What underlayment does a tile roof need?
Short answer. A clay or concrete tile roof needs underlayment that meets the residential code and the tile maker's instructions: tile is allowed only on slopes of 2 1/2 in 12 or steeper, slopes from 2 1/2 in 12 to 4 in 12 require a double layer, and steeper slopes need at least one layer. Because the tile can outlast ordinary felt by decades, choosing a long-life, tile-rated underlayment is the most important decision on a tile roof.
The code rules
- Minimum slope: clay and concrete tile need at least 2 1/2 in 12.
- 2 1/2 to 4 in 12: double underlayment, laid with a 19 inch starter and 36 inch sheets lapped 19 inches, or an approved alternative.
- 4 in 12 and steeper: at least one layer, lapped and fastened per the code and manufacturer.
- Tile attachment: tiles are secured with nails, clips or approved adhesives, with more fastening in high-wind areas.
Georgia enforces the 2024 IRC with state amendments; ask your building department about local requirements.
Matching underlayment life to tile
Concrete and clay tile can last 50 years or more, but basic felt under hot tile often fails much sooner, forcing a lift-and-relay while the tiles are still good. Spending more on a high-temperature self-adhered membrane or a premium tile-rated synthetic up front can avoid paying for that labor twice.
Other parts of the system
- Battens: treated wood or other approved battens, with drainage gaps or counter-battens.
- Flashings: pans and metal flashings at valleys, walls and penetrations.
- Eave details: bird stops and an eave riser so the first course sits at the right angle.
- Ventilation: airflow under the tiles and in the attic keeps the membrane cooler.
Converting to tile
Tile is much heavier than asphalt shingles, so an engineer or the building department may need to confirm the framing can carry it before a conversion.
See synthetic underlayment vs felt and roofing permits in Georgia.