Hetreed The Damp House

A Guide to the Causes and Treatment of Dampness
1. Auflage 2012
ISBN: 978-1-84797-403-7
Verlag: The Crowood Press
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 144 Seiten

Dampness can become an enemy when it invades our interior space, when it persists, when it spreads and damages our domestic fittings, finishes and furnishings, when it encourages the growth of moulds that threaten our health, and when it stimulates the spread of fungus and decay in the structure of our houses. A major industry has developed with specialist contractors that sometimes concentrate on a single treatment rather than offering a comprehensive diagnosis. This invaluable book provides clear information and an overview of the types and causes of dampness plus an indication of the range of treatments and remedies that are available. Superbly illustrated with approximately 150 colour drawings and photographs, The Damp House will help readers to understand and solve simple problems themselves and to evaluate solutions offered to them by professionals. Topics include: rising damp, penetrating damp, condensation and leaking services in a wide range of different type of houses. Examines damp in all parts of the house, from the roof to the walls, ceilings and floors Considers remedying the effects of damp and living with damp.

Autoren/Hrsg.

Hetreed, Jonathan

Leseproben

CHAPTER 2

Rising Damp

HOW IT OCCURS

There are three preconditions for rising damp: ground contact, ground moisture and porous construction.

Ground Contact

By definition, therefore, walls and ground floors are vulnerable. The DPCs and DPMs of modern construction work by breaking that ground contact. At a damp site, such defences have to be thorough and consistent: rising damp is remarkably successful in showing up poor construction.

Ground Moisture

Ground moisture is not so simple, since it varies markedly both in time and place. A single house can show completely different conditions; for example, between a back wall that is built into a hill and a front wall that is built up above lower ground – if the water-table (the level of ground moisture) follows the profile of the hill (often but not always so), then the back wall can be saturated while the front wall is entirely dry. The water-table tends to fall in summer and rise in winter, due to varying rainfall and to evaporation rates and transpiration from trees and plants; this can lead in borderline cases to purely seasonal damp problems, though there may be a gradual worsening of the symptoms due to the build up of mineral salts (see below).

Porous Construction

Masonry materials, brick and stone, and to a lesser extent concrete, are the best-known sufferers from rising damp, but any porous construction material, for example, timber, plaster or earth – usually found as cob, clay lump or similar in traditional houses and as rammed earth or unfired clay blocks in modern construction – may be a victim in the wrong circumstances.

Fig 17 Preconditions for rising damp.

Fig 18 Rising damp can vary within one house.

Conversely, non-porous building materials, steel, glass, very dense bricks (for example, engineering bricks) and a few impervious stones such as slate, do not support rising damp and, in the case of the dense bricks and slates, are used as damp-proof construction materials.

The relevant difference between these two groups of materials is in their pore structure: the damp-prone materials have an open-pore structure, which transmits moisture by capillary action. The extent to which this happens depends on four main factors:

1. The pore structure of the construction material.
2. The degree of saturation of the ground.
3. The rate of evaporation.
4. The concentration of salts in the material’s surfaces.

Although rising damp in walls typically reaches to around a metre above adjacent ground levels, local conditions produce wide variations. Generally, the smaller the pore size, the higher the damp rises; pores as small as a thousandth of a millimetre are not uncommon in traditional materials, including bricks. Local variations in the water-table with high ground close to a house, even though not actually touching it, can cause higher damp levels.

The effect of evaporation is again most pronounced in fine-pored material; evaporation rates increase with temperature – so in summer, and with heating, this in turn stimulates the rising damp, though its effects may be less visible because of the faster evaporation from the surface. Reduced symptoms in summer due to increased evaporation are reinforced by falling water-tables.

As moisture rises from the ground, soluble mineral salts are carried up through the wall or floor, along with further minerals dissolved from the construction materials themselves. As the moisture evaporates from the surface, it leaves the mineral salts behind in gradually increasing concentrations, which crystallize out and gradually block the pores, so encouraging following moisture to rise higher to ‘achieve evaporation’(see Fig 6).

Some mineral salts are hygroscopic, attracting atmospheric moisture; as these concentrate at surfaces, they collect dampness from the atmosphere, so that in humid conditions, walls and floors can feel clammy to the touch even though rising damp may not be present; for example, in the summer. This effect can easily be confused with condensation appearing in similar weather on cold, internal masonry (see Chapter 4).

Although surface accumulation of mineral salts is a classic symptom of rising damp, it can occur as a result of any form of dampness, since it is merely a product of evaporation of moisture – how the moisture gets into the material does not dictate how it gets out. Flooding can temporarily saturate the base of walls and floors – regardless of DPCs and DPMs – and then take months or even years to dry out. Persistent penetrating damp can, by force of gravity, accumulate moisture at the base of a wall and in floors, which in the process of evaporation can behave just like rising damp, bringing the salts to the surface.

WHEN DOES IT OCCUR?

Failure

It is unusual, but possible, for DPCs and DPMs to fail – particularly early, ‘flexible’ DPCs of hessianreinforced bitumen can become compressed and embrittled with age, so that they cease to be flexible and the hessian may simply disintegrate. The combination of ageing DPC material and even minor settlement in foundations can be enough to crack DPCs. Movement will also effect rigid damp-proof courses of slate, tile or brick, and may cause cracking of the mortar or of the material itself.

Similarly, movement in floors can produce corresponding cracks in liquid-applied membranes. Although high-grade asphalt does have considerable flexibility, and can heal minor movement cracks, many house floors, particularly in the 1940s and 1950s, were damp-proofed with thin, weak bituminous screeds of very limited effectiveness. Some modern liquid-applied membranes cure to form a tough, flexible sheet material with greater capacity for elongation than polythene, while other epoxy materials bond sufficiently to the floor to resist water pressure, as well as being strong enough for the direct application of floor finishes.

While an isolated crack in masonry, which also causes a local DPC failure, can often be simply repaired, widespread DPC failure through ageing warrants a replacement strategy.

Alteration

Many ‘DPC failures’ are simply due to careless or ill thought-out alterations to the fabric of a house that overcome the DPC by applying porous material ‘bridging’ from beneath the DPC to above it, so allowing moisture to by-pass the DPC and rise in the wall or floor above. The commonest examples would be in raised ground levels, external rendering, internal plastering or flooring (see Fig 16); once an error like this is diagnosed, it can be a relatively simple process to remove the ‘bridging’ and return the DPC to its role as an effective barrier, without by-passes.

Forming a new opening in a wall, though not in itself DPC bridging, often involves rebuilding to form masonry reveals at each side of the opening to support new lintels: where an existing horizontal DPC has been cut away, it needs to be carefully reinstated in the new masonry and lapped or bonded with the existing DPC.

Fig 19 Reinstating DPCs in alteration work.

Fig 20 DPC for protection to an opening in a cavity wall.

Similarly, in cavity wall construction, a new opening’s reveals connect the inner and outer leaves of the wall, which can allow both rising and penetrating damp to cross into the inner leaf, so vertical DPCs are needed. The same damp transfer would happen at the head of the opening – gravity-assisted, which is where the cavity tray is inserted to direct any water in the cavity to the outer leaf, usually above the lintel where it can drain out through ‘weepholes’, and at the sill to protect the wall below. All three types of DPC need to be carefully connected and lapped to shed water outwards. Wall ties are another even more widespread potential route for damp across cavities, which cannot be protected by DPCs: instead they are made with a central twist or ‘drip’ that discourages water from crossing from outer leaf to inner.

Cavity walls are a tried and tested means of keeping buildings dry but they rely on careful and consistent workmanship: without this, they can be highly vulnerable to damp. The sensible principle in minor alterations is to examine how the house was built and – providing it was built successfully – follow the same details in the alteration work – though the Building Inspector may not always agree! There is no virtue in altering a traditionally built solid wall as though it was a cavity wall, since DPCs will have a purely local effect (but see Chapter 3 for measures appropriate against penetrating damp).

Careless Construction

Amongst other problems, careless construction can lead to such a build-up of mortar droppings at the base of a cavity wall that ‘hidden bridges’ are formed, which allow moisture to reach the inner leaf of the wall: unless the poor workmanship has been unusually consistent, this fault will tend to produce patchy or intermittent areas of rising damp along a wall.

Similar effects can be produced by poorly fitted DPCs, inadequately jointed or fitted slightly out of line with a wall, so that finishes or pointing bridge over them, or simply not connected or sealed to the floor DPM. Where DPCs appear to be visibly intact and unbridged, the appearance of rising damp can be caused by condensation at...