Add about 2 inches (5 cm) to each of your measurements before calculating the area. This will give you extra felt to hang over the eaves of your roof.

Purchase an extra pack of felt in case you need more or end up damaging your first roll. Divide your area in square inches by 144 to get the area in square feet.

For example, if the width of your shed is 240 inches (610 cm) plus 4 inches (10 cm) of overlap, mark a line 244 inches (620 cm) from the left width of your felt piece. Purchase a hooked blade—which has a small hook extending from a standard blade—from a home hardware store. You can also use a standard straight blade if you don’t have a hooked blade. However, this will make cutting more difficult. Be sure the flat surface doesn’t have anything sharp on it.

Pull out nails that are holding felt to the roof using the sharp end of a claw hammer.

Use coarse sandpaper (40- to 60-grit) to remove tough dirt.

Be sure to use the same nails to fix it to the wooden beams underneath.

If you use paint, be sure to choose high-quality water-based enamel acrylic brands with rust-inhibiting properties. Select general purpose felt primer with low viscosity.

Adjust the felt as you roll it, making sure that it’s straight with no bubbling. Never install the felt vertically across your roof. Hang the lower edge of the felt over the eave—the part of the roof that meets or overhangs the shed’s walls—by about 0. 5 to 0. 75 inches (1. 3 to 1. 9 cm).

Keep each tack about 2 to 3 inches (5. 1 to 7. 6 cm) from the edge of the roof.

Take care not to rip the felt as you pull it.

Apply each row in a perfectly parallel manner. Most roof felt pieces have guidelines stenciled into them to ensure you apply them in straight, parallel lines.

Small sheds usually require about 100 to 150 clout nails. Be careful to drive nails flush. If the circular nail heads penetrate the felt they can cause leakage. Remember that each nail must be long enough to pierce 2 layers of felt during across regions that overlap. Simplex or plastic cap felting nails are also sufficient.

Move at a steady, consistent pace to ensure even application.