TLC Stove and Liners gives advice on replacing old chimneys
Old chimneys in homes carry a risk when it comes to replacement. This is due to the age of original building work. It can also be down to any related damage in the infrastructure, presenting a danger to the householder. Fortunately remedial work can happen, with secure and safe restoration work possible as long as the appropriate tests and surveys are done by a qualified tradesman.
Home owners may want to bring back some ‘life’ back into their house via a cosy, warm fire. The former chimney could have been blocked off and so using it again provides a wonderful spectacle, which is more interesting than a bland, sterile radiator! Even so, the chimney could have been over-used in former days, or not cleaned, or there can be numerous brickwork problems – the list of possible issues is endless. That is why the chimney may need modern adaptions to make it user friendly and to follow building regulations.
Tests required by law are:
- Firsthand visual inspection
- Chimney check via sweeping
- Core ball testing
- Air-tightness testing
- Smoke spill and evacuation testing
What does repairing a chimney involve?
There could be a number of structural issues, which need addressing after a survey by a qualified tradesman. The entire lining may need to be replaced inside the flue or the chimney pots at the top reworked into a firmer position. That sounds simple, but the process must be done correctly and in detailed order to eliminate risks such as carbon monoxide poisoning poisoning. Householders should employ the services of a seasoned and qualified chimney technician, who utilises industry standard safe roof-access working platforms and methods.
This is a compicated process and a look at the facts will show why a proper chimney technician is needed for such work. When external mortar on a chimney stack has reduced by a third of an inch (10mm) or is cracked, loose or crumbling – then repointing is needed. The process is strict to allow for a stable end result. Any damaged mortar needs to be raked out first until stronger, sound material is found. This should create a square profile. Next, if the remainder mortar is strong below the surface, then a 25mm depth is enough. Any cracks or crumbling though require a 35 mm depth.
It makes no sense rushing the raking out and repointing. The work must be finished one face at a time. A couple of days need to be left each time before working on the next face. The recommendation for mortar mix is a four-and-a-half parts of sand with half a part hydrated lime and one part sulphate-resisting cement. An air-entraining additive is advised to block frost attack. Certain chemical materials can support existing mortar, which is weak, to help it stick with the existing bricks and mortar. A common favourite is the compounds, which are styrene-acrylic based. Note: repointing guns must contain the exact mix needed for chimney stacks.
The last part of the repointing process is to clean the masonry and damp it down. Any new mortar used has to be pressed firmly into joints to prevent air traps. A ‘bucket-handle’ profile should finish the job to prevent rain spoiling the work.
To re-line any existing flues, be aware that there are a number of methods. It depends on your specific chimney set-up. Ask your chimney sweep for specific advice.