Building a Chimney with Fireclay Bricks and Ceramic Liner

Fireclay is a traditional heat-resistant material. It copes with high temperatures, repeated heating and cooling, and the aggressive substances in flue gases. According to manufacturers, it usually withstands far higher temperatures than ordinary home heating produces.

The ceramic liner is the flue itself, the pipe the smoke and gases travel through. Its surface is smooth, so less soot and tar sticks to it. That makes the chimney easier to sweep and lowers the risk of a soot fire. The liner also protects the surrounding masonry from the acids and condensation that burning fuel produces.

This combination suits solid fuel boilers and stoves, open fires and many gas appliances. With a condensing boiler, always check that the liner is approved for wet operation. These boilers often need a different flue system, such as a stainless steel or plastic liner, depending on what the boiler manufacturer specifies.

A chimney is a safety-critical part of the house, so it should be built by a bricklayer with real experience of chimneys. Before you build a new chimney or make major changes to an existing one, check with your local building authority which approval or notification you need. Rules differ from country to country and sometimes between regions, so do not assume that a simple notice is enough. In a block of flats, the building management or owners' association also has a say.

1. Preparing the base. A chimney is heavy, so it needs a solid, level foundation or a load-bearing slab that can carry its weight. Never build one on an old floor or ceiling without a structural engineer's assessment.
2. The bottom section. First the base unit goes in, with a condensate trap and drain, and a cleaning door above it. This door is used later for sweeping and inspection.
3. Fitting the ceramic liner. The liner sections are stacked one on top of another, and the joints are sealed with acid-resistant sealant as the manufacturer instructs. The liner must be plumb and the joints clean, with no sealant squeezed into the inside of the flue.
4. Insulation and expansion gap. A gap is left between the liner and the outer wall, usually filled with mineral wool. The liner expands as it heats up, so it must never be bricked in tightly.
5. Building the outer wall. Fireclay bricks or chimney blocks are laid around the liner using the mortar the manufacturer specifies. Ordinary cement mortar is not suitable where temperatures run high.
6. Connecting the flue pipe. An inlet for connecting the appliance is fitted at the right height. Its position has to match the boiler or stove.
7. Passing through the ceiling and roof. The clearance from timber and other combustible materials set by the standards and the manufacturer must be kept all round the chimney. Where the chimney goes through the roof, it gets flashing so that rainwater cannot run under the roof covering.
8. The chimney top. Above the roof, the chimney needs protection from rain and frost, for example facing bricks, cladding or a suitable render. A cap or capping slab goes on top to shed water away from the masonry.

• Traditional fireclay brickwork works well where the chimney has to fit an awkward spot or tie in with existing masonry.
• A system chimney made of blocks, with a ceramic liner and insulation, goes up faster, and all the parts fit together precisely. The layered principle is the same.
• Single or multiple flues: if you might add a wood burner or a second boiler later, it makes sense to build a chimney with two flues from the start. Adding one afterwards is much harder.
• Liner diameter is not a matter of guesswork. It has to suit the appliance, the fuel and the height of the chimney. In practice it often happens that a flue that is too wide has weak draught and the chimney turns damp.

The biggest factors are the height of the chimney and how much of it stands above the roof. Work on the roof needs scaffolding and fall protection, and it goes more slowly. A chimney in a new build is simpler, because it goes up together with the house before the roof is finished.

These situations make the job more complicated:

• demolishing an old chimney and taking away the rubble,
• cramped access where materials have to be carried by hand,
• going through a finished roof structure and roof covering that has to be opened up and made good again,
• more flues, extra inlets and accessories such as ventilation grilles,
• a more demanding finish above the roof, such as facing brickwork or cladding.

• Liner bricked in solid. It expands when hot, and without an expansion gap the outer wall cracks.
• Wrong sealant or mortar. Ordinary mortar in the liner joints breaks down over time because of the acids.
• Too little clearance from timber. Roof timbers right next to the chimney are a serious fire risk.
• Unprotected chimney top. Water soaks into the masonry, freezes in winter and the brickwork starts to break up.
• Poor flashing at the roof. Leaks around the chimney are one of the commonest complaints after the job.
• Chimney too low above the roof. The top must stand high enough above the ridge, or the wind spoils the draught and fumes blow back into the house.

Before the first fire, the chimney should be inspected by a qualified chimney sweep or chimney inspector, depending on what local regulations require, and you should get a written report or certificate. Do not put an appliance into use without it. If a gas appliance is to be connected, the connection must be made by a qualified, registered gas engineer. If you ever smell gas or suspect fumes coming back into the room, leave the building and call your local emergency number or 112. At handover, check:

• The chimney is plumb and the outer wall has no cracks
• The cleaning door and condensate trap are accessible and seal properly
• The condensate has somewhere to drain
• The clearance from combustible materials in the ceiling and roof structure has been kept
• The roof penetration is properly flashed
• The chimney top has a cap and a protected surface
• The liner suits the appliance you are going to connect
• You have received the inspection report and documents for the materials used