64 Making traditional and modern windows
Throating grooves
Figure 4.11(e): On traditional, double- weathered
sills (Figures 4.1(c) and 4.11(e)), the shallow,
vertical face of the 9mm step in mid- area of the
slope (and the rebated bottom rail) was originally
grooved out with a throating plane – a slim, beech
(originally hand- made) grooving plane that was
developed to produce these ‘throating grooves.’
The grooves act like a drip groove in interrupting
rainwater seepage.Capillary grooves
Figure 4.11(f ): These grooves, often opposite each
other for maximum effect, can be seen on the side-
edges of traditional and modern casement windows
and doorframes. If being pedantic, they should be
called anti- capillary grooves. This is because their
purpose is to interrupt and break the creeping
effect of water seepage between two close surfaces,
scientifically known as capillarity – or capillary
attraction.between 1 in 8 (7°) and 1 in 10 (6°). Interestingly, this
is less demanding than the unregulated, traditional
criteria.
Drip grooves
Figure 4.11(b): When rainwater runs off the slope
and down the face of a protruding sill, it tends to
become overloaded on the edge and starts to seep
back (and can be blown back) under the sill. To
inhibit this, the underside edge of the sill is grooved
with a drip groove. When rainwater reaches the outer
edge of this groove, it drips off rather than seeping in
any further.
Water bars
Figure 4.11(c): Traditionally, so- called water bars (iron
or galvanized steel bars with a 25 × 6.5mm section)
were grooved lengthwise into the underside centre
of sills to inhibit any above- mentioned seepage that
may occur. But with the development and escalation
in recent decades of modern mastics and sealants (and
the simplicity of caulking guns), these grooves and
bars are usually omitted nowadays.
Figure 4.11 (a) A double- weathered
slope of 10°; note the alternative,
un- throated shallow step between the
slopes; (b) These details show four
alternative drip grooves; (c) A likely
position for a water- bar groove; (d)
This groove is for the interior window
board; (e) A typical throating groove;
(f) Two anti- capillary grooves between
a sash and a window jamb.(a)(b) (c) (b) (b) (b)(e)(f)(d)