DOCTOR ON THE HOUSE

Most building defects come to light when the cold and wet are involved, so it may come as a surprise to hear that hot weather can bring its share of problems too. These usually centre around thermal expansion, which means that as things get hotter they get bigger, and drying shrinkage, which means that as things get hotter they get smaller. It would be handy if these two were to cancel each other out but, with building as with life, that would be too much to hope for. On the contrary, the two effects can combine to make things worse - such as when a steel hinge expands at the same time as the window frame to which it is screwed is shrinking - these conflicting movements will result in stress and, as the song says, something's gotta give.

So, thermal expansion is most significant for dry things like metals and plastics, and drying shrinkage affects materials containing water - notably timber - but also concrete, mortar and plasters, especially if they are mixed and used in hot weather conditions.

Probably the most common thermal expansion problem involves black uPVC guttering. This stuff expands and contracts more than any other building material, which is why you can hear it creaking and clicking as it swells in the sunshine - and as it shrinks when a cloud passes over. As the temperature rises from 15C to 35C - which has been happening across the country every day for the past week - a 10m length of uPVC guttering will expand by 10mm. And in the cool of the evening it will try to shrink back by the same amount. Note, I say "try"- the plastic support brackets are marked with lines indicating a recommended expansion gap between lengths - unfortunately this is sometimes misunderstood or ignored by the people who put it up, so the first summer's expansion simply pushes the whole length along to the point of least resistance, which can actually move the running outlet - the funnel bit - clean away from the downpipe. You won't notice this, of course, until a sudden thunderstorm strikes, and you've got a roof's worth of rainwater cascading down one point of the wall. Some people don't even seem to notice cataclysmic events like this, until the constant soakings show through on the inside of the bedroom wall. Then they think the house has contracted a mysterious disease called "damp".

Even when guttering is fixed according to the instructions it still may not shrink back into its rightful position, because of silt washed off the roof into the expansion gaps. Interlocking concrete roof tiles are the worst offenders here - they can erode at an alarming rate, accelerated by the mosses and lichens that love to take root on them. So the guttering expands, but it cannot contract; it gets progressively shoved along in its brackets by what is known as the ratchet effect, until, somewhere along the line, something pops out of place, and another "damp" disaster is poised to occur.

The ratchet effect can also be observed on chimney stacks, whose exposed positions mean they bear the brunt of both thermal and moisture movement. Gaps open up between the bricks, and the situation is often made worse by over-enthusiastic pointing up with sand and cement mortar. The result is the stack can only get bigger, and never shrink back again.

So remember, if cracks appear in the summer, it may just be a touch of the sun; wait to see if the change is permanent before reaching for the pointing trowel or the filler tub.