Interlocking Concrete Tiles Around a Chimney: Soakers and Fit

What makes a profiled tile different

A flat slate or plain tile presents an even surface, so a flashing can be laid close to it. An interlocking concrete tile does not. Each one is cast with a raised roll and a shallow trough, and the tiles lock side to side so that water is guided down the troughs and kept off the joints. The surface is shaped, and anything laid against it has to follow that shape.

That matters at a chimney because the junction is where water has to be handed over from the tiles to the flashing, and from the flashing back to the tiles lower down. If the lead is simply laid flat across a rolled surface, there are gaps beneath it at every roll, and wind can drive rain into them or lift the metal.

Much of the newer roofing in Runcorn is interlocking concrete tile, so this is the situation on a large share of estate stacks there. The shape of the tile, rather than the stack itself, is usually what decides how the lead has to be made.

Why soakers do not suit profiled tiles

On a slate or plain-tile roof, small rectangles of lead called soakers are slipped under each course alongside the stack and turned up against the brickwork. Each course of covering overlaps the one beneath, and the soakers interleave with them to form a stepped, watertight edge. It is a good system for flat coverings, and for natural slate nearer the Old Town it remains the right one.

With a deep-profile tile there is no flat course to interleave with, and the pattern of rolls and troughs does not allow soakers to sit snugly. They would leave gaps at the rolls, and water would find them. For that reason the usual approach on this kind of roof is different.

Instead, a continuous stepped cover flashing is run up each side of the stack, chased into the bed joints of the brickwork and dressed down over the tiles. It is then shaped to the profile where it meets the covering. This is detailed work, and it is one of the reasons a tile roof with a stack in the middle costs more thought than one without.

Dressing lead over rolls and into troughs

The front apron, the piece of lead below the stack, shows the problem most clearly. It has to cover the width of the stack plus an allowance either side, and it must sit over the rolls and drop into the troughs without leaving open channels beneath. In practice the lead is worked, or bossed, with a dresser so that it follows each roll, then turned down into the trough so that water carries on down the slope.

If the lead is not dressed down properly, wind can get under the edge and lift it. A common failure on older work is a flashing that has been bent flat on the roll and left standing off the trough, so that rain passes beneath it. Another is a lead apron that was bossed once, then flattened by foot traffic.

This is also why the choice between lead and a mortar fillet matters. A rigid fillet cannot follow the shape of the tiles at all, and tends to crack along the rolls, after which water runs straight behind it. Our guide to Lead Flashing or a Cement Fillet? How to Tell Which You Have explains the differences more fully.

Cut tiles and fixings at the stack

Where tiles meet the stack they usually have to be cut to fit around the brickwork. Concrete tiles are hard and brittle, so they are cut with a proper saw or grinder and the edges checked for chips. A tile with a damaged edge or a poor fit leaves a gap that wind-blown rain can use.

Cut tiles and those beside the stack are held down with clips or fixings, since the wind is stronger around an obstruction. The tile that sits against the upstand should leave enough room for the lead to be tucked behind it, and not be forced into place, since a tile under strain is likely to crack in the first hard frost. A tile that has been bedded in mortar to fill a gap is a warning sign, because mortar cracks away from concrete and lets water in.

The area behind the stack, where water collects, needs particular attention. Depending on the size of the stack, that may involve a back gutter or a tray of lead, formed to shed water around each side and onto the tiles below. Getting this part right is what keeps the water moving past the stack and out to the gutter instead of standing at its back.

Care, access and what to ask

Interlocking concrete tiles are heavy and brittle at the nibs. They can crack under a person’s weight, and a cracked tile can let water in for years before anyone knows. This is why chimney work on these roofs is done from a scaffold or tower and not by walking across the covering, and why we advise that you do not go up to look yourself.

If you can see a problem from below, such as lifted lead, a grey mortar line or cracked tiles beside the stack, a phone photograph zoomed in from the garden helps us see what is going on. Our own wider checklist is in What to Look For on a Chimney From the Ground.

A quotation should explain how the side flashings will be formed, whether any tiles will be replaced, and what access is being provided. For local detail on estate roofs, the chimney repairs in Runcorn page sets out how we approach them.