What Is a Lead Slate?

What a lead slate does on the roof

Every pipe that leaves a house through a slated roof creates a hole that has to be made weathertight. The hole is awkward because slates overlap in rows, each course lapping the one below. A flat sheet cannot simply be pushed in, and sealant alone will not hold for long on a surface that heats, cools and gets frozen.

The lead slate solves this by behaving like a slate itself. Its upper edge tucks beneath the course above, and its lower edge sits on top of the course below, so rainwater runs over it in the same direction as it runs over the slates around it. The pipe passes through a formed collar, or a boss, that stands proud of the sheet and is dressed tight to the pipe wall. On a vertical soil pipe, a sleeve of lead or a plastic collar finishes the top edge.

The parts of a lead slate

A typical one has four working parts. The flat apron covers the slate area. The upstand is the raised lip round the pipe, which stops water running across the sheet from entering the opening. The back tray, where the lead slate sits under a long slate, carries water around the sides. The lower lap is the bit that overlaps the slates beneath, so it must be long enough to shed water outwards over the slate surface.

Many manufacturers sell preformed versions in a mixture of lead, aluminium and plastic. Because slate sizes and roof pitches vary a great deal, a leadworker will often cut and bend a bespoke slate from sheet lead, which gives a better fit on older roofs where the slates are uneven. On a roof that is being re-covered, we can match the slate colour with a coat of patination oil so the lead stays discreet. Our page on patination oil explains why it is applied.

Why a lead slate leaks

Failures follow a pattern. The first is the collar splitting where the lead was bent too sharply, or where the pipe moves slightly in the wind. The second is a lap that is too short, so that driving rain blows under it. The third is slates around it slipping: if the slate above has dropped, the lead is left unsupported and the lap opens. A fourth cause is sealant. Mastic round a collar looks tidy for a season, then dries and cracks, and the leak starts again behind it.

A damp patch on the ceiling near the line of a soil pipe is a common sign. In the loft, a dark stain on the pipe or the boarding beneath the slate points to the same fault. We check it from inside the loft as well as from the roof, because a leak can run a long way along a rafter before it appears.

Slate roofs and the pipes that cross them

Most houses in the Basingstoke area that carry a lead slate are older or extended properties, where a new bathroom or kitchen added a soil vent pipe long after the roof was laid. The pipe is usually cut through the slates by whoever fitted the plumbing, and the weathering is an afterthought. That is why so many of these junctions turn out to be a ring of mortar, a smear of sealant or a rolled piece of bitumen tape.

A purpose-made lead slate turns that afterthought into a proper detail. It also means the slates around it can be nailed on a sound batten, which they often are not when the hole has been hacked out in a hurry. Check the gutter below for slate fragments too, since a broken slate near a pipe is often the first sign that someone has been working in the area.

Replacing or repairing one

If the lead is sound and only the slates have shifted, resetting the surrounding slates is enough. Cracked or thin lead needs a new slate, which means lifting the slates above, fitting the new sheet and re-fixing them with copper or stainless fixings. It is a short job on an accessible roof. If you want to know how lead lasts generally, read our guide on how long lead flashing lasts. The full range of our work is on the lead roofing and leadwork page.

You will get survey photos of the fitting from above and from the loft, a fixed written quote, and a plain answer about whether the slate or the roof around it is the real issue.