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...
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