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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.