How do I check a chimney is safe to use?

Most of what makes a chimney unsafe is invisible from the room. The checks you can do yourself are worth doing, but they narrow the question rather than answer it.

Safety

What you'll need

• A torch and a mirror
• Binoculars for the stack from ground level
• A carbon monoxide alarm

Step by step

1. Fit a carbon monoxide alarm first.Before anything else and regardless of what the chimney turns out to be like. Audible, to the current standard, in the room with the appliance. It is a legal requirement in rented homes in England.
2. Position the alarm properly.High on a wall or on the ceiling, between one and three metres horizontally from the appliance. Not directly above it, not in a cupboard, not behind furniture.
3. Find out when it was last swept.Annually for most solid fuel, and more often for heavy wood use. If nobody knows, treat it as unswept and do not light it.
4. Book a registered sweep.HETAS, NACS or the Guild of Master Chimney Sweeps. They sweep, inspect and issue a certificate, which insurers increasingly ask for. This is the single most useful thing you can do.
5. Look up the flue with a torch and mirror.From the fireplace opening. You are looking for daylight at the top, nests, soot build-up, loose mortar and any obvious obstruction. It is a narrow view but it finds gross problems.
6. Check for bird nests in spring and summer.Jackdaws nest in unused chimneys and can fill several metres of flue. Nesting birds are legally protected while in use, so timing matters — a sweep will advise.
7. Look at the chimney breast inside.Brown or yellow staining, damp patches, or a tarry smell means condensing flue gases or water getting in. Both indicate the flue liner or the stack needs attention.
8. Check for cracks and loose plaster.Any crack in the breast is a potential path for combustion gases into the room. This is one of the ways carbon monoxide reaches occupied space.
9. Inspect the stack from outside.Binoculars from the ground. Leaning, missing or cracked pots, eroded mortar between the bricks, and vegetation growing out of it are all signs it needs work.
10. Check the flashing where it meets the roof.The lead or mortar seal at the roof junction is the commonest place water gets in, and persistent damp on an upstairs wall is usually this rather than the flue.
11. Check the pot and cowl.A pot should be sound and a cowl clear. A capped chimney serving a live appliance is dangerous, and an uncapped disused one lets in rain and birds.
12. Make sure a disused chimney is ventilated.Capping the top without an air brick at the bottom traps moisture and causes damp in the breast. Both ends need to breathe.
13. Leave the smoke test to the sweep.A smoke pellet test shows whether gases escape into the room or into neighbouring properties. It requires knowing how to interpret it and how to seal the flue safely.
14. Check ventilation in the room.An open fire needs a substantial permanent air supply. Modern draught-proofing and sealed windows have made under-ventilation a common cause of poor draw and spillage.
15. Ask about a liner if you are fitting a stove.Most stove installations need a flexible liner sized to the appliance. An oversized unlined flue causes condensation, tar and poor draw.
16. Stop immediately if there is any doubt.Smoke coming into the room, an alarm sounding, or headaches and drowsiness when the fire is lit. Put it out, ventilate, get everyone out, and have it inspected before it is used again.

Tips

• The carbon monoxide alarm goes in before anything else, whatever the chimney turns out to be like.
• A registered sweep inspects as well as sweeps, and the certificate is what insurers ask for.
• Staining or a tarry smell on the chimney breast means gases or water are getting where they should not.

Common mistakes

• Lighting a fire to test whether the chimney draws — If it is blocked or cracked, the test itself fills the room with smoke and potentially with carbon monoxide. A smoke pellet test by a sweep answers the question safely.
• Capping a disused chimney without ventilating it — Sealed at both ends, moisture is trapped in the stack and works into the breast, causing persistent damp and staining that looks like a roof leak.

If it doesn't work

Questions people ask