The National Roofing Contractors Association recommends non-perforated, organic felt saturated with asphalt as the best underlayment for concrete tiles. This underlayment, sometimes called “felt paper,” protects your roof from the weather until you finish installing the tile, and it provides waterproofing for water that can penetrate the tile after the roof is complete.
You can find different terms used to describe asphalt felt underlayment. The terms Type 1 and Type 2 are industry terms replacing the former designations Type 15 or Type 30, sometimes called No. 15 or No. 30. The numbers 15 and 30 refer to an underlayment’s ability to bear pounds per square inch of weight.
NRCA recommends one layer of Type 2 asphalt-saturated felt applied horizontally and overlapping like shingles on roofs with a slope of 10:12 or more. Roof slopes are measured by the ratio of the amount that a roof rises compared to the horizontal stretch of roof. A ratio of 10:12 designates a roof that rises 10 feet for every 12 horizontal feet. If a roof slope is a more moderate 4:12 to 10:20, apply two layers of Type 2. NRCA does not recommend laying concrete roof tiles on roof systems with a slope less than 4:12.
Ice Dam Protection
When a gutter becomes frozen with ice, it forms a dam at the eve of a roof. Water that cannot drain backs up and freezes on tiles causing leaks. If you have an average January temperature of 30 degrees Fahrenheit or less, apply ice dam protection. Lay self-adhering polymer-modified bitumen membrane on top of the underlayment at the roof’s eaves extending up the roof at least 24 inches from the exterior wall. If your slope is less than 4:12, apply at least 36 inches of polymer-modified bitumen.
Synthetic underlayments marketed as substitute for asphalt underlayment are touted as being less likely to wrinkle, water resistant, lightweight, difficult to tear and easy to walk on in wet conditions. NRCA cautions that the American Society for Testing and Materials that assesses building materials internationally has yet to set standards for synthetic underlayments.
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