How structural movement can affect old homes
If you’re buying an older house to renovate, subsidence can be an issue that puts you off the purchase completely. However, the good news is that subsidence and house foundation problems
can be fixed.
Why does subsidence happen?
All buildings are moving all the time, but usually these movements are so small that they’re unnoticeable. They’ll move after they’ve been built or after major structural alterations, as the ground adjusts to the new weight imposed on it. They’ll also move as the ground itself moves. Clay soil, in particular, can shrink, crack and shift during hot, dry weather. Former quarry or pit sites can be unstable, and leaking drains and water mains can wash away or soften soil. All this makes the ground shift, and buildings with it.
Older properties often have shallower foundations than modern ones, so will move more as they adjust to these changes. However, if they’re built of bricks and soft lime mortar, they might also be more flexible, and less likely to be badly damaged by any movement.
If the ground under part of the foundations shrinks significantly enough, a wall can drop suddenly due to lack of support, which is when you will see cracks appear. Other possible signs of subsidence include walls that bulge outwards or lean, sticking windows or doors, and crinkling wallpaper. Don’t fear the worst, however, as not all cracks are caused by subsidence. Those that are will likely be thicker than a 10p coin, diagonal, wider at the top than the bottom, and visible both inside and out.
How to deal with subsidence
How you deal with subsidence depends on what’s causing it, and you may need to consult a structural engineer. You must tell your council’s building control department before doing any structural work.
Major subsidence is often solved by underpinning – strengthening the foundations. Cracks which extend below the damp-proof course (a layer of waterproof material in the wall of a building near the ground, used to prevent rising damp), down to the foundations, may be a sign that this is going to be necessary.
If the subsidence is caused by trees, you can get a specialist to cut them back or cut them down. Don’t do this yourself, as the ground may swell with the water which is no longer being absorbed, causing heave (the opposite of subsidence, where the ground swells up, pushing the foundations upwards). You also can be prosecuted for cutting down a tree protected by a tree preservation order, or in a conservation area, without permission. A solution which allows the tree to remain intact is to install root barriers: digging a trench between the tree and the building and inserting sheets of rigid plastic.
Minor cracks which don't affect the structural integrity of a building can often just be filled in and painted over once the cause has been dealt with. More serious cracks may mean walls need to be repointed and repaired.
If you see what you think may be signs of subsidence, talk to a specialist before acting. Bluelime Home Design have been providing architectural services in Kent
and south London for thirteen years and can offer advice on structural issues. We have offices in Dartford, Bromley, Erith, Bexley
– so call us on 01322 517632 to see how we can help.