Repairs: flues, fireplaces and chimneys

For many years chimneys were designed with a fairly straight flue profile, particularly for provincial properties.

From the Georgian period onwards flues became more convoluted as they twisted their way up the building. This makes repairs and maintenance considerably more difficult.

The modern trend for restricted-draught fires (whether radiant gas fire or wood or solid fuel burning) means that higher flue and gas temperatures are achieved than these fireplaces were designed for, so carefully consider the maintenance and repair strategy.

A sheet metal cap over old chimney pots – no ventilation is apparent

Sulphate attack

Sulphates that were present in the flue gases of coal fires or traditional coal-gas gas fires are deposited on the inside of the chimney. In the presence of moisture (i.e. condensation), the sulphates react with tri-calcium aluminate (present in Portland cement), and the sulphates expand, damaging brickwork and causing mortar to disintegrate.

This type of damage is difficult to see externally, but may cause horizontal cracks on the inner face of the wall (because it is under tension). If different types of brick have been used on the inner and outer faces of the chimney, bowing may occur.

Weathering of chimneys

A more obvious failure of traditional masonry chimneys is when they distort and bend out of plumb due to 'preferential wetting' – that is, under the influence of the prevailing wind and rain the chimney bends towards the south west.

This distortion is exacerbated by sulphation: migration of the brick and mortar joints caused by the presence of sulphates causes the masonry to expand and contract, gradually working its way towards the direction of the prevailing rain. Eventually this can significantly destabilise the chimney, requiring substantial structural repairs or replacement.

How much distortion is too much?

The BRE's Good Building Guide 2 (May 1990) suggested that about 1:100 deviation from plumb for a slender chimney stack would be about right, rather than the usual 'rule of thirds' distortion. However, many chimney stacks on smaller terraced houses are comparatively stumpy and unlikely to show such elevated degrees of distortion.