Working With Flexible Flashing

Rubberized asphalt is self-healing and sticks to a variety of substrates, but may not be stable at temperature extremes.

Butyl flashing has a wider temperature range than RA and sticks to more materials. Accordion-style butyl flashings can stretch at the corners of openings.

Solid acrylic flashings are designed to bond to substrates at the molecular level. They make the firmest bonds at the widest range of temperatures, according to some experts.

Some flashing manufacturers require an adhesive primer on certain substrates, the most common being gypsum sheathing, masonry, and the rough side of OSB. Primers come in spray-on and brush-on varieties.

Compatibility is a big issue with flexible flashing, so make sure that the flashing is compatible with all substrates it will contact. The safest route is to use a flashing and housewrap made by the same manufacturer.

Certain caulks and sealants may be incompatible with particular flexible flashings. Silicone products, as shown here, should not be used with any type of flexible flashing.

This photo was taken on a jobsite in Denver, four to six weeks after installers had done their work, and the flashing is already failing. Incompatible with the vinyl window fin and the sealant, the rubberized asphalt flashing is pulling away rom the window.

Vapor-permeable tapes keep the weather out, but allow vapor from the interior to pass through. Here the white vapor-permeable tape goes between the window and the opening, which is wrapped in black acrylic tape.

This is a good example of a common installation mistake called reverse shingling. The flashing incorrectly laps over the Tyvek (rather than vice versa), and jamb flashing incorrectly laps over the head flashings.

Flexible flashing stretches to seal around circular windows in a single pass. The housewrap then extends over the flashing.

Even the best tapes won’t work well when they have been in contact with a surface that is dirty or full of sawdust.

Small staple penetrations in a supposedly self-sealing flashing used on a rooftop parapet wall started to let water through. Just 11 years after the home had been built, the wall below that flashing had completely rotted out.

Although some installers use hand pressure to seat the flashing to the substrate, most flashings need to be rolled using a J-roller for proper adhesion.

For places that are hard to get to with a roller (such as inside corners), use the butt of a cedar shingle to press the flashing tape firmly against the substrate on both sides of the corner.

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