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Damp course

technical & practice - A new BRE book explores all aspects of damp in buildings, examines their causes and provides remedial suggestions

In an important new book, just released by the Building Research Establishment (BRE), the authors Trotman, Sanders and Harrison - an engineer, a physicist and an architect respectively - explore the causes and remediation of dampness in buildings.

Pooling their extensive experience of waterproofing, meteorology and construction, their approach is thorough and wide-ranging. The chapters break down the subject into manageable chunks pitched at just the right level for the expert and novice to appreciate and learn. They cover topics such as condensation, rising damp, driving rain; what they call 'the big three (causes)'. From the general, they delve into the specifics of contaminating salts, hydroscopic materials and bad detailing among many, many other headings.

In a new brick-block semidetached house, the authors tell us, there can often be about 8,000 litres of water used for mixing, which can take a long time to dry out. For example, 'a 150mm-thick floor slab may take about a year', but the fact that it is workable after just a couple of weeks may make people blasÚ to the hidden problem lurking within.

Soggy statistics Basing much of their data on the 1996 English House Condition Survey and the 1996 Scottish House Condition Survey, the former indicates that 3.9 per cent of dwellings were affected by rising damp and 6 per cent by penetrating dampness.

As shown in the diagram, these statistics - the incidence of damp penetration generally - have been greatly reduced during the inter-war period and reduced by a further 90 per cent since the end of the war. After 1980, rising damp, it seems, has all but been eliminated in housing stock.

However, since we currently have one of the slowest periods of house building in modern times, with only 12 per cent of the building stock being replaced in any one year, the problems arising need to be addressed in a more consolidated manner than is currently the case. Figures for Wales, and the extensive 'council housing' sector in Northern Ireland, were not available at the time of writing this report, but suffice to say they will not be any better.

Writing about the effects of moisture 'in the wrong place', they have compiled a list of location, timing (in relation to weather) and possible cause of dampness. This is a quite frivolous and yet useful table for use as a rule of thumb guide. For example, by reading across the tabulated columns, we can see that a single patch on a given surface, appearing regardless of weather conditions, is probably due to a plumbing leak.

Over the page, they list the possible visible or hidden effects of dampness and other substances that might be involved in addition to water: the blistering and flaking of paint is probably due to the pressure of vapour and the crystallisation of soluble salts.

Even though these throwaway lists are not extensive, they are handy survey aids, especially as each item is crossreferenced to a section of the text for a fuller explanation, with photographs and diagrams.

Damp underfoot The section on flooring provides guidance on bad detailing and corrective treatment. Whether addressing 'linoleum rot' and the confusing regulatory guidance on underfloor ventilation, or 'nail sickness', 'wall tie corrosion' or any other ailments afflicting the building, the authors manage to write clearly, authoritatively and helpfully. On mould, we learn that 'the most common? respiratory allergic reactions (are) to mould spores and the faecal capsules to the dust mites that are associated with moulds'.

With reference to condensation, the authors explore the nature and the behaviour of water vapour in the air in given conditions. It is all reasonably simple stuff that, in some places, is still taught as the basics of a good architectural education, but is still one of the first things to be forgotten when architects sit down to design an actual building. The fact that moulds can grow in relative humidity of just 80 per cent means, they say, that 'while air at point O (a reference point) has to be cooled to 12¦C before condensation (starts), it has to be cooled only to 15.5¦C before moulds will grow. This has important implications for detailing insulation to avoid thermal bridges that are common sites of mould.' The chapter includes information on reverse condensation, the location of VCLs (vapour control layers) and that in the 14 million houses in England with pitched roofs, 'about one in 50 showed signs of condensation in the roof space'.

The case studies are very useful examples of worst practice, with handy hints about how to avoid making the same mistakes. Details on pointing, flashing, overhangs, drips, cavity trays, etc, are all the stuff of Mitchell's, but still bear repeating in this new format, with the added clarity of explaining why things fail. I only noticed one shortfall: when examining the interminable problem of detailing a weathertight threshold, the authors explain the principles but bow out with the passing remark that 'this detail is now unacceptable in new buildings owing to access problems for the disabled'. An explanation of how to do both would have been extremely useful.

That one gripe aside, for all practitioners involved in remedial work on old stock housing, this book should be indispensable but it should not be confined to the housing sector. Anyone who wants to learn from the mistakes of the past in construction practice and thus gain an understanding of the principles of technical design and detailing will benefit from the experience of these authors. For a 216-page book about damp, of all things, this was a remarkably concise and enjoyable read.

Peter Trotman, Chris Sanders and Harry Harrison's Understanding Dampness is available from IHS Rapidoc (BRE Bookshop). Telephone 01344 404407 or email BREBookshop@IHS Rapidoc. com. Readers of The Architects' Journal should quote this article when ordering to receive the book for £37.95, free of postage (saving £6).

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