Marianne Suhr: Lime mortar repointing

The project: West End House, a Grade II listed, late Georgian brick house undergoing conservation and repair
This month's challenge: To match and patch the lime mortar repointing.

When Karen and David decided to have their Georgian home repointed they expected it to be fairly straightforward – simply remove the areas of cement pointing that were causing damage to the brick, then employ a craftsman to repoint in a traditional lime mortar. The first bit was easy, and Karen used her family builder Peter to carefully remove the hard cement using a narrow stone chisel and lump hammer. Since other works were happening at the same time, there was a break before the repointing started, providing a chance to look at some sample mortars. And that’s where the trouble started.

Recipe creating
Karen simply couldn’t get a good match with any of the standard ready mixed lime putty mortars. I suggested going back to first principles and analysing the mortar in the wall. We took out a lump and crushed it in a metal bowl, then rinsed it through with dilute hydrochloric acid (otherwise known as patio cleaner). This dissolved the lime and left behind the sand used in the original mortar. After swilling it through with water, we could compare it directly with the other available sands to try to find a match.

Back when materials were transported by horse and cart the sand used would almost certainly have come from close to home, but none of the sands available to us today seemed to be quite right. The problem was exacerbated by the presence of ‘black bits’ – little dark specks in the mortar that gave it a particular characteristic.

The black bits were probably ash from when the original limestone was burned to turn it into lime. Traditionally, coal or charcoal would have been used as the fuel in the kiln, whereas now we use gas kilns that produce a ‘clean’ lime with no residue.

Karen turned her back garden into a mortar laboratory and her garden bench into a sample table in an attempt to solve the problem. By blending various sands in different quantities she was able to change the colour of the mortar, and I helped out by pointing up sample areas in the wall. We tried adding crushed barbecue charcoal to introduce ‘black bits’ but this tended to make the mortar go rather grey. Finally, after calling every lime supplier in the country, Karen tracked down a supplier of ‘black bits’ and we had a match.

Testing times
With the recipe complete, Mike Jones and his son Matt were called in to repoint the wall. It was a hot day so they sprayed the soft brick with water to prevent the mortar from drying out too quickly. They added the ‘black bits’ to the lime putty and sand mix as they went, then pushed the mortar into the joints. After a few hours when the mortar had started to go off, they took off the surface of the pointing and pressed it back into the joint.

The result was a triumph, and it was hard to see where the new mortar had been applied. During the process, Karen had become an expert in sand and black bits, and her bench took a lot of scrubbing before it was safe to sit on again.

ABOVE (clockwise from top left): The brickwork on West End House before work started; Cement pointing is carefully removed, with care taken to avoid damaging the brick; One of Karen’s many mortar samples; Mortar is applied to the cleaned out joints; The joints are pressed back after the lime mortar has gone ‘green hard’.

The eco benefits of lime
If your period house is pointed or rendered with cement, it may be trapping moisture in the wall. A wall with high moisture content is considerably less energy efficient than a dry wall. Trapped dampness is also likely to cause any embedded timbers to rot. Lime mortars and renders reduce the moisture content of a wall by allowing it to breathe. Lime is also more energy efficient than cement as it is burned at a lower temperature. New houses built out of lime are completely recyclable as the bricks can be separated and used again; however, new houses built in cement will end up in landfill sites when they are replaced as none of the bricks are reusable.

10 tips for successful lime-pointing

• Careful repointing on brickwork is a skilled job – don’t attempt it yourself unless you’ve had lots of practice.
• Choose a builder wisely – many don’t have the necessary knowledge.
• Ask your builder to do a small sample panel – then use this as a standard.
• Never use cement with the lime.
• Use a lime putty mortar with a sharp, well-graded sand, or buy it ready mixed from your lime supplier.
• Rake out the old joint carefully and brush out the dust before work starts
• Wet the wall down thoroughly and keep the brickwork damp throughout.
• Don’t attempt lime pointing if there is likely to be a frost within six weeks.
• Use it at the right consistency: not too wet, and not too dry.
• Apply the mortar to the joint; leave it, standing proud, before finishing several hours later by ‘pressing back’.
• Once complete, fix damp hessian to the wall to prevent rapid drying out.

WORDS & PHOTOGRAPHS MARIANNE SUHR
Featured in the August 2010 issue of Period Living