Royal Hospital Chelsea

EBS was asked to inspect the timber floors and to test for structural timber decay. In particular we were asked to examine the hidden, built in beam ends in the walls. A RESISTOGRAPH decay detection drill was used to carry out the timber testing.

Non-Destructive Investigation Technology

The Resistograph Methodology, a relatively recent development involves the use of a resistograph drill which drives a 3mm diameter drill bit up to 440mm into the timber member. As the bit penetrates the wood the rotational resistance is shown on an LED display and recorded on a printout chart. This gives an immediate profile and permanent record of the internal condition of the timber.

Regions of sound wood are shown as high resistance to forward motion of the bit. As the bit enters a decayed region, the resistance to forward motion is reduced and a low signal level is recorded.

Site Investigation

The Resistograph inspection does require sound knowledge of wood science and construction (type of wood, stress grading and how the structure is put together) & mycology (knowledge of decay fungi) to decide on site where to drill and get maximum information from drilling and monitoring to make informed decisions.

Large numbers of timber members can be inspected in relatively short periods of time using the Resistograph. This is useful since the longevity and cost-effective maintenance of historic timber structures relies on early detection of decay and preservation of the members.

Timber Analysis using the Resistograph

Analysis of timbers using the Resistograph can provide the most up to date and accurate assessment of decay. If only 10 per cent of the timber is decayed, it is pointless destroying the other 90 per cent of the timber to make certain the rot is eradicated. The Resistograph inspection ensures that repairs are confined only to the decayed areas of timber and allows significantly more accurate costing and repair schedules to be produced.

Advantages

Many historic structures can no longer be considered truly contemporary of their time because much of the timber has been replaced with a modern equivalent.

Accurate information on the condition of the timbers should be obtained before carrying out expensive and destructive repairs. The use of Resistograph techniques at an early stage ensures that repairs are only carried out when and where absolutely necessary. An hour’s inspection using the Resistograph can provide an instant, precise, non-destructive analysis on site, which can save many thousands of pounds of unnecessary remedial timber treatments and exposure works.

Conclusion

Very little timber decay was found considering the amount of timber tested (54 main floor beams, 324 principal floor joists and 3240 floor and ceiling joists). Most of the problems were due to poorly executed repairs following bomb damage in the 2nd world war.

The RESISTOGRAPH allowed an accurate and rapid assessment of the structural beam ends without the need to carry out any exposure of the built in ends in the brickwork. This saved considerable time, expense and allowed the refurbishment to continue without any delays.

Environmental Building Solutions staff have been involved with similar investigation, analysis and monitoring of a large number of historic and modern buildings to provide recommendations for environmental control and sustainable conservation solutions to damp, infestation, dry rot, wet rots and timber decay, e.g. Westminster Abbey, Tower of London, Hampton Court Palace, Windsor Castle, Dover castle & Telc Castle in Czech Republic.