I wish I had a pound for every time I have had to explain the workings of this Building Regulation requirement that is much loved by Local Authority inspectors. Long ago someone on the Building Regulation committee (probably after a bad night) came up with the idea that little children could get limbs caught in the gaps between the guarding rails or the open risers of stairs. "Fair enough" you say but who really cares "that’s what children are there for" I hear you cry. Well this chap did care, and he came up with the devious idea that it should be a rule that no gap on a stair should allow the passage of a 100mm diameter sphere. Just to muddy the water a little it was further decided that this rule need not apply to stairs in buildings which are not likely to be used by children under 5. Quite who decides which buildings this applies to, is confusingly not mentioned in the Regs. As you are probably now aware I am no fan of this rule, it seems to me to create a problem that probably never existed, and then made a pig’s ear of solving it. However who am I to say, you will rarely meet an inspector who will give you leeway on the 100mm sphere rule. You have been warned, you might have seen lovely minimalist modern open tread, non balustrade stairs on Grand Designs, but forget them, they will be a non starter with the inspector. The only safe bet is a full vertical baluster stair with spacing of less than 100mm.combined with a timber or metal riser bar kit between the open treads if it is an open riser style staircase.