Brick chimneys are at high risk of rainwater penetration and frost damage, this is due to their exposed position on dwellings. Frost can damage both the mortar and bricks themselves if they are not sufficiently frost resistant. The greatest risk is the brickwork at the top of a chimney; once damage has occurred, it is often difficult and costly to rectify due to problems with gaining access to the chimney to carry out repair work.
To significantly reduce the risk of water penetration, avoid saturation and subsequent frost damage; you need careful consideration at the design and material selection stages, along with correct installation on site.
The follow things should be considered when looking at this:
- A coping should have a projection beyond the brickwork together with a drip which stops water tracking back under the projection on to the brickwork.
- Copings which throw rainwater away from the brickwork are the preferred option. Ideally, a coping should be in one unit to avoid having vertical joints through which rainwater will eventually penetrate.
- All copings should be bedded on DPCs
- If a flaunching is selected, it needs to have good tensile strength and durability. A 1:3 cement sharp sand mix is therefore recommended and should be trowelled to form a smooth hard surface.
- Bricks used in a chimney stack should be frost resistant.
- Cappings should be laid in a durable mortar mix M12 designated mix, the same mortar mix should be considered for the whole stack.
- M6 mortar mix may be suitable for brickwork protected by a coping.
- Sulfate-resisting cement should be used in the mortar if smoke billowing engulfing the chimney stack is likely to occur.
- All mortar joints should be fully filled. Where there is a risk of regular wetting of the brickwork, a low sulfate-resistant brick is advised to reduce efflorescence and sulfate attack.
- Pointing should be bucket handle or weather struck. Recessed and projecting joints which could hold water should not be used.