DOCTOR ON THE HOUSE

It's only new houses that crack up when the earth moves, writes Jeff Howell

Buildings move. They don't move around town or go to discos or anything like that, but they do expand and contract in response to heat, moisture and vibration. And when you apply loads to them, like going inside, they sink a bit, and when you come out they pop back up again. Amazing but true. Even the weight of a sparrow landing on a steel bridge causes movement; difficult to measure, but still there.

Subsidence is the house movement that attracts most publicity, and most of it is nonsense. Subsidence first hit the headlines when, due to a little-noticed change in the regulations, it became the subject of buildings insurance claims after the dry summer of 1976. As usual when the insurance industry gets involved, it didn't take long for the sound of sledgehammers cracking nuts to be heard - in this case with an accompanying chorus of chainsaws. Trees, it was decided, were the problem because they suck water out of the clay and cause shrinkage and subsidence, so they should not be allowed to grow within 30m of buildings.

This will surprise the regulars at my favourite pub, the Low House in Laxfield, Suffolk. The pub has been there for 600 years and, for the last two centuries at least, a huge sycamore has been trying to get into the tap room. The trunk is now two metres wide, and within a whisker of achieving its objective, but there is no sign of cracks in the building.

The problem with modern buildings is that they are unable to tolerate this sort of movement or, come to that, any sort of movement. They are built using techniques and materials that work fine so long as nobody slams the door. Take any 1990s house, lower a corner by 10mm, and cracks will appear: in the face brickwork; in the plasterboard ceilings; around the window frames; across the internal plaster. A house built in the 1890s rides that sort of shift without a murmur. In Victorian houses that have suffered even ten times that movement - where the door openings have become trapezoidal, and the pianola has to be tethered to stop it rolling across the parlour - there is not a crack to be seen. These buildings remain flexible. They are made from bricks bedded in lime mortar; they have lime and horsehair plaster on the walls; the window and door frames are wedged in generous rebates in the brickwork; and the timbers are chunky, close-grained heartwood, still full of preserving pine resin.

Anyway. Every dry summer since 1976 has seen a growing rash of insurance claims for subsidence and remedial underpinning. The building industry responded with its customary free-market flexibility, and hundreds of specialist underpinning firms appeared in the Yellow Pages. These gave the insurance industry what it was looking for - three estimates - and the 1980s underpinning phenomenon was born. Every time a mortgage valuation surveyor south of a line from the Humber to the Severn spotted a hairline crack, he recommended a structural engineer's report, and most of these specified underpinning.

For a few years things went a bit crazy. I've seen back additions underpinned; I've seen one half of a pair of semis underpinned; I have even - note this - seen a terraced house, in the middle of a row of five, underpinned. Still, I shouldn't grumble; some of us got a few big drinks for keeping quiet about that one.

JEFF HOWELL IS A BRICKLAYER