This case study involved the analysis of a galvanised steel framing following a house fire. A house fire had caused significant damage to the house’s roof cavity, leaving a significant degree of both soot and smoke damage. Optimech was requested to analyse samples of the debris (known as combustion product), in order to ascertain whether there was a risk of further damage through corrosion.
Samples for Testing
Samples were taken from the galvanized steel batten surfaces. We took these samples from selected locations of the house in order to assess whether soot and smoke residues had the potential to lead to long-term corrosion of the galvanized steel battens within the walls of the house. Compliance with the New Zealand building code requires galvanized steel frameworks inside walls to provide a durability of no less than 50 years.
Six positions on the roof cavity construction were analysed using a portable HhXRF Bruker Titan LE analyser.
We analysed samples of the carbon-rich deposits that were present on the steel frames. The samples taken from the south-western and north-eastern parts of the house showed slightly raised levels of chloride contamination. However, this level of contamination was similar to that of the joist, which had not been exposed to the fire. The testing also identified a raised level of sulphur, caused by the burning building paper that couldn’t be removed by cleaning.
The removal of the roofing showed that there was a brown tenacious film present on the old frames. It is likely that this deposit originates from the building paper and is high in sulphur and carbon. Carbon is cathodic to the zinc plated steel frame, in the presence of moisture this may pose a small risk of initiating corrosion.
To minimise this risk, it was recommended to clean the surface and then either spray or coat the frames with a non-toxic product that can act as a moisture barrier, such as CRC Lanocote Natural. Alternatively, paint primer or a top coat can be used to provide the steel frame with additional protection.
Chlorides can react with the zinc coating on galvanized steel and this reaction may cause ongoing corrosion of the steel framing afterwards. As long as the chlorides are present on the framing they can support ongoing corrosion, along with the sulphur compounds that were also detected.
The examination showed that there were low levels of chlorides detected on the frame and this level was close to that of the natural background.
The grime and soot deposited on the steel framing are corrosive and must be removed if the existing framing is to be re-used. Removal can be done by washing it with water and detergent and then hand-wiped. Following this, a protective coating should be applied such as CRC Lanocote Natural or an alternative product such as a paint primer or top coat.
This case study demonstrates the need to analyse building materials following a fire to diagnose the full extent of fire damage and ensure the continued compliance with the New Zealand Building Code. This is important in terms of limiting liability in the case of any future insurance claims, ensuring that you took all the required steps to restore the structural integrity of the house.