Paul Fisette
responds: Tar paper, or roof felt, is required
by most building codes. Roof shingle manufacturers
require it to maintain the warranty on the
shingles. And, as you point out, roof felt provides
temporary protection from weather while you close
in the house. Also, the fire rating of a roof
covering is assigned to an entire roof assembly,
including roof felt. If you don't install the felt,
you don't earn the fire classification that may be
required by the local code. Applying roof felt can
reduce telegraphing of sheathing seams through thin
asphalt roof shingles. However, if you install the
felt carelessly, it can create a lumpy look.
Roof felt does provide a long-term benefit
— a second line of defense if water gets
past the roof covering. Strong wind can blow
rainwater in along the edges of the roof, or even
lift off shingles. True, the roof felt does have
nail penetrations and water can leak through these
nail holes. But the holes are small and fasteners
do not penetrate most of the roof surface, so the
membrane can keep a lot of damaging water out of
the roof frame. If there's no roof felt and you
develop a leak, the water will run down under the
shingles and pour through the seams in the roof
sheathing.
Overall, water intrusion is likely to be much
less with felt than without. It may buy you time to
repair the roof before any major damage is done to
the structure.
Paul Fisette is director of
Building Materials and Wood Technology at the
University of Massachusetts Amherst and a JLC
contributing editor.