Which VELUX Flashing Kit Do I Need?

A flashing kit is the set of metal and membrane parts that join the window frame to the roof covering. It carries rainwater from the roof surface down and around the frame, so that none reaches the edges of the opening.

A kit is more than a metal skirt. It includes side and top pieces and a bottom apron that lies over the covering below, and it is either supplied with or paired with a collar that seals to the underlay. The parts are shaped for a specific covering, because the gap between the flashing and the covering has to be closed in a specific way.

A kit that matches the roof closes that gap without filler or improvisation. A mismatched kit leaves a gap that only sealant covers, and sealant is not a lasting answer.

The first thing we look at is what is on the roof. Coverings fall into a few families, and each has kits made for it.

Within a family there are differences. A deep-profile tile needs a kit with room to move up and down with the profile. A flat slate needs a low-profile apron. If a roof carries a mixed covering, because an extension has been added or part has been recovered, each slope is read on its own.

This is the reason a quote based on a photograph is a guess. The same window fitted into two roofs on one street can take two different kits.

Two measurements within the covering narrow the choice. One is the depth of the profile, which we measure from the base of the tile to its crest. The other is the pitch of the slope.

Every flashing kit is rated for a range of tile depths, and every window model for a range of pitches. The maker publishes both, and the survey puts your readings against them. Where a reading falls near the edge of a range, the sheet says so and the choice keeps the kit within what the maker allows.

The pitch reading also tells us whether the window you want is suited to the slope at all. Our page on the minimum roof pitch for a roof window explains why we measure it rather than quote a figure.

The size code of the window fixes the dimensions of the kit. A wider window needs a wider flashing, and a taller one a longer apron. The code is printed on the data plate on top of the sash, and the letters and digits in it are explained in the size codes answer.

Layout changes the kit as well. A single window uses a single-window kit. Two windows side by side, or one above another, use a combination kit that joins the frames with a shared flashing between them. The gap between the frames is set by the kit, so the spacing is decided before the rafters are cut.

Windows set apart on the same slope each take their own single kit. Only windows placed together share one. Where two windows are close but not touching, we measure the gap between them, because a narrow strip of covering between two flashings is hard to lay well, and joining them can be the better choice.

Beneath the flashing sit parts that rarely get a mention. An underfelt collar seals the frame to the roof underlay so water that gets past the covering is carried away. A thermal insulation collar can be fitted round the frame to reduce a cold path through the rafters.

These are chosen with the kit, because they have to fit the same frame and the same covering, and the maker's installation instructions show where each one goes.

Check the condition of the old kit when you replace a window, because it tells us what was there. The old kit is not reused. Flashings are shaped to the frame they came with, and a new frame is best served by a new kit fitted with it.

Once the roof is read, the kit is named on the survey sheet along with the size code, the rafter gap and the pitch. You see it before anything is ordered, and the order uses the same part.

If the covering changes between survey and fitting, say because you re-roof in the meantime, tell us. A new covering can mean a different kit, and the sheet is updated.

How the whole fitting goes is on our VELUX window installation page, and the reasons for replacing an existing unit are in our answer on replacing an old VELUX.