BUILDING BRIEFING:

Damp

A brief look at damp in UK property

To finish off the series of articles on condensation, Nigel Dann speaks briefly about issues with damp in UK houses.

Rising damp

Damp that originates from the foun­dations of a building is known as 'rising damp'. By definition, this type of damp­ness, caused by capillary action, travels upwards. The classic tell-tale is a hori­zontal 'tidemark' that can extend to about one and a half metres above the ground floor level (and generally no further). Rising damp can also be identified by hygroscopic salts brought up from the ground and deposited on walls around or below the tidemark.

Rising damp generally occurs because there is no damp-proof course (DPC), es­pecially in older properties, or because the DPC has become damaged or defective. A DPC would have been installed in most properties built since about 1900 but the DPC can fail over time.

The DPC is formed in modern properties from a plastic or bitumen strip embedded be­tween the lower courses of masonry. In older properties, slate, and sometimes even lead, was used to the same effect. A DPC can easily be rendered defec­tive or "bridged" by pointing or rendering, or by soil, paving or rubbish heaped against the wall; or internally by plaster. In a cavity wall, the cavity could be bridged by a build-up of mortar droppings.

If investigations reveal that the DPC may be faulty, it will be important to en­sure that the diagnosis is correct (a job for a specialist) as remedial work is often ex­pensive. A faulty DPC constitutes disre­pair and the landlord is responsible for arranging for remedial works.

It is also possible to cure rising damp in an older property where there is no ex­isting DPC. The cheapest and easiest method is to in­ject chemicals into the walls and to re-plaster. However, the landlord is not le­gally obliged to carry out this work under the statutory repairing obligations since this is technically not disrepair since there was no DPC in the first place! In this case, works would tech­nically be deemed to be an improvement and it is important to un­derstand the dif­ference when dealing with disrepair issues and disputes.

Damp meters

Wet materials such as brick, plaster and wood conduct electricity. Measuring a material’s ability to conduct electricity can help determine its moisture content. The most common form of moisture meter is the conductance meter. These have two metal prongs which should be firmly pressed into the material being tested. The electrical resistance between the two probes can then be measured.

Some me­ters have a digital readout on a scale of 1-100, others have coloured lights (green, yellow and red) representing ‘safe’, ‘bor­derline’ and ‘decay inevitable’ situations. The coloured light system is particularly useful when trying to assess dampness in materials other than timber. To measure dampness within a wall, not just at its sur­face, longer probes can be attached to most meters.

Most moisture meters are specifically calibrated for timber. A reading of 18 per cent in a timber skirting means the mois­ture content of the timber is 18 per cent. However, when used on other building materials such as plaster, brick and con­crete, it should be remembered that the percentage reading is relative. So, for ex­ample, a reading of 60 per cent in a plas­tered wall means the wall contains more moisture than another part of the wall with a reading of 40 per cent. It does not mean that the moisture content of the plaster is 60 per cent.

Most gypsum plasters are actually saturated at about 5 per cent or so.

Moisture meter readings cannot al­ways be taken at face value – in the wrong hands they can be misleading. Remember that:

• They do not in themselves prove the existence of rising damp or con­densa­tion but, used correctly, they can con­firm if a material contains mois­ture (at safe or unsafe levels, etc).
• It is patterns of dampness rather than single spot readings that really help diagnosis
• Certain salts present in bricks and plaster conduct electricity and can af­fect meter readings. However if these salts are present in large quantities it does suggest that the material has been wet at some time.
• Metal foil (sometimes found on the back of plasterboard) will give read­ings of 100 per cent.
• High readings may be due to hygro­scopic salts in the plaster. These salts indicate that rising dampness has been a problem at some time - it does not prove it is still occurring.

The height to which the damp rises depends on:

• The porosity of the material – in particular the diameter of the pores.
• The amount of water present in the ground and the height of the water table.
• The thickness of the wall – thinner walls are more likely to have high rates of evaporation.
• The nature of any wall finish, for example waterproof render or impervious paint. Such finishes can drive dampness higher.
• The level of heating inside the building. Steady heating encourages evaporation and limits the height of dampness.
• Chemical contamination – some chemicals can crystallise and partly block the pores.

Penetrating damp

Penetrating damp invariably occurs from the downward penetration of water - generally rain penetration. It is probably the most straightforward to diagnose and where dampness or water is penetrating as the result of disrepair in the structure or exterior of the property (in most cases), the liability will rest with the landlord.

A leaking roof is the most common cause of penetrating damp, but not by any means the only source – dampness may also penetrate walls due to leaking gutters and down pipes, pointing in poor condition or simply because of the loca­tion and ex­posure of a wall to severe weather condi­tions.

Rain penetration shows up as damp patches - usually after heavy rain – on ceilings, on the outside of external walls, or around door or window openings. It can be difficult to pinpoint the exact route the rainwater is taking - the leak may run along floorboards and joists and a damp patch on a ceiling could be the result of a faulty flashing or a missing tile some dis­tance away from the damp patch.

Remedies might involve the repair or replacement of flashings, roof tiles and mortar joints or even the complete re­placement of the roof in order to make the structure watertight. Flat roofs have a par­ticularly poor record for leaks.

Legal obligations

When resolving such disputes, the starting point is to look at each party's legal obligations. Landlords need to be aware of both their statutory repairing obligations and also any repairing obliga­tions provided under the tenancy agree­ment.

In summary, the landlord is legally responsible for keeping the structure, the exterior and the main services (i.e. the heating system, the drains and the services for the supply of water, gas and electricity) in good repair. Therefore, the fundamental issue in attributing liability is to decide whether there is disrepair.

At the same time, the tenant must care for the landlord's property and not allow damage to be caused. So, if a tenant were to block air vents he may well be found failing in his contractual duties to look after the dwelling.

Conversely, if condensation is being caused because a window has been badly painted and as a result will not open, the landlord is at fault for failing to keep the window (which is part of the exterior of the property) in good repair.

The same would be the case if the condensation has been occurring for such a period of time that it has damaged electric wiring, or any part of the structure that is the landlord's responsibility to repair.

An example of this situation occurred in a court case in 1992. The Court of Ap­peal held that the landlord was liable to replace damaged plaster in a flat that suf­fered from condensation. This was be­cause the plaster (unlike wallpaper) was held to be a part of the "structure" of the building

More information

For more information, please refer to the chapter on ‘Damp’ in House Inspector by Nigel Dann and Duncan Marshall.