Surveying equipment and tests

This method is used for determination of rising or atmospheric moisture testing in masonry walls.

Sections of the internal wall plaster and render are removed at intervals of around 150-200mm (2 or 3 brick courses) vertically in a wall above the internal finished floor level to a height just beyond the upper dampness level, where rising dampness is suspected.

Core samples (by cold drill method) are taken from the brick and the mortar to a depth of approximately 80mm at each interval and placed in a clean plastic container. The amount of sample should be the equivalent of the amount an old 35mm plastic film container could hold.

The samples can be sent to a recognised laboratory or can be gravimetrically tested at home, providing you have an accurate set of scales, an oven capable of maintaining an oven temperature of 105 degrees Celsius and a chamber that can maintain a constant 75% relative humidity. This can be achieved with common salt (sodium chloride) in a solution within a sealed chamber.

Around 2 grams of each sample is placed on a previously weighed glass or china dish (Wo), weighed wet (or as the sample was when it was first extracted from the wall or mortar) and the wet weight is recorded (Ww).

Each wet weight sample is then placed inside the constant 75% relative humidity chamber for approximately 8 hours to reach moisture equilibrium. The sample is then weighed; this is to determine the hygroscopic moisture content (W75).

Then place the sample into a preheated oven (100 degrees Celsius) and dry for around 1 hour to determine the dry weight (Wd).

The calculation is then as follows:

Rising dampness can be established when the found moisture at the base of the wall is greater than the hygroscopic moisture content, this trend will tapper the higher up the wall until there is almost a convergence between found and hygroscopic moisture contents.

Care has to be taken as to what the primary cause such a rising damp profile. BRE 245 indicates that where the hygroscopic moisture content is greater than the found moisture content, this is more likely to be as a result of internal condensation. However, the findings of Michael Parrett over several years have not found a case of serious internal condensation caused by use and occupation of a dwelling to have any great effect on found moisture content deep into a masonry wall covered internally by conventional wall plaster and renders.