Civil Engineering Reference
In-Depth Information
windows (Figure 4.17). To overcome
this, wood preservation was
recommended and now softwood
windows are normally treated with
preservative.
The hardwood commonly used in
sills, European oak, is classified as
durable, and would not normally
require preservative treatment unless
sapwood is present. Some imported
hardwoods are also naturally durable,
but others are supplied as mixed
species (eg commercial supplies of
meranti) with widely differing
durabilities, and in these cases
treatment is normally specified
(155)
.
New European standards classify
wood preservatives according to
effectiveness in different
environments. The level of treatment
to be achieved is classified by
penetration into the wood and the
retention of loading in a specified
zone, irrespective of treatment
method or wood species
(181)
.
Figure 4.17
Neglect over many years has led to progressive deterioration of this timber window - not
only paint, but also putty has failed, leading to opening of the joints
that BRE comes across with wood
windows is failure of double glazing
units. This is because most existing
wood frames do not allow for
drained-and-ventilated glazing
methods and the fully bedded glazing
used can reduce the durability of
sealed units by
allowing moisture to be trapped
between the frame and the unit.
Drained-and-ventilated glazing for
wooden doubleglazed windows
avoids this problem. If this is not
possible, glazing that complies with
BS 8000-7
(182)
, and the
recommendations of the Glass and
Glazing Federation and British
Woodworking Federation, should be
considered. If windows have to be
glazed on site they should be glazed
in accordance with BS 8000-7.
BRE is frequently asked about the
lifetime of double glazing units. A
number of variables affect lifetime,
but given the design, eventual failure
is inevitable. There have been
problems with double glazing units
where they have been installed in
frames that were only suitable for
single glazing. Such frames should
not be used. They cannot take the
weight and cannot be properly glazed
to protect the double glazing unit
from rainwater and sunlight.
Case study
Deterioration of PVC-U replacement
windows
Brown coloured PVC-U windows in a house
distorted to the extent that some of them
had to be replaced. Fifteen years of south
facing exposure had produced unsightly
whitish deposits on the windows, but
appeared not to have eroded the surface
substantially. However, the high
susceptibility of the profile to heat reversion,
combined with solar heating of the dark
surface, had led to distortion of the frame to
the extent that the windows were no longer
operable. The use of reinforcement in the
profile and greater numbers of fixings to the
wall, might have limited the distortion to an
acceptable level.
Steel windows
When steel windows were first
introduced, many were ungalvanised.
Provided maintenance was
meticulous, these windows could
have a reasonable life, but many
rusted, causing fractured glass and
broken window furniture as increased
force came to be used to close them.
Hot dip galvanising largely cured this
problem.
PVC-U windows
The material from which these
windows are fabricated has been
much improved since first
introduction. Quality of manufacture
is important, particularly in relation
to the quality of corner joint welds,
but discolouration can also occur.
Dark coloured units can suffer from
distortion under solar heating.
Glass
Glass can become etched and
aluminium frames, and zinc and lead
flashings, can suffer corrosion
staining by lime leaching from fresh
mortars onto glass or frames.
Double glazing units
Apart from rot of non-durable
timbers, one of the main problems
