When it comes to roofing materials, roofing felt is to shingles what an underlayment is to asphalt flooring, serving as a moisture barrier between the bare wood of the roof and the shingles, adding an extra protection of waterproofing. Made of fiberglass or polyester fleece impregnated with tar or some other bituminous material, roofing felt serves to protect the roof from weather conditions.
Installing roofing felt is just as important as installing the shingles, and due to the weight of the materials and handling methods, it not an easy job. Especially considering it should be installed in warmer weather to create a better bond at seams, it can be not only a heavy materials job, but also a hot job.
To install roofing felt on an existing roof, it is best to tear off the old roof before beginning, and is a necessity on a new roof. When replacing shingles on an old roof, and all the old material has been removed, verify there are no nails sticking up that could damage the roofing felt. Rolls of felt are typically 36" wide and are available from 30-feet to 100-foot lengths, and some types have a light grit on one side to keep it from sticking together in storage.
Installation Technique For Roofing Felt
Begin by laying the first layer at the eave of the roof, leaving about a one-inch drip edge over the gutter, from one end to the other. It is best if this can be a single piece but if a seam is necessary, the overlap should be four to six inches. At both edges, depending on whether it is the end of the roof or at a wall, the roofing felt should still be about two-inches longer than needed to serve as a drip edge or to go under metal flashing at a wall.
The second roll should overlap the first piece by at least six inches and if a seam is necessary, it should not be at the same spot as the seam in the first row. Seams should be at least two feet apart. If using roofing felt with grit on one side, install the grit-side up and use cold tar sealer in the overlap of the two rows.
When cuts to the roll are made, never make them on top of the previous layer of felt. Mark the spot on the roll for the cut, and then place a cutting board under it to protect the existing surface from the sharp knife. This provides a clean cut and will not bring the knife into contact with the new roofing felt being installed. Continue adding layers to the remainder of the roof surface and when you reach the peak, allow the roofing felt to lie over the top as a first layer on the peak.
