Yet Another Damp House!

Last week I had to take a quick look at a small farmer’s cottage, and was doing quite well with the survey until I looked at a small bedroom, its position backed onto a stable block, which ran parallel with the house.  My damp meter soared into the red as soon as I inserted it into the plaster for a height of about 45cm above ground level, indicating the presence of good old fashioned rising damp.  It was this eureka moment which triggered this weeks’ article.  A sad life I lead, but hopefully it should be of interest to you house owners.

When I studied Biochemistry at University, I was always told water is the most complicated substance present on this planet.  To most of you, it just looks like a clear transparent liquid, but that view belittles its composition. It is in fact made up primarily of hydrogen, oxygen, hydroxonium irons, and a range of elements or salts picked up from its’ surroundings, such as soil, sub-soil and the environment as a whole. Its pH, which is a measure of how acidic or alkaline it may be (7 is neutral) can vary from being between 1, which is highly acidic to 14 which is highly alkaline.  The scale is logarithmic, in that a movement of 1 on the acidity scale will lead to a 10 fold change in acidity/alkalinity. You often hear people talk about acidic rain; when this occurs the water falling from the sky will have a ph level of between 4 and 7.  If this lands on limestone, then the stone will quietly fizz away until the acidity of the rain water approaches neutral, leaving badly eroded stone on the property, difficult to prevent, and usually expensive to cure. One only has to look at the old sandstone buildings to see the ravages of acid rain over the centuries.

Water can move in different ways within a building. Some movements are contradictory, for instance gravity and vertical upward movement and some are complimentary for instance pressure and wind. Water also tends to be quite cohesive, in that smaller molecules will link together to form one large molecule. These forces will pull together to create a water droplet if left on an impermeable surface.  Water is also adhesive, in that it will join together with molecules from other elements such as salt. This is important in terms of understanding waters behaviour once it has entered into a building.

Another phenomenon one has to understand is the concept of capillary action. Because of its adhesive qualities, water will pull itself closer to other molecules, and thereby migrate towards a given substance.  In walls this migration can result in water travelling through a solid brick wall. Couple this with the adhesive qualities and one can start to understand why damp occurs in a given location.  Other factors affecting the migration of water include gravitational pull, and pressure differential, for instance where external barometric pressure is higher than the internal pressure within a building, water will be forced to travel into the building from the external environment.  If for some reason the external pressure drops, for instance because the weather changes, water will be forced back out from the building, so that there is always a state of flux with water levels both inside and outside the property.

As a surveyor, when ever I come across a damp wall in a property, I always have to try and diagnose where the source of the water is. Then I look to see if there is a flaw in the construction of the property, and hopefully suggest a cure.  There has to be a certain tolerance of water, and no one material is ever completely dry.  If you take mortar, 5% water content would be regarded as being relatively dry. 5% water content in bricks however would be suggested as being quite damp, and 5% moisture content in plaster would be regarded as being positively wet. In fact any moisture level greater than 1% would leave you thinking that the plaster was unacceptable wet. Timber however is much more tolerant of moisture, and it is quite normal to find a moisture level rising up to 20%.

In diagnosing a damp problem, one also has to be very aware of the buildings surroundings.  Is the building sitting on soil which has a high water table? Is it situated in an exposed position, where for instance wind whistling up from the Bristol Channel hits the building head on, driving any moisture present in the air into all sorts of knocks and crannies situated on the building?

In my survey this week the bedroom walls were damp, with moisture levels in excess of 35%. Water was rising by capillary action from the ground below, because the builder when constructing it had failed to insert a damp proof membrane at the base of the wall. All walls should have one, and over the years the materials used have varied from slate, to engineering bricks, semi engineering bricks, bitumastic felt and nowadays, good old fashioned polythene. They all have a common characteristic, they are impermeable to water.

There was no simple cure, the only solution would be to inject a chemical damp proof membrane into the brickwork, the chemical being Calcium Silicate.  A series of holes are drilled along the bottom of the wall, and chemical is injected into the brick work under very high pressure.  Once there, the Calcium Silicate blocks the air pores in the bricks, and thereby prevents further upwards capillary action from taking place. The external soil level will also have to be lowered to ensure that ground water cannot bridge the new damp proof membrane.

Internally the old plaster will have to be hacked off the walls, and be replaced with an impervious cement render. Cosmetically the wall would be finished off with a thin coat of thistle plaster and when dry be re-decorated.  An expensive cure for not inserting a cheap piece of plastic damp proof membrane into the wall structure, but it is one of the most common faults that we come across when surveying houses.

 By Tony Rowland = Partner        Timothy Lea & Griffiths   www.tlgllp.com