FEATURE TIMBER CONNECTIONS
Making the Right Connections
Pressed metal web joists use punched metal plate fasteners
Metals that are used correctly in conjunction with timber will last the lifetime of a structure under dry conditions. But metals may be at risk from corrosion in wet or damp conditions. This risk can be minimised through correct specification, design, storage, construction and maintenance.
T
he key agents of corrosion of
Specifiers should consider whether
metals by timber are moisture
these situations are likely to arise and make
therefore be of appropriate corrosion resistance
and acetic acid, a naturally
appropriate choices of materials. Different
and should be compatible with the timber
occurring compound in all
species of timber vary in acidity, so it’s
species being used and with any preservative
timber. Moisture may be present from rain,
important to specify metals with appropriate
treatment specified.
snow, condensation or vapour. Moisture
corrosion resistance and/or provide separating
content levels of 20% and over present a risk
membranes or barriers between metal and
of metal corrosion.
timber, and/or specify a less acidic timber.
Iron stain Iron stain is the result of a chemical reaction between iron and tannins or tannin-like materials
While acetic acid in timber can corrode metal, timber itself has good resistance to acids
corrosion problems. All metal fixings should
Wood preservatives and fire retardants
in the timber that contribute to form iron tannin compounds. Commonly described as iron
but is degraded by alkalis, which may form as a
Preservatives containing metals as active
by-product of the corrosion of metal by timber.
ingredients carry a risk of corroding metal fixings.
tannate, these compounds form a blue/black
The combination of acidic corrosion of metal
Manufacturers and suppliers of preservatives
stain that can develop in damp timber in contact
and alkaline degradation of timber can cause
usually advise leaving the treated timber for at
with ferrous metals such as iron and mild steel.
iron and mild steel fasteners and fixings to
least 14 days before fixing to allow it to dry out.
Iron stain is commonly associated with oak, but
loosen, with a condition sometimes described as ‘nail sickness’ developing. Salts in seawater and some salts found
Some fire-retardant salts also promote the
also with sweet chestnut, makore, idigbo and
corrosion of metals and can raise the moisture
kapur. Softwood species prone to iron stain
content of treated timber. The potential of
include Douglas fir and western red cedar. Iron stain can arise where ironworks have
in wood preservatives and fire retardant
preservatives and fire retardants to corrode
treatments will accelerate the corrosion
metals increases as the moisture content of the
been carried out near to the affected timber.
of metals in the presence of moisture. The
timber rises. The manufacturer of the wood
It can be avoided by keeping ironworks and
effects of corrosion of metals by timber are
treatment product must be consulted about the
timberworks separate and/or by carrying them
most pronounced where the two materials
appropriate fixings to be used. There are other
out at different times in the building process.
are in contact under one or more of these
fire retardants with active ingredients that are
circumstances:
not corrosive to metal fixings.
Acidity of timbers
•
Swimming pool halls
than others. pH values range across a scale of 0
Some timbers are significantly more acidic where the timber and metal are exposed to the weather, to a damp environment or to •
•
Provided the appropriate measures have
to 14, where a value of 7.0 is neutral, less than
intermittent wetting
been taken, the moisture content of timbers
7.0 is acidic and more than 7.0 is alkaline. The pH
where the timber has been treated with a
in swimming pools will not generally be high
scale is logarithmic, so that a pH value of 4.0, for
wood preservative or a fire retardant in the
enough to be the direct cause of corrosion in
example, is ten times more acidic than a value of
presence of moisture
metal fixings. However, occasional but repeated
5.0 and 100 times more acidic than a pH value
where the timber is exposed to seawater or
wetting, and/or condensation on glazing and
of 6.0. In general, timbers with a value of 5.0 or
seawater spray.
chemicals in the atmosphere, could cause
more offer the least risk of corrosion to metals.
Timber Construction Magazine
Spring 2022
14
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