Manual of Purpose-Made Woodworking Joinery

106 Designing and making stairs to current building regulations

Setting out from drawings or site

measurements

For the purpose- made staircase, a joiner or woodcut-

ting machinist takes available information from the

drawings and contract specification. But – if pos-

sible – a site visit is advisable. When the timber is

available, he proceeds to calculate the precise step

size – the rise and going. As an example, assume that

13 treads (goings), excluding (as previously explained)

the top landing- step, and a total rise of 2.688m were

indicated on the drawing, the sum would simply be

2688 ÷ 14 (13 treads plus 1 for the top landing- step) =

192mm rise.

Designing a straight flight of stairs

The logical designing approach is to first divide TR
(the total rise) by a trial- and- error intelligent number –
say between 12 and 16 – until an acceptable and
permissible rise has been found. For a Category 1 stair
in a dwelling house, we know that the regulation rise
should be not more than 220mm and we can be further
guided by knowing that a rise of between 7 and 7½
inches (178 and 190mm) was considered to be ideal
traditionally.
Next, we must consider TG (the total going). As
this dimension might include the going of a landing
(the landing’s width), this is deducted from TG
until the horizontal dimension between the faces of
the bottom- and top- risers remains. This resultant
dimension is then divided by the number of goings,
which will be (remember the rule) one less than
the number of risers – and this will determine the
step- size of the going. Once the rise and the going
are established, the degree of pitch can be checked
easily, speedily, yet accurately by drawing a precise,
large- scaled (1:2) right- angle to the rise and going;
then by forming a hypotenuse to the extremities of
these lines, thus enabling the pitch to be measured
with a protractor (preferably the adjustable, set
square type). With a 1:2 scale, this can be done on
A4 size paper.
Of course, if you have a preference for trigonometry
and a scientific calculator, the pitch angle equals the
opposite (say, a rise of 192mm) divided by the adjacent
(say, a going of 237mm), all multiplied by the shift
tangent. Such calculating results in a pitch angle of
39.01°. My preferred method with a scaled triangle
produces a pitch angle of 39° – close enough to the
trigonometry and acceptably below the regulatory
maximum of 42°.

Graphic illustration of basic design

Figures 6.31(a)(b): These cross- lined elevational grids
serve to illustrate graphically the simple mathemati-
cal relationship that exists between the division of
total rise (TR) and total going (TG), of (a) a straight-
flight stair and (b) a half- turn stair. Note that as
illustrated in the two- flight, half- turn stair, instead
of one less going than the number of risers, there must
be one less going than half the number of risers. Note
also that the stairs illustrated here are visually more
representative of concrete stairs than wooden stairs –
this is because the simpler appearance of concrete
stairs seemed to make my graphs more understand-
able – but the theory of TR- and TG- division is the
same.

TR

14

TG

13

Figure 6.31 (a) 14 risers produce 13 goings (because

the top landing is excluded).

TR

14

TG

6

Figure 6.31 (b) This design of a half- turn stair with 14

risers must have one less going than half the number of

risers.