Repointed walls aren't all they're cracked up to be
DOCTOR ON THE HOUSE; Builders love replacing old mortar between bricks with new cement. But do it at your peril, warns Jeff Howell
Sunday 11 January 1998
Anyway, repointing is the act of hacking out the old pointing and sticking some new stuff in. It hardly ever needs doing, but architects and surveyors like to say it does. Well, they have to say something to justify their fees, don't they? Pointing rarely needs replacing because it should be a sacrificial material; that is, it is supposed to be softer and more porous than the bricks, and so allow thermal and moisture movement to take place without the bricks themselves being damaged. After many years - maybe 50 or 100 - the pointing will have weathered back and may need some attention, but unless the wear is extreme, or you can see daylight between the bricks, it is usually better left alone.
Conservation pointing is a very skilled job, and the aim is to restore the integrity of a wall but leave it looking as though you haven't touched it. This is too much for some builders, who think repointing should make an 18th-century wall look as though it was put up last week. They like to rake out the soft old lime mortar and force nice thick stripes of hard sand and cement into the joints. The wall will look as if it has been redrawn using a blunt pencil, and will immediately start to suffer problems.
For one thing, the new sand and cement will be too hard, so any movement will result in the bricks cracking or, if you're lucky, in the new pointing being squeezed out. The other problem is that sand and cement is impermeable to moisture, so wetting and drying of the wall has to take place through the bricks themselves; this results in efflorescence (salt crystallisation), moss growth and frost damage. If the wall is in an exposed position, within a few years the faces of the bricks will have blown off, leaving ridges of new pointing standing proud between them. Cement pointing has probably destroyed more Victorian brickwork than the Luftwaffe.
But don't take my word for it; ask the people who repointed Hadrian's Wall. The bit of Hadrian's Wall that you can see today is the bottom half; it survived for 2,000 years because it had been buried by drifting earth. Then it was excavated in the 1920s, and the restorers decided to repoint it with a nice rich mix of sand and cement. Within a few years the stones had started to crumble. So in 1986 English Heritage began a research project to find the most suitable mortar for repairing the wall. It has now found the best one to be a mix of lime, sand and crushed brick, and the worst to be any mortar containing cement.
Oh well, as Hadrian himself might have said, Praestat sero quam nunquam.
You can contact Jeff Howell at the Independent on Sunday or by e-mail: Jeff@doctoronthehouse.demon.co.uk
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