Slope type roofs are among the most frequently built and are usually the most cost-effective option. Any structure that has a floor plan that is more complex than a single rectangle will have a roof composed of several slopes that will join at some point on their faces. This creates what are called roof valleys. Felt paper (also referred to as "tar paper") is a moisture barrier that is installed between the wood roof sheathing and the roofing material. Installing felt paper in roof valleys is slightly different but no more complicated than installing it over flat areas of the roof.
Unroll about two feet of felt paper from the roll and lay it at the top of the roof valley, with the remaining roll facing downward towards the roof eave.
Unroll the rest of the length of felt paper to the eave and cut it, leaving about a foot of extra material.
Center the entire length of felt paper over the roof valley (overlapping other felt paper strips, previously installed on the roof face) and tuck it to match the valley contour.
Fasten the top of the felt paper strip to the roof sheathing (plywood) using a roofing stapler and roofing staples.
Fasten the rest of the felt paper strip, working your way from the top to the bottom. Space the staples about 16-inches on centre, unless otherwise required by local building code or felt paper specifications.
Trip the bottom end (at the eave) of the felt paper strip to match the shape of the eave; do the same at the top of the strip.