The residential building code (R312.1.3) limits the maximum space allowed between openings in guardrails. These areas include; the space between the bottom of the rail and walking surface, as well as the space between balusters. The size of any of these spaces must be small enough to prevent the passage of a four inch sphere.
The code contains exceptions which allow for larger openings at two specific locations. The opening size on stairs can be increased by 3/8 inch to 4 ⅜. The second location is where a triangular space is formed between the bottom of the guard and stair risers. This space cannot allow the passage of a six inch sphere.
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