Moisture can sneak in through the smallest openings, and the places most at risk are the obvious ones. Windows and doors where the frame meets the wall, siding joints where two boards or panels butt together, and trim pieces like corner boards are all areas where water can get in. These entry points can see constant exposure to rain and condensation, so they need special attention. Without properly installed flashing at these vulnerabilities, water will follow gravity, slipping behind the siding or seeping into the framing until it finds a path to damage insulation, lead to mold, or rot the structure.
At its core, flashing follows one simple rule: overlap and slope. Each flashing piece must extend over the one below it and bend outward so that water naturally runs away from the house. This “over and out” principle ensures that wind-blown rain never stalls or works its way back under the cladding. When water can’t pool or reverse course, it can’t exploit gaps or work past your defenses.
Effective flashing creates a clear drainage path, giving water a way to exit rather than accumulate. In a siding joint, for instance, a thin strip of joint flashing sits behind where the two pieces butt up against each other. This way, the water won’t get through the gap and get stuck behind the siding. Instead, the joint flashing allows for the water to run down, then exit on the exterior side of the siding beneath.
Durability is critical, especially where flashing must withstand harsh elements or mechanical stress. For rooflines, above windows and doors, and above exterior fixture blocking, metal flashing is the best bet. Painted aluminum resists UV and is less prone to corrosion. Behind siding joints, high-density polyethylene (HDPE) flashing is preferred. This plastic material will hold up to the elements and is often more cost effective and easy to install.
Flashing is the front line of defense against water intrusion and rot. It’s important to take extra care to get it right, as it will lead to a long lasting and durable structure.
