I WAS rummaging through the volcano of waste paper that I laughingly call my desk when I unearthed an old envelope bearing a single word - "eyebrows". It was written in pencil and unmistakably in my own hand. What on earth could it mean?

Clearly I had written the word to remind myself of an idea. When you go on writing courses (what do you mean - "how would I know?") they tell you to write ideas down as soon as you get them, otherwise you might forget. Well, they're absolutely right; I did forget. I wrote it down and found it later and still forgot. Perhaps I got the instructions wrong - it wouldn't be the first time.

So what could it mean, this mysterious message to myself from a younger me? It was only later in the week, when I was tricked into making an ill- advised comeback for the football team, that it came back to me. For if there is any evolutionary explanation at all for eyebrows then it must be that when you are chasing a ball around Hackney Marshes in the company of younger, fitter men, eyebrows stop you from being further disadvantaged by keeping the sweat out of your eyes. And it must have previously occurred to me that the principle of eyebrows was used in former times to protect buildings from the rain. This is what the note was about. I was reminding myself to write about the houses on the other side of my street. Obviously.

My side of the street is mid-Victorian terraces. The other side, redesigned with the assistance of the Luftwaffe, is 1970s maisonettes - quite stylish, but starting to look a bit shabby now, due to the inadequacy of the weathering details. What do I mean by weathering details? This is best explained by comparing the old with the new.

All buildings have rain landing on them. As long as the rain falls vertically there shouldn't be a problem; the roof covering and gutters collect the water and direct it safely away. But rain doesn't always land vertically; quite often it is blown sideways by the wind, so it hits the walls and makes them wet. And since rain is, clearly, something to do with the weather, the deteriorating effects to which it contributes are called weathering. The Victorians knew all about this, and built their houses with regular overhangs and projections to throw the water clear of the walls. The face of a typical Victorian house will have an ornate corbel or coving detail at the top, projecting lintels or arches above the windows (which is where the "eyebrows" idea came from), projecting window sills, and sometimes projecting lines of stucco (called "string" courses). All of these weathering details intercept the rainwater and cause it to drip off, rather than letting it flow further down the walls.

Unfortunately, architects in the 1960s and 1970s decided that all that Victorian ornamentation was decidedly passe, and that houses looked much better with flat fronts. They thought the projecting details were just for show; they didn't realise that they also performed a vital function. So they designed buildings with no projecting details at all - no corbels, no window sills, and definitely no eyebrows. The result is that driven rain cascades down the walls and causes staining and moss growth. This is especially noticeable below windows; the water flowing off the glass wets the brickwork and causes a characteristic staining. Not unlike the sweat stains on my football shirt, now I come to think about it.

'Struck Off - The First Year of Doctor on the House', by Jeff Howell is available from Nosecone Publications, P O Box 24650, London E9 7XQ, price pounds 9 inc postage.