A chimney is the most exposed element of any building, so it's important to design and build it to last. We explain what has to be considered
Selection of bricks and joints
Clay or calcium silicate bricks can be used in the construction of a chimney. If clay bricks are chosen, their resilience will depend on three interrelated factors: the durability designation of the brick, the detailing of the chimney terminal and the mortar.

  • 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.

BDA Publications

The Brick Development Association publishes a comprehensive set of technical literature on its web site – www.brick.org.uk – and also in its brochure Brick Publications. This information is split into two major categories, brickwork design and detailing and structural brickwork. Data sheets have been produced to answer questions frequently asked on the BDA’s technical enquiry line. Currently, eight sheets are available which deal with the following topics:
  • Verticality of perpends: advice on specification and site inspection.
  • Recessed joints: summary of advice on the specification of joints.
  • Colour consistency: notes on the causes of patchiness and colour banding in brickwork.
  • Frog up; frog down: observations on an old dilemma.
  • Water repellent surface treatments: current BDA advice.
  • Repointing of brickwork.
  • Bricklaying in winter conditions.
  • Tinting: notes on the theory and practice of colouring brickwork.

In addition to the above, five information sheets are about to be published: Specification of Brick Retaining Walls. This is a summary of the three types of retaining wall, with notes on detailing and construction. Movement in Brickwork. This offers guidance on the many factors that should be considered in clay brickwork design. Different types of movement, movement joints and the particular problems of short returns are all considered. Specification of Brickwork in Chimneys. This document is summarised in the main feature. Thermal Insulation of Buildings. These are notes on the location and specification of insulation for solid and cavity walls. This will include a number of points on the site inspection of cavity installation. Prefabricated Brick Assemblies. A review of the range of preassembled brickwork details. It is anticipated that this will be a growth area in the future.

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