Civil Engineering Reference
In-Depth Information
can be shown that for an exposed two
storey gable wall in, say, the
Birmingham area, the run-off at the
bottom of the wall during a once-in-
three-year storm is approximately
165 l/m width. For a 10-storey
block the run-off would be about
412 l/m width.
flows parallel to the wall surface.
Therefore droplets shed from
projections tend to fall vertically
downwards to the ground. Again, a
generous sill projection of at least
25 mm should be provided.
A recommended remedial measure
for rain penetration in two storey
housing is to provide a render to the
first floor level only (including the
gable), with a bell finish at the lower
edge. This is often a cosmetically
more acceptable, and cheaper,
solution than complete cladding or
render.
The protection given by
projections at the top of a
freestanding wall, or with flat roof
overhangs at verges or eaves, is much
less certain. This is because air
movement is more turbulent at the top
of a wall. In strong winds, rain
droplets may even be moving
upwards so that projections may
become counter productive. There is
very little quantitative information on
this issue. However BRE have carried
out tests on a rig 2.25 m high with a
flat roof and positioned near the top
of an escarpment 10 m high. (Figure
1.26). Overhangs of different depths
were fitted at the top and half way
down the exposed test face
(39)
.
Some of the results are shown in
Figure 1.27. It can be seen that all the
lower overhangs provided significant
protection to the wall beneath. But
the pattern of protection afforded by
the upper overhangs was very
different. Below these overhangs,
rainfall catches could be several
times greater than for a wall with no
overhang. The simplified explanation
for this can be seen in the wind
directions indicated in Figure 1.26,
where the upper overhang was
situated in the predominantly
upwards air flow. An analogous,
though less pronounced, effect could
arise for verge overhangs on pitched
roof gables.
Eaves overhangs to pitched roofs
can provide significant protection to
the wall below, provided the roof
pitch is greater than about 25°. This is
because, in terms of wind flow
patterns, the roof acts as equivalent to
an increase in height of the wall.
Therefore the eaves projection is, in
Surface effects
Surface texture and quite minor
surface features can have a
significant influence on the
behaviour of rainwater run-off. Water
on smooth surfaces such as finely
textured concrete or smooth renders
tends to find preferred run-off or
streaming paths. This can lead to
unsightly surface staining. Heavily
textured finishes such as roughcast
rendering can break up streams of
water and give a more even wetting
pattern. Textured or ribbed surfaces
are also better able to prevent surface
water blowing sideways, an effect
which can produce heavy local
loadings on cladding joints.
Some design features tend to
concentrate water on certain areas of
the wall, in effect increasing the
exposure in those locations. An
important example of this is the use
of flush window sills. Rain water
collected from the whole area of the
window is shed onto the wall below.
Hence a spandrel of absorbent
brickwork in sheltered parts of the
country can have an exposure
equivalent to an area of the country
with over ten times the driving rain
index. It is quite common to see
frost damage in bricks located under
flush sills, whereas bricks in other
parts of the wall are in perfect
condition. A sill projection of at least
25 mm, and having a drip to allow
run-off to fall clear of the wall, should
be provided in such circumstances
(Figure 1.25).
Figure 1.25
Here, run-off from the impervious window
has not cleared the wall, leading to the
different appearance of the painted
surface below
Overhangs
H
2
/
3
H
10.6 m
Figure 1.26
Overhangs: cross-section of a BRE test rig
showing airflow patterns
Protection given by overhangs,
drip moulds etc
It is sometimes thought that in high
winds, water dripping from
projections is quickly blown back
onto the wall a short distance further
down. In fact, air close to the wall
forms an almost still boundary layer
and to the extent it moves at all, it
