Though nearly all roof jobs are done using roofing felt beneath the shingles or shakes, not everyone uses roof underlay. Though it is more expensive than roofing felt, roofing underlay, which is waterproof, helps provide additional insulation and protection for your roof and can be useful in the areas of your roof where water may stand. The application process for roof underlay is similar to that for roofing felt.
Pull up any nails on the surface of the roof with the claw of a hammer and pry up any staples left behind from old roofing felt. Sweep the roof with a broom to remove dust and debris from the surface.
Measure 35 inches up from one end of the eave of the roof and make a mark. Then, measure up 35 inches at the other end of the roof. Snap a chalk line to make a horizontal line across the roof.
Start unrolling the underlay horizontally along the line that you made. Place the first nail through the top corner at the end of the roof to secure that corner in place. Go along the roof, placing a nail into the underlayment every four or five inches, or at the distance recommended by the manufacturer of the underlay. Try to keep the underlay in line with the line you made as much as possible, but make sure at least two inches of the underlay overhangs the edge of the roof.
Measure up from the top of the first layer of underlay another 35 inches and snap a new chalk line. Attach the underlay to the roof using the same method used for the first row. Keep the underlay as lined up as possible with the line you made, but make sure the bottom of the underlay overlaps the layer before it by two inches.
Lay to the top of the roof, but do not lay underlay on the peak. Instead, move to the other side of the roof and work your way up from the eave again. At the top of the roof, roll the underlay over the peak, folding the underlay so that it tops the peak with a roughly equal amount of underlay on either side of the peak. Secure with nails as you have the rest of the underlay.