Civil Engineering Reference
In-Depth Information
vulnerable to rain penetration.
The water load on paved areas
to be provided for by the design is
50 mm/hour (see BS 6367
(200)
).
Alternative means of reducing
the water load on the area to be
drained are met by providing:
●
Paved areas outside entrance doors
should be laid to a minimum fall to
prevent water accumulating and then
freezing. A minimum fall of
1 in 80 is conventionally accepted
for this required slope. Because of
site dimensional deviations, and the
reduction of the possibility of
backfalls and subsequent ponding,
the nominal fall should be increased
to 1 in 60.
The splash zone for driving rain is
conventionally taken to be 150 mm
measured both vertically and
horizontally, although under certain
weather conditions this will be
exceeded. Ramps running parallel to
the wall in which the threshold is
situated therefore need to be kept at
least 150 mm clear of any wall over
the DPC (Figure 5.19).
Main performance
requirements and defects
Choice of materials for structure
If condensation is to be successfully
avoided, adequate thermal insulation
must be provided to link with that in
the remainder of the external wall.
The traditional stone or concrete
threshold tightly butted to stone
flagged floors, even when carpeted,
provided a massive thermal bridge
(Figure 5.18).
a shelter (eg a porch)
●
a drainage channel or gutter in
front of the threshold
●
permeable paving
If any one of the above items is
present, it is estimated that the
chances of leakage for most of the
UK will be reduced to an acceptable
level. Porches need to be cheeked to
prevent driving rain blowing in
sideways. A projection of not less
than 750 mm will prevent most
driving rain from reaching the
threshold, but it will obviously
depend on the direction of the
prevailing winds.
Drainage gutters need to have a
compromise gap of around 15 mm
for:
●
Dimensional stability, deflections
etc
The most important consideration
here is to provide adequate end
clearance for water bars cast or
inserted into stone or concrete
thresholds. This is especially
necessary on wide or double doors.
For coefficients of linear thermal
and moisture expansion etc, see
Chapter 1.2.
Waterbars
There is no doubt that a waterbar is a
satisfactory method of reducing, if
not entirely preventing, rainwater
from penetrating a threshold.
Waterbars essentially are of two
kinds; placed at the foot of the
opening leaf, creating an upstand
against which the leaf closes; the
second prevents water being drawn
through the horizontal joint between
the underside of the threshold and the
Weathertightness
Even some existing designs of
conventional raised thresholds have
proved to be inadequate in resisting
driving rain in exposed areas of the
UK, though most designs conforming
with previously relevant Codes and
Standards for weathertightness seem
to have worked reasonably well.
It is undoubtedly the case that the
higher the upstand, the better the
detail will be in resisting water being
driven over it by air leakage. In the
absence of an air seal at the foot of
the door, and therefore nothing to
resist the passage of an air stream
carrying water over the threshold,
even upstands of 25-50 mm will be
safety - for example, to avoid
trapping small section heels and
umbrella ferrules
●
reduction of risk of blocking by
detritus
Not less than 150 mm
Not less than 150 mm
A handrail is not needed if
the ramp rises 600 mm
or less, but a kerb may
be useful
Ramp slope should not exceed 1 in 12 and preferably 1 in 15
Figure 5.18
Thermal bridge at an external door
threshold
Figure 5.19
Ramps running parallel to the external wall need to be kept 150 mm away from it, and
preferably more
