Inspections and surveys for biodeterioration

Timber moisture content

Timber moisture content may be estimated at the surface by the use of a resistance-type moisture meter, fitted with insulated needle probes. The results gained will fluctuate depending on relative humidity and temperature. For example, a rafter may have a surface moisture content of 16% in summer, that might rise to over 20% in winter. This would not necessarily indicate an increased water content due to a fault in the roof; it might be water absorbed through a drop in temperature. The core of the timber will remain relatively dry and a hammer probe with insulated electrodes is recommended for measuring the subsurface moisture content.

Masonry moisture content

Estimating surface moisture content in plaster and mortar is of limited value except for comparison. A surface capacitance meter may be used on plastered walls and panelling to detect areas requiring further investigation. Absolute readings should be made by means of a carbide-type pressure meter or by the oven drying method. Moisture contours, both on the surface and within the thickness of the wall, will help to define the source and type of moisture giving rise to decay.

Detailed investigation

The findings from the initial investigations are followed up by more detailed study. The aim is to determine the distribution and extent of all significant decay organisms in the building, the distribution of all micro-environments with a predisposition to timber decay, and the building defects that cause them. The distribution of moisture and its movement through the structure is particularly important. The extent of significant timber decay should also be determined. Active decay organisms may not yet have caused significant timber decay. Conversely, there may be significant decay even when the decay organisms that caused it have been dead for many years.

Key factors to note are:

• species and viability of decay organisms;
• moisture content of materials;
• ambient relative humidity; and
• ventilation.

Timber species and previous chemical treatments may also be significant.

The results of the investigation should be coordinated with the building structure, bearing in mind the characteristics of particular periods and methods of construction. They should also be carefully recorded and quantified where possible. This allows analysis of the results by other experts, reduces the 'grey' area in which differences of opinion can arise, and forms a basis on which future investigations can be planned. This recording of data is especially important in the current legal climate, and photography can be especially valuable. A detailed investigation might take about 5 man-hours for a typical 3-bedroom house.