Applying roof felt paper before the shingles is a necessary step in the roofing process. Roof felt paper is thick paper impregnated with asphalt and tar. It helps create a waterproof barrier that keeps moisture from seeping through the shingles to the wood on the roof. Roof felt paper comes in different thicknesses. Thicker roof felt paper is better, but the price increases as the thickness increases. Roof felt paper comes in rolls that are easily cut to desired lengths and applied to the roof.
Measure 35 inches from the bottom edge of the eave of the roof on both sides near the gables, then pull the chalk-covered string tightly across the roof between the two ends, holding the string close to the roof. Pull the string upward, release the string, and a line of chalk appears on the flat surface. Repeat the process on the other side of the other roof.
Measure the roof on the bottom edge across the front of the roof near the eave. The top and bottom measurement are the same unless the house has a decorative turret or unusual shape. In areas with unusual shapes, measure across the length of the roof every 3 feet from the eave.
Line up the top of the paper on the chalk line, so the bottom edge of the paper is 1 inch past the edge of the roof. The paper has a printed line on the top edge. Make sure this line faces the top point of the roof; this lines up the rest of the sheets.
Nail the felt paper to the roof using roofing tacks. Nail a vertical row of nails across the top and bottom edges of the felt paper.
Measure and cut the next piece of felt paper the length of the bottom of the roof. Place the bottom edge of the felt paper on the printed line of the first sheet. Once the bottom edge is lined up, nail the top and bottom of the felt paper. Continue lining up the felt paper with the piece of felt paper below it, and nailing the pieces in place until you reach the highest point of the roof. Repeat the process on the other side of the roof.