Guard Baluster / Picket Count (4-in Sphere Rule)
Balusters on a guard or fence spaced so a 4 in sphere cannot pass.
Example
You enter
- Clear span between posts (in) 96
- Picket width (in) 1.5
- Maximum clear gap (in) 4
You get
- Pickets 17
- Gaps 18
- Actual clear gap 3.92 in (meets the IRC 4 in sphere rule)
Details, formula, and sources
pickets = ceil((clear span - max gap) / (picket width + max gap)); actual gap = (clear span - pickets x width) / (pickets + 1). A 96 in span with 1.5 in pickets holds 17 pickets at a 3.92 in gap; 3.5 in wide pickets take 13 at 3.61 in. The clear span and picket width both drive the count. The guard height and load are checked separately.
pickets = ceil((rail_clear_in - max_gap_in) / (picket_width_in + max_gap_in)); gaps = pickets + 1; actual_gap_in = (rail_clear_in - pickets x picket_width_in) / gaps.
Guard baluster spacing by name (the IRC 4 in sphere rule); first-principles equal-spacing arithmetic.
The equal-spacing geometry is public first-principles; the 4 in sphere limit is the code rule, stated by name.
Estimate. AHJ and licensed professional govern.
Field names used by the API: rail_clear_in, picket_width_in, max_gap_in, pickets, gaps, actual_gap_in
- Sphere rule 4 in maximum guard opening (IRC R312.1.3); 6 in at the stair triangle, 4 3/8 in below the railIRC
- Equal spacing pickets and gaps evenly distributed across the clear spanfirst principles