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Q.We are getting a lot of jobs with Mansard-type roofs. The roof covering is concrete tile, but much of the roof has a slope of only 2:12. What precautions should we take?

A.According to the National Roofing Contractors Association, the minimum slope for a concrete one-piece barrel tile or interlocking flat-ribbed tile is 4:12. For flat shingle tile, it is 5:12. The CABO One and Two Family Dwelling Code prohibits the installation of any concrete tile on slopes less than 3:12, except with the permission of the building official.

The best precaution is to use some other type of roofing on the low slopes. However, if a uniform appearance is desired, a double-layer of 30-pound roofing felt, set in mastic or hot asphalt, must be laid over the sheathing and under the tiles. This effectively forms a waterproofing membrane. The concrete tile would then be cosmetic and would protect the membrane from the sun.