Flashing is a thin, flat metal sheet made of aluminum, copper or galvanized steel used to create a water barrier when differing components of a house intersect. Step flashing and counter flashing -- two different types of flashing -- are used together when walls and roof planes meet, or when a chimney penetrates the roof. Flashing installations rely upon the "watershed" principle (i.e. that gravity causes water to flow down); properly installed flashing can provide long-term water protection without the need for caulking or sealants.
How Step Flashing Is Formed
Step flashing is formed by bending a square or slightly rectangular piece of flashing 90 degrees, creating two equal sides. The dimensions of step flashing can vary, but it is often found 10 to 14 inches square; it can usually be purchased ready to use.
How Step Flashing Works
Step flashing is installed where a sloping roof plane intersects a vertical edge, such as a wall. One side of the flashing is placed against the vertical surface, and the other side lays on the roofing material. Such flashing is typically the length of two courses of the exposed roofing. The upper part of the flashing is tucked under the upper course of roofing and lays over the lower course.
How Step Flashing Is Installed
Step flashing is installed starting at the lowest point and moving up the slope, with each additional piece overlapping the previous one by a few inches. Each new piece of flashing weaves into the roofing in a higher location, creating a stepped watershed system that essentially extends the waterproof roofing surface up the wall. Step flashing is attached with nails placed high on the vertical side only.
The Function of Counter Flashing
Counter flashing simply protects against water getting behind the top edge of step flashing, preventing it from seeping down the wall or other vertical surface.
Counter Flashing When Siding Is Used
Siding can be an effective counter flashing when it laps the step flashing and no siding nails penetrate the metal. Alternately, counter flashing can be accomplished using a continuous sheet of flashing installed following the roof's slope. The piece of flashing should overlap the step flashing to the roof surface and tuck behind the siding 4 to 5 inches, nailed only near the top edge.
Counter Flashing at Masonry
Metal counter flashing is required when the vertical surface is masonry, such as a brick chimney. A horizontal mortar joint is opened 12 to 18 inches above the roof. A 90-degree bend is made in the top edge of the flashing, creating a 1-inch lip which is placed in the open mortar joint. The counter flashing extends down over the step flashing to the roof and is trimmed to follow the slope. Pieces of counter flashing 12 to 20 inches wide are used, installed beginning at the lowest point. Each subsequent piece overlaps the piece below it, and the top lip is inserted in a sequentially higher mortar joint creating a stair-like appearance. The void above the flashing in the open joint is filled with mortar or sealant to hold the flashing in place, and the vertical edges can also be sealed to secure the flashing from movement.