If you are leaving an old roof in place, leaving the old roof is acceptable, but two layers of roof is the limit. The main advance that comes from this is that the cost of roofing is decreased and the roof gets two layers of protection from the elements. Install a second roof layer if the local building codes accept it, and if the existing roof has a smooth out feeling and an even surface to it. If there are any curled or broken shingles they need to be nailed down or take awayd. Also, if the fasteners are of adequate length to penetrate the wood deck of ¾” or penetrate through a plywood deck. If you want to install over wood shingles when the surface provides an acceptable smooth surface, you need to cut back old shingles at eaves and rakes, and install new wood edging strips as needed.
installment three layers is something that GAF-Elf does not recommend installing on any roof deck. When you nail through three layers of shingles that will not fasten the shingles properly and may result in water leakage on the roof. Also, most building codes do not allow this.
GAF-Elk has minimum requirements for installing asphalt shingles on slopes, 2/12 or higher. Slopes that are lower than that do not have enough pitch to shed water properly. Low sloped roofs must be Waterproof with a low slope roofing material.
Many master contractors feel you should remove old roofing on every project. The reasons are many; removing old shingles gives a hazard to find old, rotting roof decking and to put back it, to find and replace bad flashing that could be rusted, broken, or deteriorated, to install the proper soffit ventilation for the house, to install leak barriers at critical areas by adding Weather Watch, Storm Guard, or other GAF-Elk Leak Barriers. These areas can include valleys, dormers, skylights, chimneys, roof slope transition areas, ice dam areas, and wind-driven rain entry points. Top contractors also like to remove old shingles to comply with building codes in the area.
Reasons to tear off old shingles is when they are buckling or curling at the edges, if the old shingles are algae covered, finding new leak sources which may be an effect of flashing defects, and allowing the roof to have a smooth substrate.
Reasons to roof over old shingles is when the old shingles are all lying in a flat, smooth manner, when the flashing and roof deck is in time performing and in good condition, and if the soffit is still acceptable.