For anyone who cares about historic buildings, one of the most depressing sights these days is the proliferation of mega-skips outside period houses, sometimes overflowing with original fittings which their new owners have casually chosen to discard.

They may invest £1m (€1.27m) or more in a sought-after Georgian, Victorian or Edwardian house, only to set about wrecking much of what made it attractive in the first place. More often than not, such travesties are the result of ignorance, stupidity or downright bad advice.

There's a lot of ignorance about, says Frank Keohane, who compiled and edited Period Houses: A Conservation Guidance Manual, which has just been published by the Dublin Civic Trust. So many people are acquiring very valuable properties and yet they know so little about them.

"They're buying period houses, which are marketed as such, and the first thing they want to do is `de-periodise' them. I know attitudes are changing as people become more aware of the significance of grand houses in Merrion Square or artisan dwellings, but so few are left intact."

Even quite simple interventions can cause "an awful lot of damage", such as the removal and substitution of joinery, doors and windows or the re-pointing of brickwork with cement rather than lime mortar. And in most cases, he insists, these works are "completely unnecessary".

The 148-page manual, copiously illustrated with examples of what to do - and what not to do - is designed to dispel ignorance among the owners of historic buildings and even, it must be said, among architects, structural engineers and other professionals as well as building contractors and tradesmen.

It aims to provide sound guidance on the principles of repair and conservation of period houses, distilling the current wisdom in this important area from "best practice" in Britain and other European countries. And though it concentrates on Dublin, its advice could be applied anywhere in Ireland.

It's the sort of advice that has kept buildings going for two or three hundred years, Frank Keohane says. As a result, the manual is quite cautious about including some technical innovations which have not yet been proven. Neither does it, nor could it, set out to be categorically prescriptive in individual cases.

One of its most important messages is that older buildings breathe. In winter, they take in moisture and in summer let it out again. "It's a natural cycle", as he explains. The use of cement mortars or renders which are very dense hinders that breathing process and can lead to damp being trapped within the building.

Dampness is, of course, a perennial concern among the owners and occupiers of buildings in Ireland - not just because it is unhealthy in itself, evoking grim associations with poverty and tubercolosis - but also because it often results in the decay of materials in the building by feeding such horrors as wet or dry rot.

The manual emphasises that owners should recognise that their buildings "breathe" and should be allowed to continue doing so. That's why soft, porous lime mortar was commonly used in pointing brickwork so that water would evaporate through the mortar rather than through the brickwork or masonry.

"A lot of the methods used for dampproofing don't actually get rid of moisture", according to Mr Keohane, who is studying for a masters degree in urban and building conservation at UCD. "To treat rising damp, it might be much more useful to improve drainage around a building than to insert a full damp-proof course."

Timber decay is also a major issue - the dreaded dry rot. But it's only a plant and if you don't water a plant, it dies. Dry rot doesn't grow in dry conditions, but in wet conditions. So, if you remove the moisture, you can kill off dry rot and minimise the removal of timber as well as the use of harmful chemicals and sprays.

"Other issues relate to the cleaning of brick or stone. This is often done quite poorly, using abrasive materials which can remove the outer surface of brick and damage the tooling and decorative details of stonework. Someone may come up with a magical solution, but until then we're better off to take a `play safe' approach."

The manual adopts an informed, organic approach based on "minimal intervention". It seeks to inform those who have bought period houses about what they have taken on board and also to persuade them to think twice before they embark on ill-advised "improvements", such as junking original windows for hideous PVC.

Before doing anything, they should first try to understand the building and how it works. "Good advice is invaluable because in most cases it will save you money," says Mr Keohane. "But you need to get it from architects or structural engineers who really know about the conservation of historic buildings.

"There are a lot of good conservation architects and engineers out there who can provide good advice. They understand that a period house copes with structural and thermal movement much better than modern buildings because it's not a concrete straitjacket."

BUT many people in Ireland seem to have a problem about paying for advice, he says. "Some may have invested £1.5 million in a desirable period house and may be prepared to spend another few hundred grand on renovations. Yet if they sought advice, they might find that they don't need to carry out the works at all."

Frank Keohane says he is not such a purist that he would be opposed to the installation of fitted kitchens or conservatories. "But using resins, cement and chemicals in the `restoration' of historic buildings is not a good idea." It also pains him to see money spent badly, particularly where the work will have to be reversed.

Putting the manual together was a daunting task, but he managed to do it with help from some of the leading practitioners in conservation, such as architects Paul Arnold and James Howley, structural engineer Chris Southgate, Colm Murray of Duchas the Heritage Service, and Ian Lumley, heritage officer of An Taisce.

Mr Keohane also `served his time' recording historic buildings for the Historic Heart of Dublin, a successful joint venture between the Civic Trust and Dublin Corporation. The manual he has produced is not laden with technical detail and should therefore be intelligible even to the most misguided owners of period houses.

It marks a step forward from the leaflets on building conservation published by the Department of the Environment some years ago by relating its advice to particular house types. And it will be revised and updated in later editions to reflect changes in thinking about what constitutes "best practice" in conservation.

For those who purchase period houses with the best of intentions, this conservation guidance manual will prove invaluable; those less benignly inclined would be better off building bungalows. Copies of the manual, at a modest £20, are available from the Dublin Civic Trust, 4 Castle Street, Dublin 2, telephone (01) 475 6911.