Civil Engineering Reference
In-Depth Information
other openings and cause
dermatological problems for human
occupants. Roosting prevention is
usually by means of sprung wires
placed just above ledges, or by
spikes at suitable intervals. BRE
Digest 418 gives further guidance.
paint films. On internal surfaces, as
well as being unsightly, mould is
thought to cause respiratory problems
in susceptible individuals.
There is further discussion of rot
and insect damage in
Roofs and
roofing
. Guidance is available on the
recognition of wood rot and insect
attack in buildings
(77)
and on remedial
treatments
(78)
.
Air seals
Inside
Mould and fungus
There are two kinds of building
fungi - those that cause wood rot, and
those that do not. Wet rot occurs
mainly at the bearings of timber
joists in external walls; for example
at the sole or head plates rather than
in the studs. Wet rot decay of timber
and timber based board materials can
take place only where these are
maintained in persistently damp
conditions (Figure 1.69). Initiation of
attack generally results from
microscopic airborne spores, but can
also occur where pre-infected timber
has been used; in this situation, very
rapid decay can occur in new
construction. Under appropriate
conditions damage may be rapid and
severe, and therefore of structural
significance. The dry rot fungus,
Serpula lacrymans
, can also occur,
which is more devastating, though
less common than wet rot.
Surface moulds be found on
external and internal surfaces of
walls and partitions, very often
adjacent to ceilings and usually
accompanied by persistent
condensation or other form of
wetting. Moulds are unsightly and
may also cause premature failure of
Space to allow
air pressure to
equalise each
side of baffle
Rain baffle
Joints
Joints are often the weakest points of
walling, and it is usually the
durability of infrequently occurring
joints that receive least consideration
by designers, fabricators and
constructors. Joints between like
components are more likely to
receive adequate consideration than
joints between unlike components
from different sources. Extra
attention therefore must be paid to
joints between unlike components.
For the same reasons, careful
attention needs to be given to unusual
or rare joints or those that occur at
changes of plane.
Two-stage joints can be formed in
both single and two-skin walling. The
underlying principle for such design
is that the water and air barriers are
separated. Provided water is
prevented (by geometry, and by
having an air path past the water
barrier) from reaching the air seal at
the rear of such joints, performance
can be reasonably assured (Figure
1.70). Two-stage joints are more
likely to tolerate variations in
Outside
Figure 1.70
Separation of the functions of water and air
exclusion
Figure 1.69
Fruit bodies of Paxillus panudides and decayed wood. This fungus prefers softwoods in
very damp situations
