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.