The short answer
Usually, yes. An unused flue left open lets rain, birds and debris in, so it is normally capped. But the flue should still be ventilated, with a vented cap at the top or an air vent at the base of the chimney breast, because a sealed, stagnant flue collects condensation and can send damp stains into the room. How to cap depends on the stack, the pot and what the room below is used for.
Why an open flue that is not in use causes trouble
When a fireplace is blocked off and the chimney is forgotten, the top of the stack is still wide open to the weather. Rain runs down the flue and soaks the lining and the brickwork inside the breast. Birds nest on the pot. Leaves, twigs and small debris drop in. Over a few winters, that damp works its way through the plaster and shows as stains, peeling paper or a musty smell in the room.
The effect is stronger where the flue has soot or tar on its walls, because those deposits hold moisture and can bring brown staining through the wall. If you are already seeing marks, our page on damp patches on a chimney breast helps you work out whether rain or condensation is the cause.
Capping the top is the straightforward answer. It keeps the weather and the birds out, which protects the flue, the breast and the room beneath.
Why capped flues must still be ventilated
A flue is a tall, thin, cold column of air. Left sealed at both ends, the air inside stagnates, warm moist air from the house meets the cold flue walls and condensation forms. With nothing moving the air, the damp builds up. This is a common reason for stains on a chimney breast in a house where the fire has not been lit for years.
The fix is to let a little air through. A gentle flow along the flue carries moisture out of it and keeps the masonry dry. That is why a cap for an unused flue is a ventilated one, with a gap or a slotted opening beneath a hood that stops rain but lets air pass.
The other half of the arrangement is a vent into the flue at the bottom, a small grille in the breast. Air then moves up from the room, through the flue and out at the top. Where a flue has been sealed top and bottom, it usually needs one end opened up again. Ask your building control body or us what suits your house.
The capping options for an unused flue
The choice depends on the pot, the stack and how permanent you want the change to be.
- Vented cap on the pot. A clay or metal cap sits over the pot. It keeps rain and birds out and leaves a gap for ventilation. This is the most common choice for a flue you may use again.
- Capped and vented stack. Where there is no pot, the stack top is finished with a slab or tile on short supports, leaving gaps for air, and the flaunching is made good around it.
- Cowl. A cowl is a hood that sits on the pot. Some are made to be open for ventilation and others to suit a working fire. Our guide to chimney pots, cowls and caps compares the types.
- Removing the stack. If the stack is no longer wanted, taking it down to the roof and covering the flue is another route. That alters the roofline, so read whether you need permission to remove a chimney first.
Whichever you choose, the flaunching and the pot should be sound before a cap goes on, or the cap will be fixed to a failing base.
When not to cap, and what a sweep or roofer checks first
Do not cap a flue that serves a working appliance. A gas fire, a stove or an open fire needs a clear flue, and the type of cowl on top must suit the appliance. Capping a live flue can send fumes back into the room, which is a safety matter and not a damp one.
Before capping any flue, it should be swept and looked at. A sweep clears soot and loose debris and lets the sweeper see whether the lining is intact. A roofer then looks at the pot, flaunching and brickwork at the top. A flue with a nest, a collapsed liner or a heavy deposit of tar is a bad candidate for sealing in, because the problem stays in the stack.
If the flue was blocked off at the fireplace, check that the blocking has a vent, and that no pipe or appliance still connects to it. An unexpected connection is not something to find after the cap is on. For a diagnosis of obstructions, see our note on how to tell if a chimney is blocked, which uses the same checks at the pot.
Getting it done, and what the quote shows
A cap is a small piece of work, but it still needs a safe way up to the stack. Whether that is a roof ladder or a scaffold decides much of the cost, which is why the quote prices the access on its own. The cap, any flaunching repair and the ventilation work come next, each with a figure of its own.
Our survey photographs of the pot and the stack top go with the quote, so what we describe is there to be looked at. A 10-year workmanship guarantee is given in writing, and we leave nothing behind on the roof or in the garden. Other stack work is covered in chimney repairs in Stroud.