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What is Damp?


Of all building defects, high moisture levels give rise to the largest number of problems that we identify. Water, in places where it is unwanted or uncontrollable will almost always lead to damage of some or many parts of a property. In excess it can cause mould to grow on walls, reduce the strength of timber through rot and decay, cause plasterboard to weaken and decay, damage decorations and in some cases weaken structural components by allowing destructive chemical reactions.

We will therefore look at some of the causes of damp, the issues they cause and the solutions. This is not an exhaustive list and each scenario will be different, therefore if you have any specific concerns, please do give us a call.

What is damp?

This is a difficult concept because essentially it is somewhat subjective. Timber for instance will have moisture in it at all times, it never dries out and is considered stable at around 12% moisture content. In general terms it is therefore normal to expect some moisture content in most building material; timber; plaster; concrete and brick for instance due to their porous nature. Not only do materials contain moisture naturally, their content will vary dependant on the relative moisture content of the air around them. Humidity is a measure of moisture contained in the air and this varies dramatically dependant on the location of the area that is measured, time of day, temperature etc.

Given the varying moisture content of building materials and their propensity to hold water at all times to some degree, the best measurement we have of assessing whether an material is at risk is measured relative to timber. When timber reaches 20% moisture content for prolonged periods of time this will allow fungal decay to begin so we say that any material is damp when it has a moisture content of 20% or more.

This is however misleading as brickwork can be considered wet with much lower relative moisture content and so each case must be taken in isolation.

How do we measure damp?

This is a varied subject with much discussion in the field however most building surveyors will attend site with either one or two types of moisture meter, the first being a resistance meter or conductance meter which will have two metal prongs which check the conductivity between the prongs. Because water conducts electricity, a material that contains water will allow current to pass between the two and dependent on its resistance it will give a reading. These metres are usually calibrated to timber and therefore readings in other materials are often inaccurate. It should be noted that even these meters are calibrated to softwood rather than harder woods such as oak and ash.

Modern damp meters are non-invasive and take moisture readings beneath the surface of the substance being tested but are similar in that they test conductivity. An image of each type is at the head of this blog.

Testing in this way can be misleading particularly if not testing timber softwood as there are a number of materials that are conductive but not necessarily wet. For example, lead-based paints used historically are conductive, and in some instances deposited on the surface of a wall due to a historic leak can also show conductivity but remain relatively dry. Therefore care should be taken at all times when using these metres and a patent should be looked for and correlated against the area being tested.

More accurate means of testing are possible however the equipment is much larger and more specialist and very rarely carried by a normal building surveyor. For example carbide meter will take samples of a substance and run them through a process to check precisely the moisture content.

Where does damp come from?

Rising damp: this is a much debated area however broadly speaking rising damp can be described as moisture from external source, usually the ground, rising up through the porous holes in a building material such as brickwork, stone or block work. This is usually arrested in modern construction with the use of a damp proof course, an impermeable material laid within a wall at low-level meaning that the can remain wet below but will be dry above.

  1. Penetrating damp: this is a series of causes where water is able to enter a property for various reasons such as through cracks, loose fitting tiles, wind pressure, driving rain and generally speaking is one of the more common causes of water staining seen in properties. In other words the external envelope of the building is not acting correctly to keep out water through some form of defect which has arisen or been present since construction.

  2. Condensation: we have looked briefly at humidity, this is the ability of air to hold water vapour. Where warm moist air cold surface such as a window or cold external wall, the water vapour in the air will condense on the surface of that cold material. This is often seen in the winter as misty windows however if left unchecked, condensation will cause a buildup of mould on the face of a wall which can be hazardous to health. When humidity levels are very high within a property, the moisture content of timber such as in doors and skirtings can increase to above 20% and if left at this level will allow rot to occur.

  3. Leaks: this is relatively self-explanatory, where pipework has burst this will cause localised areas moisture buildup and is commonly seen as rotten timber below bath or shower enclosures.

As long as the damp proof course remains intact, rising damp will not normally be a problem however where a damp proof course is damaged or inadequate, water will rise through the wall to the internal space of the property.

How do we cure damp?

We will take each of the four sources of damp mentioned above in turn:

  1. Rising damp: this is usually caused as a result of ineffective or damaged damp proof courses and therefore is usually cured in many cases with the introduction of a new chemical or physical damp proof course. Chemical damp proof courses are undertaken by injecting a fluid into the brickwork at low-level which then percolates through generally and creates a barrier for water ingress. This is not always 100% effective and so is usually undertaken in combination with an internal water proof render to a height of 1100 mm which is generally understood to be the height that water will rise to.

  2. Penetrating damp: this is normally cured by ceiling any susceptible areas. For example re-pointing cracked or missing areas of pointing in existing masonry, sealing the perimeter of Windows with external grade silicone sealant or securely fixing down tiles and increasing the amount that each tile laps over the other. Each case will need to be reviewed in turn to provide a relevant remedy.

  3. Condensation: this is usually improved in four ways:

  4. Leaks: needless to say where a leak occurs it should be repaired. This may be by repairing pipework, mending broken tiles, filling cracks et cetera.

  • Improving insulation on external walls meaning that they will be warmer to the touch and so water will not condense upon them.

  • Increasing the air temperature within a property by heating.

  • Improving the ventilation within a property although care must be taken as too much ventilation can make a property cold.

  • Educating users of a property. For example when using a shower, the bathroom door should always be closed and extractor fan on the window open. Clothes should be dried outside an extraction should be on when cooking. All of these things will reduce the humidity in the air thereby reducing the likelihood of condensation occurring.

Where a property has been badly affected by damp, the building materials will normally need to be replaced after a period of drying out. Timber for instance can sometimes be salvaged if rot has occurred however it is more common to remove all affected timbers and treat the area with fungicide or simply dry it fully and replace in a like-for-like manner with pretreated modern equivalent timber.

The Hopps Partnership will be able to advise you in each instance and if you have any concerns, please do give us a call.

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