Repairs: flues, fireplaces and chimneys

Distortion – including that caused by sulphation – can be so severe that the chimney will need to be carefully taken down and rebuilt.

If the building is listed or in a conservation area, not only will listed building permission be required but careful reconstruction on a like-for-like basis will be essential.

Many older chimneys do not incorporate any damp-proof material at all (such as a tray or adequate flashings). If they do then often the detailing is inadequate or semi-functional. This could easily be improved upon by incorporating a proper sheet lead tray through the complete thickness of the stack together with properly fixed in flashings.

• Even on repair works this is frequently omitted, and contractors effectively replace the defective masonry like-for-like, missing the perfect opportunity to improve performance to moisture resistance.
• While the chimney is down, you could take the opportunity to improve the flue lining as well (see below).

Where repairs will suffice, a number of options are available.

Solution 1

Frequently the chimney fissures vertically as the masonry separating the flues expands, partly under the influence of contaminative sulphates.

If the fissuring is comparatively slight it may be possible to stitch back the masonry either by:

• using twisted helical bars (see Walls, problem 7); or
• putting a belt around the chimney at intervals of courses.

The latter involves winding stainless steel or copper wire through the mortar courses that have been cut back and are then face pointed. This effectively puts a simple truss or corset around the chimney to restrain future movement.

When carrying out repointing repairs, ensure that moisture entrapment is not made worse – the use of rich modern Portland cement mortars in older chimneys that had lime mortar joints would not be appropriate; a breathable lime mortar would be the ideal option.

Where it is possible to use a Portland cement mortar, use a sulphate-resistant specification rather than a conventional Portland cement.

Where frost damage and erosion has affected the brick, possibly coupled with chemical attack, given care, it should be possible to carefully cut out and let in new bricks.

If the degree of cracking through the chimney is too bad it may be possible to carefully let in new bricks, stitching across the crack, although some of the mortar joints at the perpends will necessarily be rather thicker than in the original work.

It may be tempting to render the chimney, but consider issues of sulphate-resistant mortars.

Where render is used it is important to use properly gauged coats in accordance with the British Standards (see Walls, problem 15). If there is any risk of the chimney cracking or much moisture penetrating (in a severely exposed position for example) rendering may have the effect of trapping more moisture within the masonry. Hairline cracks would soon develop under exposed conditions, and after a couple of years the chimney would be likely to be in a very sorry state.