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.
How to install tar paper on a roof.
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 roofing tar paper.
Lay the first course of tar paper lengthwise on the roof so one edge is covering the drip edge nailed to the roof.
Unroll the paper on the roof and cut it to the correct length with a utility knife.
Whether you re building a new house or putting a new roof on your existing house installing tar paper first helps waterproof the roof and prevent leaks.
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.
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.