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• Durability: clay bricks with British Standard durability designations FL, FN, ML, MN are all suitable.
  
• Terminal: chimneys are usually exposed on the top and all four sides, so the brickwork is often saturated. This means that they are more exposed to frost and sulphate attack than any other part of the building.

For this reason, it is desirable to put a coping or capping on top of the brickwork to reduce its exposure to moisture.

Cappings are made from brickwork with tile creasings, which are then flaunched – filleted with mortar – to help throw water clear of the brickwork. However, cappings cannot be relied on to keep out moisture indefinitely and so require a dampproof course beneath them.

• Copings usually comprise a single precast concrete unit with a weathered top and a 55 mm throated overhang to help shed water. It is recommended that a dpc is installed beneath the coping. This is particularly important if a coping is to be assembled from sections.
  
• Copings, cappings and flaunchings should be constructed using only FL or FN class clay bricks with BS designation (i) mortar made using sulphate-resisting Portland cement.
  
• Calcium silicate bricks are naturally frost resistant and are free from soluble salts, so their specification is slightly less onerous. For copings, cappings and flaunchings, bricks should be BS class 4 to 7, with BS designation (ii) mortar made using sulphate-resisting Portland cement.
  
• The chimney pot or flue terminal should be bedded in at least three courses of brickwork to ensure that it is well anchored and unlikely to be dislodged by high winds.

Mortar
The most appropriate mortar mixes for chimney construction will depend on the durability classification of the bricks and the risk of the brickwork becoming saturated. It is strongly recommended that sulphate-resisting cement is used because of the presence of soluble sulphates in flue gas condensation. Table 14 in BS 5628 part 3: 2001 describes the most suitable mortar mixes. Flush, bucket handle or weathered mortar joints are all suitable for chimney stacks and terminals but recessed joints should be avoided.

Flashings and dampproof courses
To prevent water that has penetrated the chimney brickwork above the roof from permeating down into the building, a preformed lead dpc tray should be installed. To prevent the risk of corrosion when in contact with the mortar, the lead should be coated with a solvent-based bituminous paint on both sides before installation. The tray should have upstands at the back and sides and a projection at the front to dress over an apron flashing. It should also be taken through a joint in the flue lining and dressed up on the inside of the liner by 25 mm (see detail).

Where a steeply pitched roof has been used or where the chimney is exposed to severe wind-driven rain, an additional dpc should be installed above the lead tray to reduce the amount of water reaching it.

Flashings should be installed around the base of the chimney to prevent water entering the building at the junction between the roof and the chimney stack.

Note: This feature is an abridged version of Brick Information Sheet No 6.9, which is available from the Brick Development Association. For guidance on the design and detailing of fireplaces and flues, reference should be made to other sources of information.