Which flashing does a roof window need on slate or tile?

Flashing is the set of metal and sealing pieces that fits round the frame of a roof window. It joins the window to the roof covering so that rain runs over the frame and onto the slates or tiles below, rather than finding the gap between them. A roof window, which the trade calls a rooflight and a homeowner may call a skylight, arrives with its frame and glass; the flashing is the piece matched to the roof it goes into.

Makers supply flashing kits as products of their own, listed by covering type and window size. That is why the kit is chosen with the window.

Slate lies flat and thin, so the flashing for it is low and close to the surface. The kit has a flat apron and side pieces that lie under the slates, and the slates round the window are cut to meet it. A red-brick Victorian terrace in Edgeley or Heaton Norris may have a slate roof, and slates of that age can lie unevenly, so the survey includes a look at how they sit where the window will go.

On slate the kit needs to be one the maker lists for flat coverings, which is a different box from the one for profiled tiles.

Concrete tiles, often found on interwar and post-war semis like those across Bramhall, Cheadle Hulme and Hazel Grove, can be deeply profiled, with ridges that stand proud of the roof. The flashing for them has to allow for that ridge, with a shaped apron so that water runs off over the tile rather than collecting behind the frame. A kit made for flat slate will not sit down on a ridged tile, and the other way round.

Plain clay tiles are in between: smaller, overlapping and thin-edged, and some makers list a kit made for them. The covering type is confirmed before anything is ordered.

A roof window can sit level with the covering, or be set lower into the roof so that it sits closer to the line of the covering. A recessed window needs a kit made for that depth. Some makers offer a recessed flashing kit for this; not every window or covering has one, so it is a check at the survey.

When a worn window is replaced, the new unit goes in with a new kit made for it rather than the old flashing. Makers size the kit to the window, which means a replacement of a different size code needs a kit to match, and the opening may need to change. The covering round the opening is looked at during the survey, because a kit for one tile profile will not seat on another. A replacement roof window in an existing home should meet the energy limits for a roof window, and the kit is part of how the unit joins the roof.

The sequence on the roof follows the kit. The covering is lifted back to the rafters around the opening, the frame is fixed, and the flashing pieces go on from the bottom of the window upward, each lapped over the one below so that water runs down and off. The slates or tiles then close up round the kit. Where the work is notifiable, we handle the Building Regulations notification with the council's building control team or a registered building control approver. Pricing waits for the free survey, and afterwards you receive a fixed written quote. Behind the fitting stands a 10-year workmanship guarantee.

Here the window itself is not the issue. A kit that does not suit the covering leaves gaps along the side or at the top, because pieces made to lie on a flat slate cannot follow a ridge. The result is water travelling where it should not. That fault belongs to the kit and its fitting, so it is chosen before the window is ordered. How the kit enters the price is covered in what a roof window costs, and the roof windows page follows the job through to handover.