Should I put a felt underlayment under my shingles?

HVAC & Fireplace Bucket EXPERT April 01, 2008

Felt underlayment for asphalt shingles is common practice and is required by some authorities. There are several reasons one might use underlayment and some problems associated with it.

Possible Advantages 1. Some people say it is a second layer of protection for the roof against leakage. However, once the building paper has had several hundred holes punched through it by the roofing nails or staples, it's difficult to believe that it provides great leakage protection. 2. Some people maintain that the building paper adds insulation. In the first place, there isn't much insulation value to a piece of building paper. In the second place, it makes no sense to try to insulate between the outdoors and an unheated attic space. 3. Some people say that the building paper provides a cushion for the shingles. While this may be true, it certainly doesn't seem to help much. In tests that were done to determine hail resistance, it was found that shingles with underlayment were less resistant to hail than those with no underlayment. 4. Another argument for the use of underlayment was a possible chemical reaction between the asphalt shingles and plywood sheathing, which was introduced in the early 1950s. We now know that there is no chemical reaction problem and, as a result, could do away with the underlayment in this sense. 5. Some people say the main advantage of underlayment is to protect the building before the shingles go on. While there is some validity to this, if left to weather for very long, roofing paper often is blown off a roof. 6. Some experts maintain that, in severe weather, if the roof covering material is blown off, much of the underlayment remains intact, protecting the building. A counter point is that it's unusual to have shingles blown off the roof and the felt paper remain intact.

Disadvantages 1. Roofing felts do increase the cost of the roofing job. Many roofers believe that the roofing felts make the job more dangerous because the underlayment is often more slippery than the roof deck or the shingled surface. 2. Some people maintain that 15-pound felts are actually detrimental to the performance of the shingles. As the felts get wet, they tend to swell and any wrinkles in the felts often telegraph through the shingles. Wrinkled felt underlayment is a common complaint. No matter what your personal opinion is, you should be familiar with whether underlayment is used in your area. In non-freezing climates, the underlayment should extend over any drip edge at the lower edge of the roof. Where a drip edge is used along the rake, the underlayment should be tucked under the drip edge. The underlayment should overlap any eave protection by at least four inches.