Jonathan Hetreed, author of The Damp House, explains how to recognise and deal with damp in your home

The causes of dampness in houses are often difficult to diagnose, because problems are hidden, unlike symptoms, or because several causes act simultaneously. Once cured, a damp problem may take more than a year to dry out, so making good to finishes needs to allow for this.

Dampness in houses

Rising Damp

The classic form of ‘damp’ consists of moisture from the ground rising in porous wall construction by means of capillary action. Typical signs are the 'tide marks' of damp staining, often reaching to around a metre above local external ground levels.The only sure solution is to insert a physical damp course in walls above ground level, which tends to be disruptive and costly.

Injected damp courses guaranteed by specialist contractors can be effective in consistent walling materials; electronic anti-capilliary and evaporative enhancement installations are also marketed with guarantees; external land drain installations to lower moisture levels in the ground can be equally effective in some circumstances.

Penetrating Damp

Moisture penetrates under wind or gravity. Each part of the external skin of a house, roofs, walls, windows, doors, chimneys, etc – has its components: damaging or displacement can allow water penetration, though complex modern construction with its multiple layers, means the symptoms may appear in unexpected places.

The signs may be similar to those of rising damp but relate more to local sources:  a sheltered wall may appear dry whereas one exposed to driving rain may be seriously affected, or a damp patch worst at the top of a wall from a leaking gutter.

The key to success in preventing penetrating damp is correct diagnosis in the right weather conditions over time: once identified, many of the problems are simply resolved by repairs, though several contributory causes can make remedies slower and more complex, and sometimes the side effects of long term damp can require more repair than the damp problem itself.

Construction Moisture

For a typical masonry house,  8000 litres of water are embodied in its construction.. Although half will evaporate by completion, drying out will continue for 6 months or more if delayed by damp weather, or poor construction management. The same process happens with extensions, improvements and alterations.

For drying out, a common rule of thumb is a month of 'good drying conditions'  for every 25mm (1") of 'wet construction' such as concrete.  Symptoms of damp may well appear as serious faults and damage decorations but should be short-lived.

The clues to dampness from construction are its association with new (wet) work and it's diminishing over time. Avoiding or reducing the problems is a matter of good construction management and correct timing.

Condensation

Condensation occurs when warm moisture-laden air (water vapour) meets a cold surface such as the inside of a window in a steamy shower room. It sometimes occurs as 'interstitial condensation' between the layers of a buildings structure: invisible but potentially problematic. In winter with houses least ventilated and  uninsulated walls and windows at their coldest, it is particularly affected by people's management of their homes, for example in ventilation and control of moisture sources such as cooking and washing.

Other signs of condensation are often grey or black spotted mould growth on walls and ceilings, particularly in corners or recesses, and colder areas of construction around windows and doors or below uninsulated roofs.

Solutions to condensation problems, in sequence:

• Reduce the amount of moisture vapour.

• Extract or vent residual vapour near its source.

• Improve insulation of surfaces affected.

• Improve surface absorption.

• Improve level and distribution of heating.

• Improve level and distribution of ventilation.

Leaking Service and Appliances

Improved convenience in houses included bringing water supply and  drainage inside the 'building envelope', bringing the risk of failure in these 'services' –leaking pipes or tanks – causing dampness. Leaking external pipes and drains, above or below ground, can cause damp problems by supplying the water for penetrating or rising damp.

‘Wet appliances’ – principally washing machines and dish washers – have a much shorter life than piped services; most of our 'sanitary ware' – baths, basins, sinks, showers and lavatories – is reliable and long lasting but small leaks can go undetected, especially from shower cubicles and trays and this can lead to cumulative damp problems, often hidden initially in floor structures.

Once correctly diagnosed, these leaks are often simply cured, though dampness may prove persistent long after its source is cut off; for underground services, specialist drain surveys, leak detection and repair systems may be required.

Remedial finishes after damp problems need to be specified for long term damp tolerance and permeability to allow for gradual drying.

Illustration: Jonathan Hetreed