Unless you live in the desert, your roof will need more than just a layer of shingles to protect it from the elements. Roofing felt, also know as felt paper or tar paper, provides a waterproof barrier that will protect the roof sheeting from any water that seeps under the shingles. When applying roofing felt it is critical to always work up from the bottom of the roof, overlapping the paper as you work your way to the peak. This will insure that water will not be able to get under the paper as gravity pulls the water down the roof.
Unroll the felt paper, 5 feet at a time, beginning at the bottom of the roof and at the end farthest from any valleys. Always work from the edge in, toward valleys and dormers.
Nail or staple the felt at 2-foot intervals, smoothing out any wrinkles.
Cut the paper at an angle when you reach the valley. Use a straightedge under the paper as you cut so as to not damage the roof underlayment.
Overlap the bottom edge of the second row of paper 2 inches over the top of first row so water will run off the upper sheet and onto the lower.
Continue working up the roof until you reach the peak. Do not cover over the roof's peak.