My niece has sent me a joke. If a building is finished, she wants to know, then why is it still called a "building", and not a "built"? Good question, especially if you're in that glorious age of innocence when someone else is still providing your accommodation for you. But in a few short years, Becky will come to realise, like the rest of us, that a building is never finished; no sooner has the last lick of paint been applied, than the forces of nature are upon it, and the whole thing starts falling to pieces.

The speed of the inexorable process of decay depends on two things - the quality of the original construction, and how much effort is put into maintenance. The quality has nothing to do with the "quality assurance" claptrap on the contractor's letterhead; it comes down to materials, chiefly. Big solid lumps of stone tend to last for a long time - witness Stonehenge and the pyramids. Good hardwood timber is also durable, as used in medieval oak-framed houses. Lime mortar and plaster can perform well for a few hundred years, accommodating slight movements as a building settles into place, and slowly hardening back into the limestone from which they were made. All these materials have a long and honourable history, and require skill and understanding to use properly.

Modern building materials, on the other hand, are mostly chosen for the ease with which they can be assembled by unskilled labour, rather than for their long-term performance and durability. So there will be plenty of 1880s houses surviving for another couple of centuries. Few of those built in the 1980s will be keeping them company.

Cement mortars are too brittle to absorb movement without cracking and, funnily enough, neither are they particularly good at withstanding the eroding effects of wind and rain. Gypsum plasters, too, make mountains out of molehills by transforming the slightest movement into a fearsome crack. And the only stone used in building these days is likely to be the block engraved with the architect's name at the entrance to the estate.

Whatever vintage your home, however, regular maintenance can do a lot to slow the rate of decay. Buildings, like cars and horses, need regular servicing to keep them in top condition, and neglected faults can lead to more serious problems later on. This was often not appreciated by the generation pushed into home-buying during the Thatcher years. They were not told that the delights of property ownership were balanced by an obligation to clean out gutters, paint window frames and replace slipped roof slates. Hence the mounting sales appeal of the many "permanent cures" for cold, wet cracks and flaking paint. This quackery rarely results in anything more than a hole in the bank balance and, in many cases, the cure turns out to be worse than the complaint.

Here's hoping that when Becky comes of age and buys her own home, she can still see the funny side of it.