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How Much Felt Is Needed for a 378 Square Foot Roof?

Roofing felt, sometimes called tar paper or roofing underlayment, provides a layer of protection between the roof sheathing and the shingles. Builders commonly apply the material in a single layer with a 2-inch overlap for each 36-inch-wide course of felt applied to the roof. A 378-square-foot roof, for example, would require about 400 square feet of felt to allow for the overlap and some waste where the material is cut to size.
  1. Installation

    • Begin by placing the roofing felt parallel to the eaves along the base of the roof. Fasten this first course in place with 1-inch shingle nails or 3/4-inch staples. Apply the second course, overlapping the first course by about 2 inches. Overlap end splices, such as at the end of a roll of felt, by 4 inches. Apply the roof felt to the entire roofing project at once before starting the shingling process.

    Felt Grades

    • Roofing felt is available in 15- and 30-pound grades. Installation practices are the same for both grades of material. Use the heavier felt for steeper roofs where the material will be subject to more stress from the movement of construction workers and materials to prevent damage.

    Why Apply Roofing Felt

    • Roofing felt serves a number of purposes. It provides a waterproof layer between the shingles and home if the shingles should fail. It also provides better workplace traction on the roof, helping to prevent slips and falls by construction workers. Roofing felt also provides some level of rain protection to the home in the event of precipitation during the construction process.

    Beyond Felt

    • Felt is the standard underlayment for the majority of roof projects. Some builders choose to include waterproof membranes in areas of the roof at a high risk of leaking. Use these membranes along eaves where ice dams can occur or in roof valleys, areas where two segments of roof join, to prevent moisture from entering the house if the water gets under the shingles at those points.