Railing Code in Washington: Heights, Gaps and the 4-Inch Rule

Most railing arguments on a jobsite come down to four numbers. Here they are, with the section they come from, so you can check us.

Everything below is from the 2021 Washington State Residential Code, which is what applies to houses in Clark County. If you are building anything commercial, you are under a different code and different numbers.

The four numbers

Requirement	Number	Code section
Guard height, open-sided walking surfaces	36 in. minimum	R312.1.2
Guard height, open sides of stairs	34 in. minimum	R312.1.2
Handrail height above tread nosing	34–38 in.	R311.7.8.1
Maximum opening in a guard	4 in. sphere	R312.1.3

Guard height: 36 inches, not 42

Required guards at open-sided walking surfaces — stairs, porches, balconies, landings — must be not less than 36 inches measured vertically above the adjacent walking surface or the line connecting the nosings.

Two exceptions matter in practice:

• Guards on the open sides of stairs may be not less than 34 inches, measured from a line connecting the nosings.
• Where the top of the guard also serves as the handrail on the open side of a stair, it must be not less than 34 and not more than 38 inches.

The 42-inch number people repeat is commercial. It comes from the International Building Code, not the Residential Code, and it does not apply to your house.

Handrail height: 34 to 38 inches

Handrail height, measured vertically from the sloped plane adjoining the tread nosing, must be not less than 34 inches and not more than 38 inches (R311.7.8.1).

There is a useful exception: where fittings or bendings provide a continuous transition between flights, at winder treads, or from handrail to guard, the height is permitted to exceed 38 inches at those fittings. That is what makes a properly detailed continuous handrail legal around a landing.

The 4-inch sphere

This is the one that fails inspections. Required guards shall not have openings from the walking surface to the required guard height that allow passage of a sphere 4 inches in diameter (R312.1.3).

Two stair exceptions:

• The triangular opening at the open side of a stair, formed by the riser, tread and bottom rail, may not pass a 6-inch sphere.
• Guards on the open side of stairs may not pass a 4-3/8 inch sphere.

In practice, vertical pickets land around 4 inches on center. On a horizontal-bar rail the spacing is tighter than people expect. On cable, the sphere rule is not really about cable spacing at all — it is about deflection. A cable spaced correctly at rest can still be pushed apart into a passing gap if the posts are undersized or spaced too far apart. That is the single most common reason a cable railing fails inspection.

Where this actually goes wrong

Almost none of the failures we see are a misunderstanding of the numbers. They are measurement problems.

A stair that is a half inch out of square across a 16-foot run puts the far end of a guard below the minimum even though it was cut to the right height. A landing that was framed 3/4 inch high does the same thing. If the rail was fabricated from a hand measurement taken before drywall, nobody finds out until it is installed and the inspector puts a tape on it.

This is the entire reason we 3D-scan the space with a Trimble X9 before we cut anything. The scan tells us what the stair actually is, not what it was drawn as, so the finished guard height is verified against the real geometry before steel is cut. It is the difference between passing inspection and rebuilding a railing.

The short version

Guards 36 inches, 34 on open stair sides. Handrails 34 to 38. Nothing that passes a 4-inch ball. Then measure the space you are actually building into, not the one on the plans.

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Source: 2021 Washington State Residential Code, sections R311.7.8.1, R312.1.2 and R312.1.3, published by the International Code Council at codes.iccsafe.org. Code adoption changes — confirm the current edition with your local building department before relying on these figures for permitted work.