How to Build a Roof - Felt

Applying the Roofing Felt

Roofing felt acts as a waterproof barrier between the sheathing and the roofing material (shingles, etc.). We recommend using 15-30 lb. roofing felt. It is necessary to apply the roofing felt (tar paper) to a clean, dry surface immediately after the sheathing is completed to protect it from the weather. If, however, this is not possible and the sheathing gets wet, allow it to dry for a couple of days before applying the felt so as not to trap any moisture that may cause damage to the sheathing. Felting applications vary according to the type of roofing so determine the proper application before beginning.

Most Common Mistakes:

1. Applying felt to a wet surface.
2. Not overlapping the layers.
3. Not applying smoothly.

Construction:

1. If you have chosen to apply a metal drip edge along the eaves, this must be in place before laying the felt. A drip edge is usually made of 26 gauge galvanized steel with a top flange of 3 to 4 inches that extends in from the roof edge.
2. In heavy rain or snow areas we also recommend eave flashing, a strip of smooth or mineral faced roll roofing. This is cut to extend from the edge of the roof to a point 12 inches inside the wall line. Place the lower edge of the strip even with the drip edge.
3. The roofing felt is then applied in accordance with the type of roofing to be used. We will assume here that you are using asphalt shingles.
4. Snap horizontal lines on the roof sheathing to align the upper edge of the felt. Snap the first line at 35 and 5/8 inches above the eave. (The 36" felt should overlap the eave by 3/8".) Snap succeeding lines at 34 inch intervals, assuring a 2 inch overlap of each course.
5. The felt must be flush at the rake (side) edges, have 4 inch overlaps where two pieces are joined, and 6 inch overlaps on hips and ridges.
6. Ensure that the felt lies flat and smooth before securing it.
7. Tack the roofing felt down with 1/4" staples using an air compressor with a pneumatic stapler, a hammer-tacker, or a staple gun every 12 inches.