Damp-Proof Trays and Cloaks Where a Stack Meets the Roof

What the tray is for

Rain does not only run down the outside of a stack. It soaks into the brickwork from the top, through the joints and through the bricks themselves, and gravity takes it downward. Where the stack leaves the roof, that water arrives at the point where the masonry meets the house, and without a barrier it carries on into the roof timbers and ceiling below. A damp-proof tray is the barrier that stops it.

The idea is simple: lay an impermeable layer across the full thickness of the stack, with an upstand at the edges and a fall to the outside. Water that has worked its way down inside the brickwork meets the layer and is turned outward, where it leaves through weep gaps or over the lead. It is the same principle as a damp course in a wall, applied to a vertical shaft.

A tray does not stop water entering the stack, and it is not a substitute for a sound head or sound joints. It deals with what has already got in. If you want to see how damp shows itself below, Damp on a Chimney Breast: Leak, Condensation or Salts? explains the signs on the inside of the house. Think of a tray as a safeguard for the inside of the house. A roof can have fresh lead and a handsome head and still let damp into the ceiling if the middle of the stack is a sponge, and the tray is the part that answers for that middle section. Our chimney page for chimney repairs in Ellesmere Port describes why exposed stacks need this extra line of defence.

How it is detailed

The detail matters more than the material. A tray is laid in a bed joint just above the point where the lead apron and flashings finish, so that the highest point of the lead sits comfortably below it. It runs through the full depth and width of the stack, with a turned-up edge around the flue linings and any other voids, so that water cannot slip past the sides.

In older practice the layer was slate or bitumen. Today it is usually a flexible, purpose-made membrane or a lead cloak, chosen to bend over the flue-liner area and survive being built into mortar. At the front it overhangs the face of the stack, so that water drips clear instead of tracking back. Weep holes or open perpends in the course just above allow drained water to escape rather than collect.

The lead flashing is then linked to the tray, so that water coming down either route ends up outside the roof. Lead Flashing or a Cement Fillet? How to Tell Which You Have covers the lead side of this junction in more detail. A tray built in without being linked to the lead leaves a gap through which damp can still travel. Spacing and position count. Too low and the tray sits under the lead, where it can trap water behind the flashings; too high and it leaves a wet stretch of brickwork between tray and roof. The usual aim is for the tray to sit just above the top of the lead and to sit clear of the roof covering, with a small gap of brickwork showing below it.

Why older stacks usually do not have one

Many stacks pre-date the habit of building damp-proof layers into chimneys. They were built with lime-rich mortar, which is soft, breathable and lets moisture pass back out. The stack dried between showers, and a lack of tray did little harm. Thick walls, open fires and a constant flow of warm air through the flue also kept the masonry drier than it is now.

The conditions have changed. Cement mortar is harder and holds water in the brick, appliances run cooler than open fires, and unused flues stop moving air at all. A stack that coped for decades can become a permanent sponge, and the water finds its way to the roof junction. That is why damp staining on the ceiling by the chimney breast so often turns out to be a stack problem.

Spotting this from the ground is not straightforward, since the missing tray is hidden in the brickwork. What you may see is damp in the loft or a patch on the ceiling, and a roofer can help tell a stack leak from other causes. Another reason is cost control in the original build. A tray was an extra operation for a bricklayer working to a budget, and when stacks stayed dry enough in practice, nobody noticed it was missing. The omission shows up decades later, once the mortar and the use of the flue have changed around it.

What happens during a rebuild

A rebuild is the ideal moment to put in a tray, because the courses at roof level are already taken down and open. The tray is set into a clean bed, the brickwork goes on above, and the lead is chased in over it. Nothing has to be cut into existing work, so the detail can be done the way it ought to be done.

If a stack is only being repointed, adding a tray is not usually possible without taking down at least some courses, which tends to push the job towards a partial rebuild. We will say so plainly in the written quotation, and set out the options. Our work is on the outside of the stack only, and we do not line or alter flues; that is for a HETAS or Gas Safe registered installer or a sweep.

Once the rebuild is done, the tray is hidden and forgotten, which is as it should be. What you notice is the absence of damp. Free written quotations make this part of the job visible, so ask for it to be listed, and see Repoint, Rebuild or Remove? How the Decision Gets Made for how to weigh a rebuild against other options. After the rebuild, ask for the detail to be described in the quotation and in the final account of work. A tray that is mentioned in writing is a tray someone has thought about, and it gives the next owner of the house something to rely on when they come to inspect the stack in future.