Dry Rot or Chemical Attack
Neil Marsden Snr
Managing Director @ Neil Marsden Associates Ltd, Independent Damp and Timber Surveyor (PCA), Director Neil Marsden Property Care Training Company
For the less experienced surveyors, it can sometimes be confusing when they see evidence of what appears to be dry rot affecting the roof timbers. Over the years I have been asked to investigate outbreaks of dry rot within roof voids on numerous occasions only to discover that the defect is the result of what is commonly referred to as a 'chemical attack' of the timbers. So what is the 'chemical attack'?
Nearly all old roofs were lime torched (see opposite). Whether slate, clay tile or stone flags, lime was?used to bed the tile, or beneath them to prevent undue wind, and hold condensation. Lime acts as a sponge to hold water and release it as conditions change and torching is still used today in Heritage properties. The torching on clay tile roofs also contributed to securing them in the days before nibs were added to hold them on the wooden battens. The torching normally consisted of a mixture of lime mortar and horsehair although the practice largely became outdated in the late 1930s when bitumen underfelt was used more extensively.
As the mineral salts in the mortar are dissolved by rainwater, during periods of drier weather the salts can crystallise on the surface of the timbers and between the fibres of the timber as the moisture evaporates and it is commonly mistaken for dry rot where the build-up of salts is significant. As the salts crystalise that can result in the separation of the timber fibres to the point that the damage, although largely superficial can appear to be significant. The salts often become more apparent following the renewal of the roof if a sarking felt is installed. As the timbers dry out the build-up of mineral salts can accelerate as the moisture contained within the timber evaporates and the salts migrate to the surface and then crystalise. The white fluffy clumps of mineral salts are often mistaken for dry rot mycelium.
领英推荐
Before the renewal of the roof and the installation of sarking felt the salts may not be apparent as they will generally be held in solution if rainwater is penetrating the roof coverings. It is when the timbers start to dry out that the salts become visible.
A simple test is to pinch some of the white fluffy material and put it on your tongue, the salts will dissolve instantly.
I hope this is useful to the less experienced surveyors among us.
Best Regards
Neil
Director at Riconstruction
3 年Thanks for sharing
Head of Building Facilities at ERYC
3 年Danielle Fleming Jamie Carter Lorraine Ward Connor Humphrey very interesting read and something to be aware of.
Director at Silverstone Building Consultancy Ltd
3 年Ross Haggie
MD TASC at TASC ENVIRONMENTAL SERVICES LIMITED
3 年very informative as usual I complete Asbestos surveys and sampling and am often asked to sample the torching between slate tiles as they see the animal hair and worry about it being Asbestos. So you have yet again provided me with useful information that I can use. One word of warning is your method for testing on your tongue horse hair can contain Anthrax spores that can reactivate when in a warm environment and get into cuts and the bloodstream. I came across this information when I was asked to test Heritage sites horse hair plaster for Anthrax. Again thanks for your always informative clips. Ray Doyle