Roofing felt is a type of roofing layer made out of felt materials that have been soaked in asphalt and allowed to dry. It is a typical underlayment material that roofing contractors use before nailing shingles into the roof. Some contractors also use roofing felt or similar materials in flooring construction as well.
Identification - Roofing felt may be referred to as tar paper as well, but true felt versions are also called builder's felt, a nod to its multiple uses in construction. When the asphalt is combined with felt, it creates a water-resistant surface that protects the home from leaks and moisture damage. Some types of roofing felt include breathable layers that allows trapped moisture to escape as well.
Use in Flooring - Despite its name, roofing felt is used in flooring -- especially wood flooring. Fiber cement backer board is a thicker, tougher subfloor material, but it is heavy, more expensive and must be installed in panels. Roofing felt is less expensive, still protects against moisture damage, and can even be used in addition to the fiber cement boards for better protection. Of course, the backer board does not need to be leveled, but with uneven plywood or for mortar-bed preparation, roofing felt can be a leveling tool as well.
Best Locations - Roofing felt is best used when you have minor floor problems and need a moisture barrier between the primary floor material. Bathrooms are a common location for roofing felt installation, since they are more susceptible to moisture damage than other areas. For large cracks or depressions, however, you should level the floor with backer board or an epoxy compound before installing the felt.
Felt Choices - Contractors use two main types of roofing felt for flooring: 15 pound and 30 pound felt material. The 30-pound type is a thicker material with greater moisture-resistance and strength, and is used for leveling a slightly uneven floor that needs protection from moisture. The 15-pound version works best for more generic applications.