Manual of Purpose-Made Woodworking Joinery

Stair regulations guide 99

Special stairs

Tapered treads

Figures 6.11 and 6.23(a)(b)(c): For a stair with tapered

treads, 2R+G should be measured as follows:

● If the width of the stair is less than 1m, 2R+G

should be measured tangentially on the centre- line,

where the notional curved pitch line touches the

nosings (Figures 6.11 and 6.23(a)).

● If the width of the stair is 1m or more, 2R+G should

be measured at 270mm from each side, tangentially

where each curved pitch line touches the nosings

(Figures 6.23(b) and (c)). The minimum ‘tread- width’

of tapered treads, from the face of a riser to the outer-

edge of the adjacent tread, should not be less than

50mm (as shown in Figure 6.11). Where consecutive

tapered treads are used, a uniform going should be

Pitch

Figure 6.18: The maximum pitch for a Private Stair
is 42°. Note that the recommended pitch angles for
the other two categories of stair are not given in AD
K1. However, using the criteria that are given, if the
maximum rise and the minimum going were used
in these categories, the maximum possible pitch for
Category 2 would be 33° and for Category 3 would be
38°.
Note that if the area of a floor of a building in
Category 2 (Institutional and Assembly Stair) is less
than 100m^2 , the going of 280mm may be reduced to
250mm. Therefore, with the maximum rise and the
minimum going, the maximum possible pitch would
be increased from 33° to 36°.

The 2R+G design formula

Figure 6.22: In all three categories, the sum of the
going plus twice the rise of a step (traditionally estab-
lished and commonly referred to as 2R+G) should be
not less than (NLT) 550mm, nor more than (NMT)
700mm (subject to the separate criteria laid down for
tapered treads).

Rise

Going

2R+G = NLT 550 mm/NMT 700 mm

Rise

Figure 6.22 Graphic illustration of Stair- Design Formula.

Wall string

Stairway

1

2

3

4

Pitch

line

DL

2

Figure 6.23 (a) Deemed length (DL) divided by two to

give a single pitch line (for measuring 2R+G) if the stair-

way is less than 1m wide.

Stairway

270

270 Wall string

Complusory

inner handrail

Long pitch line

Short pitch line

Deemed width line

Outer string

Centre line of compulsory

outer handrail

Centre/radial point

270

270

Newel

post

1

2

3

4

5

6

Figure 6.23 (b) Two pitch lines required for measuring

the minimum and the maximum 2R+G for stairways of 1m

or more in width (deemed length).

NMT 16

risers per

flight

NMT

250 mm

No pitch

regulation

NMT

190 mm

Figure 6.21 Category 3 Stair, indicating the maximum
rise and risers per flight, the minimum going and – again

• an unregulated pitch.