The short answer
A disused chimney should normally be capped to keep rain, birds and debris out, but the cap needs ventilation. A sealed flue traps moist air, and the damp then shows as stains, peeling paint and salts on the chimney breast. A ventilated cap, with a vent set low in the flue, lets the chimney breathe and keeps it dry.
Why leave an unused flue open at all?
A chimney is a tall brick tube, and brick takes in moisture. Rain soaks the upper courses of the stack. Condensation forms inside from warm, moist house air that drifts upwards. A moving column of air dries all of that out. Seal the top and the air stops moving, so the damp sits in the brickwork and soaks slowly downwards into the chimney breast.
The effect often appears in rooms with a disused fireplace. Wallpaper lifts and paint blisters, brown tide marks spread up the breast, and white or yellow salts, carried by moisture from the brick and old soot, creep to the surface. Our page on why a chimney breast goes damp covers the causes in more detail.
Leaving the pot wide open is not the answer either. An uncovered flue lets rain run straight down, gives birds a place to nest and collects leaves, so the sensible middle course is a cap that shields the opening but leaves a gap for air. Fitting one is usually a short visit, though a stack with a high or awkward pot may need a tower or scaffold, which is worth asking about before you agree a date.
What a ventilated cap does
A ventilated cap covers the top of the pot or flue but leaves a gap or louvres for air. It stops rain falling straight in, keeps out nesting birds and blocks leaves, while the column of air carries on moving. Caps are made in clay, concrete, stainless steel and aluminium. Some sit on top of the existing pot, and some replace it and are bedded into the flaunching.
A well-chosen cap suits the stack. On a Victorian terrace, a clay cap in a matching colour looks right. On a modern house, a steel or aluminium one is less obvious. Before altering the look of a stack on a protected house, ask the council, because pots and caps form part of what it guards.
The cap alone does not finish the job. Ask your roofer to look at the flaunching and pointing at the same visit, since a new cap on a crumbling crown is a short-term fix.
The vent low in the flue
Air needs a way in as well as a way out. When the fireplace opening has been bricked up or boarded over, the flue should still have a small vent low down, usually a grille set into the board or the wall just above the hearth. Air then comes in at hearth level, rises through the flue and leaves by the cap, taking moisture with it.
Plenty of damp breasts turn out to have a cap but no lower vent, or a vent that was painted over or hidden behind furniture. Checking takes seconds and costs nothing, and it is worth repeating each autumn. Hold the back of a hand near the grille on a breezy day and you should feel air moving. If you find a sealed-up fireplace under a sealed-up stack, the cure is to open one end or both, and that need not cost much.
Some people close the vent because it feels draughty, or because the grille looks dated against a newly decorated chimney breast. A small, well-made grille lets through very little air, and the damp it prevents does more harm than the draught ever could.
When capping is not enough
Some stacks need more than a cap. If bricks are loose, joints are open, the stack leans or the lead round the base has split, water will get in whatever sits on top. In that case repair the stack first, with repointing, new flaunching or, in the worst cases, a rebuild, and cap it afterwards.
Do not confuse capping with removal. Taking down a chimney breast always needs Building Regulations approval, and a neighbour may need to be served a notice if the wall is shared. Our page on permission to remove a chimney explains what is involved. Capping keeps the stack in place, and for most disused chimneys it is the smaller job by a long way.
If you want the flue gone for good, ask a structural engineer and your building control body before any bricks come out, because a stack often supports more than it appears to.
Before you cap: a short checklist
If the flue is blocked with a nest or debris, clear it first. Our page on the signs of a blocked chimney explains when to call a sweep. If you might use the fireplace again, pick a cap that lifts off, and have the flue swept and checked before any fire is lit. If a gas appliance or boiler uses the flue, do not cap it without advice from the installer.
Steer clear of caps that are fully sealed, or that rely on a flat slab and a bead of silicone, and be cautious if someone offers to seal a chimney "to stop the damp" without asking about ventilation. A ventilated cap with the right vent below is a small job, and we would rather recommend that than something bigger. Send photographs through the enquiry form, ring 01793 626096 or use WhatsApp, and our survey will show your own stack in photographs, followed by a written quote. For related faults, see our chimney leaks page, or return to the chimney repairs overview.