The basic answer to the question is “YES”, but if you are interested in the science behind it, read on.
There are three reasons for installing roofing felt, or tar paper, under your shingles. I will discuss them from greatest to least in importance.
The primary function of roofing felt is to act as a slip sheet between your roof decking and shingles. From day to night there are vast temperature differences impacting the surface of your roof. These temperature differences cause your shingles to expand and contract. Shingles tend to get sticky as the heat up during the hot summer months causing them to sometimes stick to the wood decking. The problem is that wood and shingles expand and contract at different rates. When the shingle gets hot, it sticks to the wood, then rapidly contracts as night falls while the wood is still warm. This can cause the shingle to buck or lift because of this differential. I have observed this to be a predominant cause for nail “POPS” or backed out nails. The addition of a slip sheet or roofing felt layer to the equation is the answer to the problem, as the shingles bond to the felt and not the decking allowing a small amount movement without damage or distortion.
The next function of felt is to act as a moisture barrier for condensation. Due to the same day/night heating discussed above, condensation has been observed to form on the back of the shingles. Without the felt paper barrier, this moisture would be trapped against the wood decking, causing potential rot or mold problems. Felt also allows a small amount of airflow below the shingles
Lastly, felt provides a temporary barrier in the event of shingle loss due to storms or winds. Most people believe that felt’s primary purpose is to protect the roof in these circumstances, however during shingle installation, the average roof receives more than 10,000 nails applied through the felt surface, rendering it a poor barrier to the elements in a stand alone situation.
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