When you are repairing or replacing an existing roof there are a few important tips about each roofing type that homeowners should know. In general, it is always best to remove old roofing before installing new shingles; however there are exceptions to the rule.
The following types of roofs allow for installation over a single layer of asphalt shingles when the existing shingles are flat enough to allow for proper installation of the new roof. If there is more than one existing layer of asphalt shingles it is never okay to install a new roof over them, they must be removed before installation:
Asphalt Shingles – In order to install an asphalt shingles roof over one layer of existing asphalt shingles, the existing shingles need to be flat and smooth and longer nails need to be used when repairing the roof.
Wood Shingles – When you choose wood shingles, it is always best to remove any existing shingles before installing the wood shingles, however if there is a single layer of asphalt shingles it is acceptable to install the wood shingles over the asphalt shingles.
Concrete Tiles – Since concrete or clay tiles can be quite heavy, there might need to be some modification to the structure before the new roof is installed. In many cases it is acceptable to have a single layer of smooth asphalt shingles below the concrete tiles.
The following types of roofs never allow for installation of the new roof over existing shingles:
Slate Roof – This unique type of roof should never be installed over existing shingles.
New Asbestos Cement Shingles – Another unique roof type that is not used very often, asbestos cement shingles should always be removed prior to installing new asbestos shingles. Special care needs to taken when disposing of the asbestos cement shingles because of the asbestos content.
Metal Roof – This roof type should never be installed over any other type of shingles.
Built-up Roofs – Even though built up roofing systems proclaim that it is okay to install them over the existing built-up roof system, it is not recommended. The moisture that is trapped in the old roofing system will deteriorate the new roofing system faster than normal.
Single Ply Membranes – Plastic and synthetic rubber roof membranes should never be installed over asphalt shingles or built-up roofs. Some manufacturers say that it is okay to install over an existing single layer, however most agree that stripping of the existing roof is required.