Does a Gas Fire Need a Chimney Liner?

A chimney liner is a tube that runs up inside the old flue, from the fire to the pot at the top. Its job is to carry the gases from the fire safely out of the house. Older brick chimneys were built for coal and wood, with wide flues that draw hot, strong smoke. A gas fire sends up cooler gases in a smaller volume, and in a wide flue they can cool further, which affects how well the flue draws.

Gas also produces moisture. In an unlined flue that moisture can soak into the brick and mortar, and over time it shows as staining on the chimney breast, crumbling mortar or damp patches on the wall. A liner keeps the gases inside a sealed channel and protects the masonry from them.

There is a second reason. Many old flues have cracks, gaps or loose joints, and a liner closes them off.

Not every installation uses one. Some gas fires are designed to work with a particular flue, and some flues are already lined. The point is that you should not guess. A fire fitted to the wrong flue is a safety matter, and the person who judges it is the gas engineer, not the roofer or the homeowner.

By law, paid gas work must be done by a Gas Safe registered engineer. That covers fitting or moving a gas fire, connecting it, and testing that the flue clears the gases properly. Check the engineer's Gas Safe registration and that it covers the type of appliance. Ask to see their ID card, and ask for a certificate when the work is finished.

A roofer is not the right person to decide whether your gas fire needs a liner, and should not sign off the flue. What a roofer does is the work outside and above: surveying the stack, repairing or rebuilding the brickwork, fitting the pot and the top plate, and making the stack watertight.

So the sequence matters. If you are having a gas fire fitted or replaced, ask the engineer first what the flue needs. If a liner is required, they or a specialist will often arrange it, and we then work with them at the top of the stack. If you are unsure where the fire stands, start with the engineer.

For blocked or smoking flues of any kind, our guide on whether to call a sweep or a roofer shows where the lines between trades fall. Anything that smells of fumes is an emergency for a gas engineer, not for a roofer.

When a liner goes in, the top of the stack has to be dealt with. The liner usually ends at the pot or a plate fixed at the top of the stack, and that detail must shed rain and stop birds. The flaunching, the mortar slope that sheds water around the pots, is often renewed at the same time, because the work disturbs it.

We check the condition of the stack before the lining is fitted. Loose bricks, crumbling joints and a failed flashing, the metal weathering where the stack meets the roof, are all easier to put right with access already in place. Our guide on chimney pots and cowls describes the parts involved.

The cap or cowl is chosen with the engineer. A cowl for a gas flue must let the gases escape, so the fitter says which kind is acceptable. We fit it, seal around it and leave the stack sound.

If the stack turns out to lean or show open cracks, we tell you with photos, and explain when repair is no longer enough. Because the chimney is also a shared item in many semis, check who owns the stack before work starts.

The costs divide cleanly. The gas engineer prices the fire, the flue test and any liner. We price the stack end. For our part, Checkatrade's April 2026 guide gives typical UK ranges of £250 to £1,000 for chimney flashing, £300 to £1,000 for repointing and £180 to £520 for a cap. Treat them as yardsticks. Our end is priced after a survey of the stack, as one written total, because height, access and condition all move the figure.

Scaffolding is often the largest single item. If a liner is being fitted from the top, the access that the engineer needs can serve the roofer too, so tell each of them what the other plans. It is cheaper to share one set-up than to hire it twice.

A sensible order is to get the engineer's advice first, arrange the roof survey, then agree one date when both can work together. Ask each tradesperson to put the plan in writing.

Finally, if the fire is out of use and you would like it sealed rather than lined, that is a different job, and our guide on capping a disused chimney covers it. Our chimney repairs page lists the rest.