What is roof flashing

Roof flashing provides extra protection for roof areas that are prone to water seepage. The purpose of flashing is to direct the flow of water that leaks into intersections down and away from the interior of the structure to the topside of the roofing material. Most flashing is made of galvanized metal or aluminum and should be installed around the chimneys, skylights, soil and vent stacks, HVAC units, roof valleys and also where roof decks and walls intersect and where water is prone to collect.

All roofing systems are comprised of a number of different components: roof sheathing, underlayment, roofing material, roof intersections, flashing details and ventilation. Each of these systems must be installed correctly for the system to work as designed. Correct installation is even more critical when it comes to flashing. In every type of roof, the top edge of the flashing passes underneath the underlayment, the upper pieces of flashing pass over the lower pieces, and the lower edge of the flashing always passes over the top of the roofing material. Ideally, the flashing direct the flow of water toward the top layer of the roofing material so it’s not in direct contact with the wood structure below.

Valley flashing protects the valleys where two roof planes meet. This material is usually created in a v-shape and is placed over the felt before the roofs finishing material is installed. Step flashing is used to protect the joins between the roof deck and chimney or dormers and steps up the wall or chimney to channel water away from the weak spots. Vent pipe flashing fits over flues or pipes and is typically a cylindrical shape with a wide flange at the base. Finally, drip edges are strips of flashing that run along the roof eaves and rakes to prevent water from seeping under the finished roof edges.

Roofs with a steep or normal pitch have fewer problems with water accumulation than flat roofs, because water will run off more easily. Gravity doesn’t carry the water away on a flat roof, but rather forces it into every imperfection in the roof system. That’s why flashing is even more important on flat roofing systems like built-up, spray foam, modified bitumen, single ply and metal roofing.

Flashing details, especially the vertical welds that are sometimes necessary, are a very technical aspect of roof construction, and this type of work is best left to experienced professionals.