How Are Dormer Cheeks Leaded?

What the cheeks are

A dormer is a small structure that projects from a sloping roof and carries its own window. It has a front face, a roof on top, and two side walls, called cheeks. Those side walls are normally framed in timber and covered in boarding, and they stand next to the main roof slope. The cheeks are exposed to rain and wind, and the joint where they meet the main roof is a complicated junction, which is why dormers are among the more leak-prone parts of a house. Lead is the traditional covering, since it can be shaped to the angles of the structure and lasts a long time.

The covering step by step

The first job is the base. The cheek boarding is checked for rot and made sound, and a layer of building paper or a separating membrane goes over it so the lead does not bear on bare timber. The lead is then cut into pieces sized to the cheek. Because a large area of lead moves a lot, the cheek is covered in sections that are lapped down the face instead of one big sheet. Each section is fixed along its top edge with clips or copper nails, and the lower part hangs free. The vertical joints are lapped or turned into a welt, depending on the design. Our page on the maximum length of a lead sheet explains why the sections are kept short.

The top edge of the cheek covering is tucked under the dormer roof edge, or under the covering of the dormer roof, so water running off the roof falls onto the lead and not behind it. The corner where the front face meets the cheek is dressed round carefully with no cuts left open, because that corner is where many dormers leak.

The junction with the main roof

This is the detail that matters most. Along the bottom of the cheek, where it meets the slates or tiles of the main roof, the lead is turned out over the roof covering in a stepped form that follows the pitch of the roof. In older work, small pieces called soakers, each lying under a tile and turned up the wall, do the job; our guide to soakers and step flashing explains the methods. In current practice, a continuous stepped flashing is dressed over the tiles and tucked into the cheek. The lead should lap the roof covering by enough to throw water onto the tiles, not under them.

Where the dormer window itself meets the cheek, the frame is sealed with a flashing and the detail is checked for any gap. If the dormer has a small flat roof, the lead there is laid with falls and with joints formed across the flow of water, with a drip at the front edge.

Fixings and movement

The way lead is held matters as much as the lead itself. Lead expands in warm weather and contracts as it cools, and over many years the cycle can make a rigidly fixed sheet creep or crack. That is why only the top edge is nailed, with copper or stainless fixings that will not corrode and stain, and why the lower edge is held by clips that let it slide. A detail that pins the sheet at several points across the face looks tidy and fails early. Underneath, the paper or membrane gives the sheet a surface to slide on.

If the dormer is old, expect to find that the cheeks were built with little overhang at the top and a flashing that runs behind a rotted bargeboard or fascia. Repairing the lead alone then fixes nothing. The timber has to be sound first, and the roof edge has to shed water clear of the cheek.

Why cheeks fail

The usual problems are lead that has split because it was fixed too rigidly or in sections that were too long, lifted laps where wind has worked underneath, and rotten boarding behind the lead. Cheeks that have been covered in felt, mortar or sealant in a previous repair are often hiding such faults. Painted or tile-hung cheeks are an alternative, but lead is the neat answer when the design calls for a metal finish. Our page on signs of failing lead flashing shows what to look for from the ground, and lead bay roofs and canopies describes similar work on other small structures.

We look at a dormer from the roof, photograph each junction and the condition of the boarding, and give you a written price for the repair or the recovering. Visit the lead roofing and leadwork hub, the homepage, or ring 01256 213956.