By now, most conscientious builders know that window rough openings need to be carefully flashed before a window is installed. For residential builders, the most common way, by far, to flash window rough openings is with peel-and-stick flashing.
However, an increasing number of builders are taking a closer look at something different: liquid-applied flashing.
What is liquid-applied flashing?
Liquid-applied flashing comes in various forms. Some products are dispensed from cartridges like caulk; others come in a pail and have the consistency of mayonnaise.
If you are using liquid-applied flashing to flash a window rough opening, you squeeze or spread a generous amount on the surfaces that need flashing — generally the rough sill, the rough jambs, the head, and a 6 to 8 inch wide band of the wall sheathing around the perimeter of the rough opening — and then you spread the material out with a trowel or a plastic Bondo spreading tool. (Some products have a thinner consistency that can be applied with a brush or roller.) Most manufacturers advise adding enough material to make an opaque layer — one that you can’t see through.
In addition to being useful for flashing window rough openings, liquid-applied flashings can be used to flash penetrations through wall sheathing — for example, vent pipes or plumbing pipes.
Liquid-applied flashings are stick tenaciously to a wide variety of materials. Most manufacturers of liquid-applied flashing claim that their products stick to plywood, OSB, framing lumber, concrete, CMUs, brick, aluminum, painted steel, vinyl, rigid foam, glass, and EPDM. (Note, however, that some manufacturers warn that their products don’t stick to housewrap.)
Most brands of liquid-applied flashing can bridge cracks up to 1/4 inch wide without the need for any caulk or reinforcing mesh. Once cured, these flashings form a rubbery layer that is vapor-permeable yet waterproof and airtight.