Before applying roofing you ll need to cover the sheathing with roofing felt also often called tar paper most local codes call for using 30 pound felt.
How to install tar paper on a roof.
It improves look and uniformity because your roof deck might not lie perfectly straight or flat a layer of roof paper gives an even uniform surface upon which you can install your shingles.
Some roofers prefer to attach felt underlayment with 1 inch roofing nails or special nails with plastic washers but most codes allow staples which are easier to drive.
The idea is to make sure water that falls on the roof drips down from paper to paper and off the roof rather than up under the tar paper potentially damaging the roof s wooden structure.
Staple it to the plywood with a staple gun placing the staples at 6 to 8 inch intervals along the edges and in the middle of the paper.
Unroll the paper on the roof and cut it to the correct length with a utility knife.
Roofing felt prevents the deck from being exposed to the elements before or while the shingles are being applied.
Place a final piece of tar paper over the top of the roof attaching it so it overlaps the tar paper on both sides of the roof.