Marianne Suhr: Repointing with lime mortar

The project: Corner Cottage, an 18th-century rubble stone house undergoing restoration.
This month's challenge: Removing the external modern masonry paint and cement, then repointing using lime mortar.

I met Becky on a course I ran three years ago, and we have since become good friends. Back then, she had just bought a gorgeous little cottage in north Oxfordshire and was looking forward to the challenge of turning it into her dream home.

The previous owner had attempted a whole host of DIY jobs, but left most of them unfinished. So over the past three years, Becky has been sorting out a series of tricky leaks and damp patches, not to mention rewiring, insulating, fixing new gutters and replacing the kitchen.

Perishing walls
One of the last jobs left to tackle was the outside walls. These had been patched with cement and coated in masonry paint and the soft stone underneath was now rejecting this impermeable coating and starting to crumble. But the job was so enormous that even Becky didn’t know where to start. Now engaged to Robin, the couple were really keen to get it sorted before the wedding, and the winter.

It suddenly occurred to me that the Society for the Protection of Ancient Buildings (SPAB) scholars might just relish the experience of removing paint and learning about lime repointing. These are a group of four architects and surveyors that are currently touring the country on a scholarship arranged by SPAB. They are getting hands-on experience of old building repair working with craftsmen and professionals.

Becky’s face lit up at my suggestion and she immediately said, ‘Great! They can all stay here and I’ll cook!’ However, this makeshift working party was lacking one thing – a leader and I knew just the chap, Anthony Goode, friend, colleague and lime mortar guru. Having only recently returned from a working party in Romania, I knew that this would be a project he’d enjoy.

More the merrier
Keen to dust off my own steel-toe-capped boots, I bundled my toddler Max off to the nursery for the day and joined the party. With the scholars around for just 48 hours, there was a lot to do. We started by removing the old masonry paint using ‘scutch’ hammers, tapping at the surface to remove as much as we could.

The wall was so saturated beneath that in some areas the stone and mortar just crumbled. The task was then to repoint the large gaps between the stones with lime mortar. For larger voids, greater than 15mm cubed, the joint was packed with small stones, known as ‘gallets’ or ‘pinnings’. Luckily our hosts provided lashings of tea and biscuits to see us through.

ABOVE (clockwise from left): The team complete the pointing to the gable end which awaits five coats of limewash; SPAB scholar, Meriel, discovers the void at the head of the window – finally the leak is diagnosed; party leader Anthony Goode re-bedding loose stones to the corner of the house.
BELOW (clockwise from left): Getting to those tricky, hard-to-reach places; SPAB scholar, Maya, tackles the corner of the building; Becky and Robin embark on the daunting task of repointing their 18th-century cottage before the winter sets in.

Marianne's Checklist: Repointing rubble stone walls

• Prepare the wall. If removing crumbly lime mortar, an old screwdriver is an ideal tool. For removing hard cement mortar, a plugging chisel works well. Rake out as much loose material as is necessary to get back to sound mortar. Once removed, clear the dust using a dry paintbrush.
• Wet the joint before the work begins with a hose and trigger spray.
• Apply the mortar to a clean, damp joint, working from the top down. Bring the mortar slightly proud of the stone face. A plasterer’s small tool is the ideal implement.
• Wait for the mortar to go ‘green hard’; test this by prodding with a finger and see if it makes an impression – if it doesn’t, the joint is ready to be finished. Be sure to catch the mortar in time; if it has started to go white, you have left it too long.
• Once green hard, scrape the excess mortar off the surface back to a flush finish. This will take the lime rich layer off, leaving the joint ‘open textured’ and able to evaporate more moisture.
• Finish the joint by hitting it with a bristle brush. This will force the mortar to the back and close up any shrinkage cracks.
• Give the joint a fine mist spray and cover with hessian to slow down the drying and minimise cracking. This is particularly important on warm or windy days when evaporation is increased.

Lime mortar recipe

You will need:

• One tub of mature lime putty*
• 3 x 25kg bags of sharp, well-graded sand, fairly dry
• A large board for mixing on
• A shovel
• Wellington boots
• Gloves and goggles

1 Spread out three buckets of sand on the board and spread one bucket of lime putty on top. Use your shovel to mix and ‘chop’ the putty, integrating it fully with the sand.
2 Resist the temptation to add water at this stage – this will make the mix too sloppy. To make light work of the job, stand on the putty and sand and do the ‘twist’, using your feet to knead the mix to a pliable consistency.
3 When the mix is right, you should be able to form it into a ball, throw it and catch it. If the mortar is too crumbly, add a tiny bit of water and keep mixing.
*Alternatively, buy your lime putty mortar ready-mixed from a lime supplier, either by the bag or, for larger jobs, the tonne.

Step 1: After the crumbling old mortar has been raked out, the wall is wetted to slow down the drying process.
Step 2: Lime putty mortar is pushed into the joints and brought proud of the surface.
Step 3: After a few hours, the joint is finished by scraping off the top then beating with a stiff bristle brush.
Step 4: The finished joint.

WORDS & PHOTOGRAPHS MARIANNE SUHR
Featured in the December 2009 issue of Period Living