Capping and disused chimneys

Most chimneys in Lewisham are no longer used. What you do with a redundant stack matters more than people expect — and the most common approach, sealing it up tight, is the one that causes damp.

If you have a chimney you do not use and you are wondering whether to cap it, cowl it, leave it or take it down, this page sets out the options and the one rule that matters more than any of them. If you have decided to get rid of it entirely, see chimney removal instead.

Why redundant chimneys give trouble

A working chimney is a warm, dry, self-ventilating structure. Combustion gases keep it warm, and the draught keeps air moving through it. Stop using it and you remove both.

What follows is predictable:

• The stack stays wet longer. Nothing dries it from the inside any more, so rain that gets in stays in.
• Mortar erodes faster because it spends more of the year saturated.
• Frost damage accelerates. Wet masonry in a freeze is what blows brick faces off.
• Old soot deposits attract moisture. Soot contains hygroscopic salts that pull water out of the air; they migrate through the brickwork and show as staining on the breast indoors. See damp around a chimney breast.
• Nobody looks at it. An unused chimney gets no sweeping, no service visit and no attention until something falls off it. An unswept flue that is later brought back into use is also the classic setup for a chimney fire.

Disused stacks are frequently in worse condition than used ones, which surprises people who assume that not using something preserves it.

The options

1. Leave it as it is

Perfectly reasonable if the stack is sound and the pots are in good order. It still needs the same maintenance as any other stack — pointing, flaunching, flashing — and it should be inspected periodically. Doing nothing is a decision, not an absence of one.

2. Fit a capping cowl

The usual answer for a flue that is genuinely finished with. A capping cowl sits on or replaces the pot and keeps rain, birds and debris out while allowing a controlled amount of air through. That ventilation is the whole point — it is what distinguishes a cowl from a seal.

Cowls also solve some specific nuisances:

• Birds nesting in an open flue, and the debris and blockage that follow
• Rain coming down the flue and appearing at the fireplace
• Draughts down an open chimney into the room
• Debris falling down a flue that has never been swept since it was abandoned

3. Cap the stack with a slab or cover

Where the pots are damaged or you want a lower-profile finish, the top of the stack can be closed with a weathered cover. It still needs a ventilation path, and it still needs to shed water properly with a fall and a drip edge, exactly as flaunching does.

4. Reduce the stack

Taking a redundant stack down to below the roofline and weathering the roof over it. This removes the maintenance liability permanently and is sometimes the sensible answer for a badly deteriorated stack on a property where the chimney is not an architectural feature. It changes the look of the building, needs the roof properly made good, and on a shared stack it is usually not possible without the neighbour's agreement. Covered in more detail under chimney removal.

5. Full removal

Stack and breast taken out entirely. A much bigger job with structural implications. See chimney removal.

Ventilation: the part that gets missed

Capping the top is only half of it. A disused flue needs air moving through it, which means ventilation at both ends:

• At the top — a vented cowl or cap, rather than a solid seal
• At the bottom — an air vent in the fireplace opening or in the blocked-up breast, so air can pass through

Where a fireplace has been blocked up with plasterboard or brick and no vent fitted, and the top has been sealed with a solid cap, you have created a sealed damp trap inside the wall. This is one of the most common causes of unexplained damp on a chimney breast that persists after roofing work — because the problem was never on the roof.

What about insulation?

People often ask about filling or insulating a disused flue to stop heat loss. Be cautious. Filling a flue with insulating material removes the ventilation path and can trap moisture inside the stack, producing exactly the damp problem described above. Draught-proofing at the fireplace opening with a product designed to allow background ventilation is a different and much safer proposition than filling the shaft.

Before you cap: check the condition first

Capping a stack that needs repointing simply means paying for access twice. If access is going up anyway, that is the moment to sort the pointing, the flaunching and the flashing. See roof and chimney surveys if you would like the condition established before deciding.

Capping questions