Tile roofs need a minimum slope to shed water effectively and prevent underlayment failures.

For clay and concrete roof tiles the common thumb rule is a minimum slope of two and a half over twelve to three over twelve depending on the tile interlock and profile. Measure slope as the vertical rise per horizontal twelve inches and confirm with the tile manufacturer because some interlocking profiles allow the lower end of that range. Where slope is lower than the preferred value use a full waterproof underlayment applied according to the underlayment instructions and lapped shingle fashion from the eaves upward to ensure a continuous water barrier. Install battens or counterbattens as required by the tile system to provide airflow, create a secure fixing substrate and control tile alignment and water shedding paths. Flash all penetrations and eaves with metal or compatible flashing sealed to the underlayment and fasten tiles per spacing guidelines so wind uplift and point loads do not concentrate at single fixings. For reroofs confirm existing deck condition, rectify any sag or deflection that can trap water, and adjust pitch where practical rather than relying solely on heavier underlayment.

Common mistakes are assuming all tiles behave the same; different interlock types and profiles change minimum slope needs so always check the specific tile instructions and adjust slope or underlayment accordingly. Another issue is skipping battens or ventilation which leads to trapped moisture and accelerated underlayment wear; follow the tile system layout and include counterbattens where needed to promote drainage and airflow. A third problem is improper flashing and poor laps at transitions which causes leaks even at acceptable slopes; ensure metal flashing is integrated with the underlayment and tiles and that laps are oriented to shed water outward.

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