Timber Industry Yearbook 2012
www.trada.co.uk
Discoloration:
prevention
and cure
TRADA Technology’s Senior Technical Consultant Peter
Kaczmar discusses a TRADA research project into the
causes of discoloration of external cladding.
The large numbers of cladding-related enquiries from architects’
practices, handled by the TRADA advisory line, show that timber
cladding continues to gain popularity. In some cases, and particularly
for cladding involving species such as oak and Western red cedar, the
norm continues to be to specify cladding in the unfinished state.
Many architects choose to do so because they want the wood to
‘weather’ to a silver-grey appearance as part of their design aims and
the absence of a finish avoids the need for regular re-painting, and
minimises maintenance costs.
The timescales for the development of such ‘weathering’ can be
unpredictable. This depends on factors such as exposure aspect,
the degree of exposure to sun and rain, and the time of year when
erected. The rate of colour change may also vary between species
(hardwoods tend to take longer to achieve a silver-grey colour than
softwoods, particularly if it is a very dark timber to begin with) and can
sometimes vary within the same species.
Wood containing a high proportion of tannin (a water soluble extractive),
such as Western red cedar, European oak or sweet chestnut, will exude
this for some time. With unfinished timber, that part of the tannin
fraction which is water-soluble may become redistributed by successive
cycles of rain to leave unsightly stains or ‘tide marks’ beneath areas of
preferential water runoff such as occur beneath discontinuous eaves
details or beneath cills.
In the case of unfinished cladding, the ‘weathering’ process will give
an even appearance, provided that the detailing is correct and the
local environmental conditions are considered in the original design.
Weathering generally tends to be more even on vertical rather than
horizontal boards. Unfinished wood, particularly species which are
acidic, can also suffer iron staining from pollutants such as brake dust
and exhausts in close proximity or downwind of a contamination source.
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In most cases leaving cladding unfinished works reasonably well, but it
is important to detail it carefully. Large overhangs may lead to uneven
‘weathering’ and sometimes can create areas of differential colour
beneath shaded areas. Overhangs will also prevent rain water washing
the surface clean of dirt particles. Nor should unfinished wood be used
in locations where there may be pollution from nearby traffic or other
forms of atmospheric pollutants.
Given these trends and uncertainties, TRADA Technology is undertaking
a research project entitled ‘Investigation into the discoloration of external
unfinished timber cladding, its prevention and remedial treatment’ which aims
to identify the causes of the various forms of unpredictable discoloration of
unfinished exterior cladding in service. In addition, by means of site-based trials
it seeks to establish the effectiveness of preventative treatments to prevent
discoloration occurring in service and, finally, to develop appropriate strategies
for the remediation and long-term prevention of discoloration on exterior
uncoated cladding. The ultimate aim of the project is to improve the aesthetics
and consumer perception of using uncoated timber cladding in service.
In our experience, surface disfigurement of cladding is caused by the
growth of surface fungi and yeasts which can lead to a non-uniform,
dirty appearance of the exterior facade. The initial phase of the research
involved collating and re-examining more than 40 technical reports
Western red cedar cladding showing characteristic ‘tide’ marks in areas receiving differential amounts of rainfall
Timber Industry Yearbook 2012
www.trada.co.uk
CLADDING
97
of disfigurement of cladding boards in service around the UK. Of the
32 reports that contained a detailed analysis of causation, mould
colonisation, extractive staining and iron staining were found to be the
three main causes of discoloration.
This work has identified areas of buildings where cladding is likely to
be more susceptible to mould colonisation, including areas where wood
remains wetted for longer, or on areas of sapwood. By understanding
where the mould is likely to occur, it should be possible to design out the
problem, or to use a treatment that inhibits mould growth.
Many clients wrongly associate mould growth with wood decay.
Although the presence of mould may indicate that the moisture content
of the wood in the affected areas is elevated periodically, it does not
mean that decay has been established.
Sweet chestnut cladding test area showing difference in performance after one year between panels
cleaned and treated with different methods
Typically mould growth on exterior cladding is caused by the growth of a yeast
fungus from the genera Aureobasidium or Hormonema which exist in a highly
melanised or black form producing the characteristic black discoloration.
This type of fungus can use naturally occurring sugars present in the wood, as
well as the breakdown products from delignification of wood (produced on the
surface of cladding which has weathered). The surface discoloration produced
in this way is confined to the surface of the wood and does not penetrate it and
should not be confused with disfiguring agents such as blue-stain fungus.
Disfigured sweet chestnut cladding being cleaned with power washer
Discoloration caused by these surface yeasts is more prevalent on
unprotected aspects which are more liable to periodic wetting from
rainfall and which are subjected to high levels of solar radiation. The
surface weathering provides a food source in the form of breakdown
products from wood delignification. This will happen less on sheltered
elevations and beneath areas which are sheltered from direct sunlight
and rainfall – under eaves, overhangs and canopies.
Often mould discoloration can be seen as a distinct zone immediately
above overhanging cills or above ground level within the splash zone
where cyclic and repeated splashback can contribute to the moisture
regime of the cladding in these areas.
Dicoloured oak cladding following cleaning with power washer
colonisation. We are examining the efficacy of preparatory options that
may offer long-term protection against these forms of disfigurement.
Differential porosity or differences in the availability of nutrient sources can
be responsible for differences in disfigurement between individual wood
components which may lead to discoloration from disfiguring moulds or yeasts.
This can be a particularly troublesome form of discoloration if the cladding
is made from smaller finger-jointed sections where the porosity and water
absorption characteristics of adjacent pieces differ. This can create areas of
differential mould growth in clearly visible boundaries between adjacent pieces.
We are now into the final stages of the research project, to investigate
cladding at sites where mould colonisation is known to be a problem
and are exploring a number of different cleaning and surface treatment
options aimed at removing the existing discoloration and preventing
in-service build up of surface nutrients as a measure to prevent re-
Peter Kaczmar
Senior Technical Consultant,
TRADA Technology Ltd
To these ends we have set up a number of sites
where experimental test areas consisting of
disfigured cladding have been cleaned using
a variety of methods. Some of these areas
have been left untreated, while others have
been treated with different, colourless products
in order to test their relative effectiveness in
preventing future discoloration. In doing so, we
will gain a far better understanding – based
on an experimental protocol using actual field
sites – of the most effective and commercially
viable methods of controlling mould and yeast
disfigurement on unfinished cladding.