Under the lamp

Case study

Islington, London

In June of 2024, we carried out a damp assessment and investigations into the condition of structural timbers forming the roof and floors, as well as the condition and suitability of historic waterproof tanking.

Author

Patrick Hughes and Matthew Amis

Service

Timber Condition Survey, Damp Survey

Completion

June 2024

Client

Buyers (pre-purchase)

The property was a four storey, mid-terrace Georgian house located in a desirable area of Islington that had been subjected to several periods of refurbishment and modernisation. Our client had contacted us due to concerns highlighted during a previous RICS survey, surrounding various issues, including the suitability of structural roof timbers and the condition of historic waterproof tanking.

We carried out several survey packages to provide a thorough assessment of the condition of the building using a holistic approach to identify defects, latent defects, and areas of increased vulnerability due to material failure and any subsequent moisture ingress.

The main survey packages included a preliminary damp survey and condition surveys of structural timbers forming the butterfly roof and the suspended timber floors, to address specific concerns highlighted in the RICS report. Historic fungal decay was detected to several original roof timbers, as well as other minor structural issues. Moisture readings of all investigated timbers were well below the threshold for the survival and growth of biological decay organisms; therefore, these instances of decay were considered to be no longer active.

Specialist imaging equipment was used to visually assess encapsulated floor timbers that were positioned in vulnerable areas, as well as to determine the materials and structural arrangement.

The original external roof finishes would have likely been slate, originating from quarries within the UK. However, these had been replaced in the last few decades with artificial slates made from fibre cement and/or slate composite, and these did not display any critical defect at the time of survey.

External allowance for ground-surface drainage did not appear to be adequate at directing surface water away from the structure. This had contributed to damp conditions arising within the lower-ground floor level of the property.

Internally, cementitious render had been applied to the masonry walls in the lower-ground floor kitchen and dining area, in an effort to previously tank (water-proof) the habitable space and prevent damp penetration from the ground below, due to the absence of a damp-proof course. The tanking was deemed to be beyond its typical service life and was beginning to show signs of failure in localised areas.

Context and materials – Fire resistance of timber

Whilst investigations were being carried out in the void of the Butterfly roof, assessments of individual timbers identified light charring. It should be noted that whilst timber is a combustible material it can still display behaviours favourable towards its use in the built environment, such as the maintenance of strength as time and temperatures increases. In this area timber often outperforms other common structural materials. As the surface temperature of the timber element increases the chemical bonds of the compounds and constituents forming the timber begin to break down and release flammable volatile gases. As combustion, pyrolysis and thermal decomposition continue a carbonised charring will form. As more volatile gases move to the surface from below the charring this causes transpirational cooling, which provides some amount of cooling to the heat affected timber and allows the charring to act as an insulative layer to the sound, unaffected timber beneath.

It was thought that the charring may have been caused by a fire during the London Blitz of 1940 – 1941, however desk-based analysis using historic maps of bomb-damaged architecture in the local area did not identify this as such. Despite this, all charred timbers were of adequate section size and were deemed to be structurally sound and suitable for retention.