1. Figure your roof area, and calculate how much felt you will need for the job . One roll of 15# (pound) felt has 432 square feet, but you must allow for overlaps on joints, In most cases this will leave 400sq ft net coverage.
2. Clean the roof decking of any debris, protruding nails, or other objects or materials that will damage the roof felt.
3. Make sure the surface is dry. Never apply roof felt over wet or damp decking, as this condition will cause the decking to warp or buckle, and possibly rot.
4. Replace any damaged or rotten material on the roof decking.
5. Begin at the bottom of the decking, on one end, and roll the felt length way, along the lower edge of your roof. For hip type shed roofs, begin at the hip, allowing a foot or so of the felt to hang over the hip.
6. Allow the lower edge of the felt to overhand the eave about 1?2 to 3?4 inch (1.3 to 1.9 cm) so water will drip off without wetting the decking underneath.
7. Tack the end of the felt where you begin rolling it to hold it in place while the rest is unrolled.
8. Pull the felt tight from the opposite end, taking care not to rip it, but making sure it lies flat on the roof surface.
9. Nail the felt with simplex or plastic cap felting nails. These should be spaced at about 12 inch (30.5 cm) centers along the bottom edge, and 30-36 centers along the top, since subsequent overlaps will be nailed through the top of the last layer applied. Be careful to drive nails flush and not penetrate the felt, or you will have a leak.
10. Cut the felt at the opposite end of your roof, and roll felt back in the opposite direction, allowing about 4 inches (10.2 cm) of overlap to the first layer. Most roof felt has guide lines stencilled on it to ensure you apply it in straight, parallel lines.