Civil Engineering Reference
In-Depth Information
Abutments, external and re-
entrant corners
For horizontal timber boarding it is
normal to provide a vertical batten
at external corners to give some
protection to what would
otherwise be exposed end grain or
mitred joints. This is also beneficial
for plywoods. On the other hand,
vertical boarding can be turned round
corners quite satisfactorily using a
plain butt joint instead of a tongued
or grooved edge
(Figure 9.27).
Weathertightness, dampness and
condensation
Timber feather edged shiplap has a
comparatively good record for
weathertightness provided it does not
rot, warp or split. PVC-U sidings are
unlikely to suffer too much from the
two latter defects in the absence of
deliberate vandalism, but there can
be surface deterioration.
Vertical boarding is frequently
seen on agricultural buildings,
spaced apart to allow ventilation.
Provided the joint widths are not
excessive, little driving rain,
especially when it comes at an
inclined angle to the wall, will enter
the building. Wind-driven fine dry
snow, however, is another matter.
Figure 9.27
One way of making an external corner joint
in timber boarding
Main performance
requirements and defects
Choice of materials for structure
Both hardwoods and softwoods have
been widely used for claddings,
though one of the most popular
species has been western red cedar.
The characteristics of species are
discussed in more detail in BRE
Digests 417
(248)
and 407
(155)
.
Plywoods, particularly class M or of
Douglas fir with suitable WBP
adhesives, have been used in external
situations with good results. See also
BS 6566-7 and BRE Digests 323
(249)
and 394
(250)
.
Thermal properties
Although timber boarding and
extruded PVC-U sidings have some
value as thermal insulation, this can
be negated by wide joints which
permit air leakage, or even
deliberately ventilated cavities for
the removal of moisture migrating
through the wall from within the
building (see
Thermal insulation:
avoiding risks
, clause 3.18).
Fire
Timber boarding may be treatable to
obtain Class 0 surface spread of
flame. Without treatment, it is not
likely to better Class 3 (Figure 9.28).
PVC-U in fires behaves quite
differently from either wood or
metal, and any necessary assurances
must be sought from manufacturers.
See also Chapter 1.8.
Strength and stability
Timber boards, whether of
softwood or hardwood will normally
fare no worse than doors constructed
of similar materials in situations of
heavy wear and tear. See the general
discussion on impacts in Chapter 1.1.
Figure 9.28
Horizontal timber cladding under test
Dimensional stability, deflections
etc
Some plastics may be subject to
shrinkage or bowing from loss of
volatiles.
Plywoods used in fascias and
soffits can suffer from undulations if
the centres of the fixing battens or
noggings are too far apart. The
thinner the boards are, the closer the
centres ought to be; even 300 mm
may not be excessive.
For coefficients of linear thermal
and moisture expansion of timber,
plastics and metal cladding materials,
see Chapter 1.2.
Noise and sound insulation
None of the products dealt with in
this chapter are likely to add much to
the sound insulation of an external
wall.
Durability
When considering the durability
requirements of timber used
externally in buildings, a distinction,
for example in building regulations,
may be drawn between 'permanent'
and 'non-permanent' buildings, with
minimum standards for both species
and treatment necessary for adequate
durability. In the past, of course, such
