How Is Lead Flashing Fixed to a Roof?

Lead moves. It swells in sun and shrinks in frost, and over a long run the difference can be several millimetres. A flashing nailed hard along every edge would have nowhere to go, so it would stretch each summer, then crack along the folds. For that reason a flashing is fixed in one place, near the top, and left loose at the other edges.

The idea is simple. Only the upper edge is held firmly. The rest of the sheet hangs or lies over the roof, resting on the covering and clipped so the wind cannot lift it. As the metal changes size it slides a very small distance over the surface below, and nothing is pulled apart.

This is also why lead used as flashing is laid in short lengths, not one long sheet. Lengths are lapped and, on long runs, broken by movement joints. A single piece that is too long will split at its weakest fold, however well it is fixed. If you have seen a crack that returns after repair, our answer on why lead flashing cracks explains the cause in more detail.

Against a brick wall or chimney, a flashing has to be anchored in the masonry. A groove is cut into a mortar course, level with the line the lead will follow. This groove is the chase, and it is usually a joint's depth into the wall. The upper lip of the lead is turned into it.

Lead wedges hold it there. A wedge is a small folded or rolled strip of lead, tapped into the chase at intervals along the flashing so it grips the top edge and the brick. Lead is used for the wedges because it is the same metal as the flashing and does not set up corrosion between two different metals. After the wedges, the chase is pointed with mortar or filled with a suitable sealant to close it against rain.

The wedging spacing and the quality of the pointing decide how long the flashing stays put. A loose wedge or a crumbled joint lets the top edge drop out, which is the most common reason a flashing comes away from a wall. Our answers on lead flashing coming away from a wall and repointing lead flashing cover the repair.

Without something to hold it, the lower edge of a flashing would flap in a gale. Strips of lead or stainless steel, called clips or tacks, are the answer. A clip is nailed to the roof or wall at intervals, and its end is folded around the free edge of the flashing. It stops the wind lifting the metal but lets it slide.

Fixings matter. They should be copper or stainless, never plain steel, which rusts and stains the lead. Nails go through a clip, not through the face of the flashing, because a hole in lead is a hole in the roof. The only point where a flashing is nailed directly is usually the top edge on a stepped detail, and there the nail is covered by the lap above.

Under a chimney or against a wall, the flashing is often made in two parts: a soaker or apron under the covering and a cover flashing over the top. This keeps the join flexible and gives two layers of defence. Each layer is laid to shed water over the one below.

Standards govern the thickness, the lap and the fixing spacing. If you see a flashing held by a continuous bead of sealant alone, it has not been fixed properly.

You can judge a flashing from the ground, with binoculars if needed. The top edge should sit tight and straight in its chase, with continuous pointing and no gaps or dark shadow lines. The lead should lie flat over the roof covering and follow its profile, without ballooning or sitting proud of the tiles. The lower edge should not hang away from the wall.

Signs of trouble are easy to see once you know them. Pointing that has dropped out in lumps, wedges that lie loose on the roof below, a flashing that rattles in the wind, or lead that has slid down and left a gap at the top. Where the lead has been bent or squashed into position by hand, it often works loose again.

On the survey we photograph the chase, the wedges and the clips, so you see how your own flashing was fixed. If it has to be redone, the written price covers the whole detail, not a patch of sealant.

Next step: email a picture of the spot where the lead meets the wall, with the side of the house it is on. The lead work page covers our other flashing jobs.