The short answer
A flue sealed at both ends traps still, damp air, and that air condenses on the cold brickwork inside. The result is damp patches on the chimney breast, salts and stained wallpaper. Letting air move through the flue stops that. This is done with a vented cap at the top, a vent in the room at the bottom, or both. The roofer fits the top one, and the bottom is a small job for the room.
How air movement keeps a sealed flue dry, where the vents go on a capped stack, and what to avoid when blocking an old fireplace.
What goes wrong in a sealed flue
When a fireplace is no longer used, it is common to board up the opening and put a solid cap on the pot. That keeps the rain and birds out, but it also stops the air moving. Warm air from the house leaks into the flue through gaps in the old fireplace, meets cold brick, and turns to water. With nowhere to go, the moisture sits in the stack and soaks into the breast.
The signs are indoors first. A brown tide mark on the plaster above the fireplace, wallpaper that bubbles, or a musty smell that gets stronger in damp weather. White salt deposits may appear as the water dries out of the wall. Outside, the brickwork of the stack may spall faster, because the stack is being wetted from the inside. A stain that appears in a dry spell points to condensation inside the flue, not a roof leak.
Capping is not the problem here. A sealed flue with no air movement is, and a small gap at each end puts that right at modest cost. Letting the flue breathe is what keeps it healthy.
Top vents and bottom vents
Moving air needs an entry point and an exit. At the top, a capping device sits over the pot or flue opening. A vented cap has a cover to keep the rain out and gaps or louvres to let air through, while a cowl does a similar job over a pot. Our guide to chimney cowl types shows what the options look like and where each suits.
At the bottom, a ventilation brick or a vent plate is fitted in the blocked-up opening or in the chimney breast itself, low down in the room. Room air enters there, climbs the flue and escapes at the cap. That gentle draught is all that is needed. The vent is small, and it is often painted to match the wall or fixed behind a fire-surround.
Avoid a bottom vent that opens into a cupboard or a void without a route to the room, as the air then has nowhere to come from. And do not stuff the opening with newspaper or insulation, because that blocks the flow again.
The top vent is roof work and the bottom one is a small indoor job. Our answer on capping a disused chimney covers how a cap is fitted.
When a flue should not be sealed at all
Not every flue is disused. If a gas fire, boiler, stove or open fire still uses the chimney, it must stay open and be capped and vented as that appliance needs. The right arrangement depends on the appliance, and the engineer who installs or services it should say what is needed. A liner may be fitted for a gas fire, and that is the engineer's call.
If you are unsure whether a flue is live, ask before capping it. A neighbour's flue in a shared stack may also be in use, and a stack serving both homes is not yours to seal alone.
Birds are another reason for a proper cap. An open pot invites nesting, and nests block flues. Wild birds are protected, and it is unlawful to deliberately wreck an active nest, so a cap goes on only after checking that the pot is empty. Our answer on birds nesting in a chimney covers what to do if one is already there.
What the roofer checks and what it costs
On the roof, the checks are simple. The roofer looks at the state of the pot, the flaunching around it, and the brick below. A cap fitted over a cracked pot or crumbling flaunching will soon fail, so these are sorted first. The cap should be secure, sit clear of the flue for air to pass, and match the pot size.
As a national guide, Checkatrade's April 2026 chimney repair table gives a cap at £180 to £520. Treat this as a yardstick. Your figure turns on the height, the type of cap and the access, and a table price can leave VAT out. If scaffold is needed, ask whether other stack work can share it.
A vent in the room is something a handyperson can fit, and we can advise on where it should go at the survey. Expect the stack-top photos first, and after them one total price in writing, with the cap type named. Our chimney repairs hub shows how capping sits with repointing and flashing. A picture of the pot and the fireplace tells us what to check on the roof before anything else.
More on this in Chimney Repairs in Crawley
Two capped flue questions
Will a vent in the breast make the room cold?
A small vent lets in only a gentle flow, much less than an open fireplace did. Because the air is drawn up the flue and out of the house, it keeps the breast dry. A vent plate in a sensible place, such as low on the breast, is usually barely noticeable.
Can I just seal the pot to stop rain and birds?
A solid cap does keep rain out, but it also stops air moving and invites condensation inside. A cap with ventilation gaps keeps out rain and birds while letting the flue breathe. Add a bottom vent in the room, so air has a route in as well as out.