A reader's Stairs question: Stair riser gap?

Peter B.   from   Gold Coast, Australia   had this Stair riser gap question.

As I understand stair design, the BCA requires that the vertical gap between the top of one tread and the underside of the tread above (in an open stair) must not permit a 125mm ball to pass between the treads.
What type of obstacle would you recommend to comply with this requirement?

Bill's reply

Hi Peter, This is a real pain, you would have to say that 99% of stairs of this type already built, don't comply with this rule. A typical 40 thick tread and a fairly average rise of 175 gives a gap of 135. Just 10mm too much.

  • The only method that I have used for this is to screw a timber batten, 20 to 35 thick to the underside of the treads. Set back 25 to 35 from the edge of the tread. That is in a similar position to where a vertical riser would be.
  • We have done this and only nailed the battens on, knowing that the owner would remove them after the final inspection.
  • With external precast concrete steps at 50 thick, we have just tried to keep the rise to no more than 165.
  • I once considered using cut lengths of 8 or 10 SS rod in a stair that had SS balustrades, but in the end we still put the batten under the treads. Just simpler and people just don't notice them all that much.

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