Deck Flashing

If you’ve dismantled an old deck, you’ve probably had the opportunity to inspect areas of inadequate flashing. The vast majority of decks, especially if they were built more than a few years ago, are not properly flashed, yet we now know that proper flashing can have a huge effect on a deck’s longevity. With the advent of self-adhesive membranes, the job of proper flashing has gotten much easier (see photo 1 below). Be aware, however, there is a learning curve when working with this type of flashing—for example, the ability to consistently find the backing release cord (photo 2). If you decide to use metal flashing instead, you can choose coated aluminum, stainless steel, or copper (photo 3). The best flashing applications are often layered systems that combine self-adhesive membranes with metal flashing (photo 4). Whatever flashing situation you face, you will have a better chance of success if you assume that water will find a way in; the job of the flashing is to direct it back out again. That said, in extreme environments a strip of self-adhesive flashing applied over the joist top can provide the primary defense against water infiltration (photo 5). In this way, the flashing adds a protective layer between the decking’s underside and the joist’s top, closing a potential point of entry for rainwater (photo 6). Flashing also creates a seal around the fastener shank (photo 7) because the adhesive will stick to the screw as it penetrates the material (photo 8).

1. Self-adhesive flashing comes in rolls of varying widths and tackiness. In hot weather, choose a less aggressive adhesive.

2. A release cord that cuts the backing paper is embedded under the flashing to help you apply the flashing in stages.

3. The composition of the metal flashing determines how well it will withstand the corrosive effects of the weather.

4. Layering flashing takes extra work up front but pays dividends down the road in avoiding expensive repairs.

5. Flashing the joist top prevents water from penetrating into the middle of the joist, where there is less chemical protection.

6. Seen in cutaway, flashing applied over the top of the joist seals the fastener shank.

7. When rainwater collects on the joist top, the flashing prevents infiltration into the joist and directs it to the side.

8. The gooey flashing adhesive seals around the shank as it is driven in.

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