All of our roofs require solid decking. Whether the installation is over old shingles or directly over wood decking, underlayment should be used. Roofing underlayment is required by the International Building Code and it helps protect the back side of the metal from any rough surfaces below the roofing material. Generally, one layer of synthetic underlayment is used or one layer of 30-pound felt. In certain areas, as required by code, ice and watershield products or fire-retardant underlayments will be used.