When replacing a roof, leaving the old roof is tolerable, but two layers of roof is the limit. The main advantage that comes from this is that the cost of roofing decreases and the roof gets two layers of protection from natural elements. Install a second roof layer if the local building codes accept it, and if the existing roof has a smooth feeling and an even surface to it. If there are any curled or broken shingles, they need to be nailed down or removed. Also, the fasteners need to be of adequate length to penetrate a wood deck of ¾” or to penetrate through a plywood deck. If you want to install over wood shingles, make sure the surface provides a suitable smooth surface. You will also need to cut back old shingles at eaves and rakes, and install new wood edging strips as needed.

Never install a third layer to your roof. GAF-Elk does not recommend this practice. Most building codes do not allow this type of installation either.

GAF-Elk has minimum requirements for installing 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 waterproofed with a low-slope roofing material.

Many professional contractors feel you should remove old roofs on every project. The reasons are many: removing old shingles gives you a chance to find old, rotting roof decking to replace it, to find and replace bad flashing that could be rusted, broken, or deteriorated, to install the proper soffit ventilation for the house, to put in 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: if they are buckling or curling at the edges, if the old shingles are covered in algae, 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: when the old shingles are all lying in a flat, smooth manner, when the flashing and roof deck is still performing and in good condition, and when/if the soffit is still acceptable.