Design and Construction ~ because of the legal requirement of not
having a gap between any two consecutive treads through which a
100 mm diameter sphere can pass and the limitation relating to the
going and rise, as shown on the previous page, it is generally not
practicable to have a completely riserless stair for residential
buildings since by using minimum dimensions a very low pitch of
approximately 27^1 = 2 ° would result and by choosing an acceptable
pitch a very thick tread would have to be used to restrict the gap
to 100 mm.
Timber Open Riser Stairs