How far apart can balusters be on a railing?
Openings can't allow a 4-inch sphere to pass through (IRC R312.1.3).
Required guards must be built so that no opening between the walking surface and the required guard height allows a 4-inch (102 mm) sphere to pass. For typical vertical balusters, that means clear spacing of less than 4 inches.
Two exceptions apply on stairs: the triangular opening formed by the riser, tread, and bottom rail may pass up to a 6-inch sphere, and openings in a guard on the open side of a stair may pass up to a 4⅜-inch sphere.
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Need this scoped to your jurisdiction?
Adopted edition and local amendments vary by city and county. Ask Donnie checks your jurisdiction and cites the exact adopted section.
Ask Donnie →General information based on the International Residential Code (a model code). This is AI assistance to verify — not legal advice. Confirm the adopted edition and any local amendments with your local building department (AHJ) and a licensed professional before you build.