22 Joinery joints
the tenons, then the mid- haunch area would be a
half ATW, as shown in Figure 3.4(b) above); and
the fourths rule for wide bottom rails, as shown in
Figure 3.4(a).
Tenon thickness
Figures 3.6(a)(b): Again, using entrance doors as the
criterion for mortise- and- tenon rules, because the
tenoned rail’s thickness is the same as the mortised
stile, the tenon’s thickness should normally be one third
of the material’s thickness. In reality, however, this
rarely works out precisely, because – whether making
the mortises by hand with traditional, joiners’ mortise
chisels or by machine with so- called square, hollow
chisels with integral auger bits – the available hand- or
machine- chisels are limited in their range of sizes.
Therefore, in practice, the calculated one- third has to
be increased or decreased to the nearest chisel size.
The difference is usually no more or less than a few
millimetres – and whether to increase or decrease is at
the craftsperson’s discretion – but it makes good struc-
tural sense to go to the nearest chisel size above, not
below. A reduced chisel- size would mean a thinner,
weaker tenon, but an increased chisel- size, whilst
resulting in a slightly thicker tenon, does not signifi-
cantly weaken the equally- distributed and supportive
timber fibres remaining on each side of the mortise.
Even so, the mortise should not be too much wider
than a third.
A structural proviso to the above rule concern-
ing doors is that the maximum tenon thickness should
not exceed 16mm. Therefore, if a door exceeds the
standard thickness of 45mm (and is over 48mm),
twin tenons should be used, meaning that each
tenon’s thickness should now be one fifth of the mate-
rial’s thickness. Thereby, a standard 60mm- thick
door should be divided by five to give twin tenons of
12mm thickness. As illustrated at Figures 3.6(a) and
(b) below, the haunches can be single or separated. (I
believe twin tenons should have twin haunches). This
traditional maximum 16mm rule seems likely to be
related to the comparative strength of twin- tenoned
joints to single- tenoned joints when doors increase in
size and weight.Maximum width- of- tenon rule
Figures 3.7(a)(b): There is another established wood-
working rule of bygone years, which prescribes that
the width across the grain of a tenon should not exceed
five times its thickness (i.e., a 16mm- thick tenon × 5
should not be wider than 80mm). This rule should be
used generally, but is more relevant when tenoned rails
equal the same thickness as the mortised stiles. The
reason for this is that a wide, single tenon used, say, on
an ex 225mm- wide bottom- or middle- rail of a door
is not only adversely affected by additional shrink-
age across the grain, but the greater width would also
make the side- encasement material of the mortised
stile relatively thin and weak on each side of the wide
tenon.
Also, as illustrated in Figure 3.7(a) below, buck-
ling and snaking of a thin, over- wide tenon can occur
when the wedges are driven in. And if such a deform-
ity develops, the compressive forces created by the
wedges have changed their direction to some extent
and have resulted in a sideways pressure on the thin,
pliable sides of the mortise, causing them to bulge
with a risk of the stile splitting beyond one or both
ends of the mortised area.
Of course, common sense should prevail regard-
ing the width of a particular tenon – and slight(a) (b)
Figure 3.6 (a) Double twin tenons on the middle rail of
a 60mm- thick door (with single haunch) (b) Single twin
tenons on the top rail of a 60mm- thick door (with twin
haunches).
Splits in stile
caused by
buckling and
snaking of
over-wide
tenon(a)Figure 3.7 (a) Damage caused by violation of the Width-
of- Tenon Rule.