The condition of the existing roof determines whether you can add another layer of roofing materials. Relaying is not recommended for roofs with existing shingles that are curling, moss covered or splitting. The contractor should inspect the roof for sagging or rotting decking, which indicates problems requiring more than a simple addition of new roofing materials.

The type of shingles for the new roof must be of the same material as the original roof. When placed over heavyweight shingles, lightweight shingles reveal all the ridges and lines from the original roof. Metal Roof-overs may also be an option.

The Uniform Building Code states that only two layers of roofing material will be allowed on any residence, so there can only be 1 layer of existing roof. It is typically better to remove and replace the roof, this will allow any existing sheathing damaged to be exposed and corrected whereas a Roof-over does not permit this.