1. The old roofing will provide additional insulation.
2. You can lay the new roofing without exposing the interior of the building or the sheathing to the weather.
3. You avoid the labor, expense, and mess of removing the old covering.
The roof framing must be strong enough to support the additional weight. If your roofing is exceptionally heavy, you may have to brace the rafters, or if they cannot be properly braced, you may have to remove the old covering.
Rigid shingles and metal roofings may be laid over old roll roofing and asphalt shingles if the surface is not puffy or badly wrinkled. Puffy areas should be slit or cut and the old roofing nailed flat. If the new roofing is metal, cover the old roofing with rosin-sized paper (asphalt-saturated felt for aluminum).
Metal roofings may be laid over old wood shingles. Nail 2- by 4- inch nailing strips over the shingles, parallel to the eaves. Fasten the strips to the decking. For lightweight aluminum roofing, space the strips 16 inches on center. End laps of the metal roofing sheets should be over strips. If the new roofing is aluminum, cover nailheads in the strips with aluminum-pigmented mastic or asphalt-saturated felt.
New wood shingles can be laid over old. First, nail flat and secure all curled, badly warped, and loose shingles, and hammer down all protruding nails. Use five-penny nails1 ¾ inches long for the new shingles. The old shingles may have been laid on lath or strips. However, in nailing the new shingles, it is not necessary that the nails strike these strips.
Estimate your roofing needs.
After careful canvassing, researching, and having selected the type of roofing you will want to determine the amount of roofing material needed. This should be done someone experienced in estimating roofing. The following is offered as a guide.
Roofing materials are commonly sold by the square, which is 100 square feet. The number of squares needed will be determined by the area of the roof in square feet. Extra material is required for overhang at eaves and gables and for fitting around chimneys, dormers, and valleys. Include this in your estimate. Allow also for waste.
It is better to consult experts rather than do guesswork. Roofing is something you will be having for a long time. So make a wise decision and make it quick before the old roof disintegrates.