When people see timber or wood products such as decking, cladding etc exposed to the elements e.g water, rain, sunlight, wind etc that have begun or have changed in appearance mostly turning to a silvery, bleached, grey colour this is the weathering process. The appearance of weathered wood can be “marmite” It is either you love it or hate it.Occasionally you see weathered timber that has been overpainted to try and cover the weathered look. You can try and get rid of the weathered look by sanding or using a bleaching solution e.g mild concentration of oxalic acid.
Weathering of timber is a natural process. It is a combination of processes both physical, chemical and also involves biological organisms (surface mould/fungi/algae). Weathering in timber products is believed to occur in five stages.
Every situation or exposure yields different results this helps me answer the question when people ask. When my cladding will turn grey as it looks patchy or it looks too clinical? My answer is how long is a piece of string? Only the Timberbossman tends to get away with a response like this.
It is essential for designers to consider weathering during the design process in the detailing, selection of wood species;- in design taking into consideration elevations exposed to the elements, e.g. south facing will tend to get hit hardest also allowing for roof overhangs.
Timber exposed to weathering will change colour, the rate of colour change will vary based on some exposure situations; – Freezing and thawing, Wetting and drying, ultraviolet rays (sunlight), fungi/mould. Every species is different some species will change colour quickly, and some species surface will erode faster, as surface fibres separate leading to splitting and cracking.
Leaching of water-soluble extractives is another factor different in every species very evident in species like Western Red Cedar. In Western Red Cedar the soluble extractives are- thujaplicins, polyphenols and in Oak- tannins which are water soluble and can be seen of the surface of timber especially when the wood has been exposed to wetting. A typical result of this is on exterior cladding having defined marks typifying an image of shadows/ lightning marks.
The bleaching of lignin, one of the structural chemicals in wood. The latter is photosensitive and changes colour on exposure to UV light present in solar radiation.
The subject aspect of appearance/ weathering in wood has contributed to the demand of some of the most recent technologies where clients want to have more control of how the building looks due to weathering e.g using
Modified timbers – Accoya, Lignia, Kebony, Thermowood
Charring or burning ( a Japanese technique)
Use of chemicals and finishes e.g nanotechnologies, Sioo X, Organowood