Can Synthetic Roofing Underlayment Be Doubled?

Doubling Underlayments

• Doubling an underlayment does not mean taking the felt and folding it over on itself to create two layers instead of one. You should not fold a roof underlayment. This creates uneven layers that you cannot shingle over properly and cannot be nailed down effectively. Underlayments were not designed to be doubled like this, but there are ways you can increase underlayment amount or protection without sacrificing quality.

Overlap

• Roofing felt and other underlayments are installed in strips nailed or stapled down across the roof surface. When properly overlapped, these strips can essentially create two layers of protection instead of just one. This double layer is created on low-slope rooftops where water pooling might be an issue. By laying down one strip at least 19 inches over the previous strip, the double effect is created without any folding or awkward felt placement.

Doubled Products

• Even without overlapping, you can double your synthetic underlayment options simply by buying thicker or dense materials. Most roofing felt comes in two options: 15 pound and 30 pound. By switching to 30 pound, you are doubling the amount of protection your roof receives while keeping installation procedures nearly the same. Heavier felts tend to cost more than thinner versions, but can save money by the end of the installation.

Doubled Fasteners

• You may also want to double the number of fasteners you use on the roofing felt. Synthetic underlayments have a fastener schedule that shows how far apart fasteners should be applied. Doubling these fasteners can help protect a roof from damage in areas that are prone to very high winds.