What is a Velux flashing kit?

A roof window cuts a hole in a surface that is built to shed water. The window frame is waterproof, but the gap between the frame and the roof covering round it is not. A flashing, in roofing language, is a strip of weatherproof metal that bridges that gap, tucked beneath the covering on one side and lapped over the frame on the other.

A flashing kit puts those strips together as a single set, cut and folded at the factory to suit one frame. Fitting a window with a made-up kit is a more repeatable job than forming each piece by hand on the roof, because corners and laps arrive already shaped. That is its value, and it is why almost every new roof window in a tiled or slated roof goes in with one. The wider skylights and roof windows page shows how it fits into the whole job.

A typical kit has a bottom piece, often called the apron or cill flashing, which crosses the lower edge of the window and lets water run out over the tiles below. Side pieces run up either flank, sometimes as a stepped set so each course of tiles sits over its own lap. A top piece goes above the window and parts the water coming down the slope so it passes round both sides.

Fitting follows the way water travels. Work starts at the bottom edge, the sides come next and the top piece goes on last, so every piece overlaps the one beneath. The insulating collar and the underlay detail round the frame complete the picture. Water running down the roof should never meet an upward-facing joint, because rain will find that joint in a gale. Our answer on condensation in roof windows helps you rule out a leak that is not one.

Different coverings need different flashings. A deep-profile interlocking tile needs a flashing that follows its high and low ridges, a flat plain tile needs a thinner one, and slate needs another design again. A kit made for one will not seal against another, and mixing them is a recipe for leaks. It is also why a window that behaved on the old roof can start to leak after a re-cover in a different tile.

Tile profile varies more than you might expect. Marley's concrete interlocking range alone has tiles with minimum pitches from 12.5 degrees to 25 degrees, so two roofs that look alike from the road can need different kits. We check what is on the roof, note its make and profile where it can be identified, and order the kit that matches. Where the covering is old and cannot be matched, a lead flashing made on site is often the better answer.

Lead is the traditional flashing material and can be shaped to any profile on site. It comes in codes that set its thickness: code 4 is 1.80 mm and is listed for flashings, soakers and pitched valley gutters, and code 5 is 2.24 mm. A lead surround takes longer to make than a factory kit takes to fit, but it suits an odd profile, a historic roof, or a window set among slates that will not take a standard kit.

On most ordinary tiled roofs, the factory kit is the tidier and right choice. On a roof with hand-made clay tiles, old slate, or a building in a conservation area, lead is often the better match, and we put our recommendation in the quote. Our guide to lead roofing and leadwork explains how lead is dressed, and the answer on repairing a roof window shows when only the flashing needs attention.

When an old roof window comes out, the flashings around it are exposed and usually need a decision. If the new window is the same size and the old kit is undamaged, it is sometimes possible to reuse the flashings, but removal often distorts them, and a fresh set is a small cost beside a second visit to put things right. If the new window is a different size, a new kit is needed, because kits are cut to a single frame.

Our rule is to renew the kit when in doubt and to tell you which parts could stay. Photographs of the old flashings, taken as they come off, show what was wrong and what the new set replaces. Our guide to replacing an old Velux window goes through the checks, and the page on what fitting costs explains what moves the figure. The written quote arrives once the roof has been seen, and carries a single price.