ONE OF the great building myths is that older properties must be periodically "modernised" to bring them up to scratch. Naturally, new technology offers increased comforts, and people want to take advantage: gas, electricity, indoor toilets, telephones, central heating - who would be without them?

But take a closer look and you will see that this list is really only one of building services - that is, things involving wires and pipes which are independent of the building itself. The basic structure and fabric of a building - walls, windows, ceilings - are timeless, and don't need reinventing. I mean, it's not as if we all lived in open boxes for a few thousand years and then someone came up with the idea of roofs, is it?

Actually, roofing is a good example of the modernisation myth at work. Many surveyors think that an old slate or tile roof is substandard if it hasn't been lined with roofing felt, or sarking felt, as it is more accurately known. But sarking felt is a relatively modern innovation. It makes life easy for roofers because it means they can strip off all the old slates or tiles in one go, and cover the exposed roof with this weather-proof wrapping, so it doesn't matter if it rains that afternoon. Before sarking felt came in, roofers had to work more carefully, stripping off and re-covering small sections at a time.

But in terms of the finished roof, sarking felt serves very little purpose, and can actually cause problems. Surveyors will argue that it provides a second line of defence in case of slipped or broken slates, but such occurrences are rare, and should be dealt with by immediate proper repair. In the vast majority of cases, sarking felt, after doing its initial job as a temporary cover, serves no other purpose than to cut down ventilation to the rafters and battens, and act as a convenient surface on which condensation can form. Few of our European neighbours use it.

The worst problems are caused by the latest plastic types of sarking felt, and several readers have reported that their new roofs are dripping internally with condensation. What is happening here is that moisture evaporating from the water tanks in the loft is condensing on the cold under-surface of the plastic. The building materials industry has responded to this problem by introducing a vast array of patented roof ventilation devices, such as ridge vents, eaves vents and anywhere-in-between vents, which add around pounds 5 per square metre to the cost of a new roof. These gadgets are now finding their way into the Building Regulations - the official, Government-sanctioned guidelines - because, it seems, the tile manufacturers, by setting up and financing performance tests, are now dictating the standards. One tile manufacturer, for example, has a wind tunnel far more sophisticated than anything available to the Building Research Establishment, and so its research tends to set the agenda.

Personally, I don't see the need for special roof vents at all. In my house, I have simply cut the sarking felt out from between the rafters, from the inside, and been gratified to feel the gentle draught coming through the slates to air the roof space. Of course, if ever I have to sell, then I will be castigated by the be-suited junior surveyor who tells me that the roof has to be brought up to "modern standards" by re-roofing, including the provision of sarking felt. No problem with that - I've always loved arguing with surveyors.