The short answer
The clearest signs are smoke or fumes coming back into the room, a fire or stove that burns badly and draws poorly, soot, twigs or crumbs of masonry falling into the fireplace, and scratching or chirping from the chimney. If you see any of these, stop using the appliance and call a chimney sweep or a roofer to look at the pot and flue. Fit a carbon monoxide alarm if you do not have one.
Signs inside the house that point to a blockage
The first sign is usually smoke. If a lit fire sends smoke into the room rather than up the flue, something is stopping it leaving. A stove that used to burn well and now needs more air, or goes out, is telling you the same thing. Other signs include:
- twigs, straw, feathers or a bird's nest falling into the grate
- a heavy fall of soot or tar flakes that is greater than usual
- crumbs of brick, mortar or clay coming down
- scratching, fluttering or chirping from behind the fireplace
- a smell of old smoke or damp when the fire is not lit
None of these proves a blockage on its own. A cold or wet day can make a fire draw badly, and a new appliance may need different fuel. But a pattern of them, or a sudden change, is enough to stop using the fire and have the flue checked.
Treat any sign that fumes are entering the room as urgent. Carbon monoxide has no smell, so a working alarm in the room is the only way to know. Open a window and do not relight the fire until the flue has been looked at.
What blocks a chimney
Birds are the usual cause in spring and summer. Jackdaws build large nests of sticks on top of an open pot, and these can fill the flue pot to the brim. Nesting birds are protected, so if a nest is active the work has to wait. Leaves, windblown debris and small animals can also drop into an uncapped pot.
Inside the flue, soot and tar deposits build up from burning wood and coal, and a damp, smouldering fire leaves more of them than a hot, dry one. Over time a hard layer can narrow the flue.
The third group is damage. A flue liner can crack and slump, a pot can break and fall in, and loose pieces of mortar and brick from a deteriorating stack can drop down and lodge in a bend. Debris of this kind is a repair problem, not a cleaning one, and a sweep may stop work and say so. If this is the cause, our damp chimney breast page explains how a damaged flue can also show up as stains indoors.
What a roofer checks at the pot and the stack top
A sweep works from the fireplace upward. A roofer looks from the other end. We check the pot and its flaunching, whether a cap, cowl or bird guard is fitted and in place, and whether the opening is clear. We look at the state of the stack's brickwork and whether any loose pieces have fallen into the flue.
With a camera or a torch, we can often see how far down the obstruction sits and what it is. A nest at the pot can be removed in minutes once we are up there; a collapsed liner cannot, and that is where a repair quote comes in.
Many open pots have no cowl at all, which makes it easy for nests and debris to get in. A cap or cowl solves that, and our guide to chimney pots, cowls and caps explains the choices. Deciding to close off a disused stack altogether is a separate question from clearing a blockage, and we cover it under capping a chimney.
When to call a sweep and when to call a roofer
Call a sweep when the problem is soot, tar or loose debris inside the flue, or before you start using a fireplace or stove again after a long gap. Sweeping clears the deposits and lets the sweep see the flue from below. For a stove or a fire you use regularly, it belongs in the yearly routine.
Call a roofer when the trouble is at the top: a nest on the pot, a missing cap, a pot that has cracked or slipped, flaunching that has gone, or damaged brickwork. A roofer has the access, the materials and the skill to fix it, and a sweep cannot repair a pot from below.
Often both are needed. A sweep clears the flue, and a roofer fixes whatever let the debris in. Neither replaces the other, and a roofer who tells you your chimney is fine without looking at the flue is only half checking it.
If the cause is nesting birds, wait until the nest is no longer in use before removal, then fit a guard to keep the pot clear.
Keeping the flue clear afterwards
Prevention is cheaper than a repair. A cowl or bird guard keeps nests and debris out. Burning dry, seasoned wood leaves less tar than wet logs. A yearly sweep before the season starts is the simple routine for a working fire.
A flue you have stopped using should be capped to keep rain out, though it still needs air moving through it. A sealed, unventilated flue is a common cause of damp indoors, which is why our page on damp patches on a chimney breast is worth reading before you cap anything.
When we survey a stack, we photograph the pot, cap, flaunching and brickwork, and our itemised quote keeps any repair apart from any fitting. Work is covered by a 10-year workmanship guarantee in writing, and we leave the roof and gutters clear. For stack work generally, see chimney repairs in Stroud.