A Good Flashing

The weakest link in any roofing system is transitioning from the plane of the roof to another surface. Whether it’s the wall of a dormer, a chimney, or a skylight frame, these transition areas require flashing, especially on slate or clay roofs. Use the best flashing materials you can afford, preferably 16-gauge copper or better, and copper fasteners. Never use roofing compound to patch or repair flashing; it can become brittle and fail within a year.

Flashing locations include perimeter edges, areas of penetration (for example, where exhaust vents, pipes, and plumbing stacks come through the roof), valleys (where roof surfaces meet at an inside angle), and areas where the tile abuts a vertical surface.

Perimeter edges should be flashed relative to the severity of the climate. For example, a slate tile roof in a frigid climate might get an especially deep band of edging, while a clay roof in an area where the temperature rarely falls below freezing may need no perimeter edging at all.

All penetrating pipes and vents should be flashed into the roof with a flat flange that extends outward around the penetration. Valleys can be open or closed. An open valley is lined with metal over which slates or tile lap only at the sides. Closed valleys are covered with tile and have either a continuous metal lining or metal flashing built in with each course. Open valleys are easier to install and maintain, and are generally more watertight than closed valleys. Whichever type you have, the flashing should be fastened to the roof with cleats, not nails (as nail holes are a source of water leaks).

Protecting areas abutting vertical surfaces from water intrusion calls for flashing techniques including step flashing, familiar on chimneys that project through a sloping roof. Certain kinds of clay tile require counterflashing. As the name suggests, counterflashing is intended to mate up with other flashing to divert water from vulnerable parts of the roof. (See “Flashing Clay” on p. 45.)