Handrail & Baluster Spacing
Baluster counts and gaps for guards and stairs — with the three different sphere rules, and why thinner balusters need more of them.
Guard, handrail and the checks that go with them
Thinner stock needs more of it
The regulated dimension is the gap, so a slim baluster uses up less of each pitch. Same run, same rule, every stock size.
Three sphere rules, not one
Everyone knows the 4″ sphere. The code writes two exceptions for stairs. A guard generally must not pass a 4″ sphere. The open side of a stair is allowed 4⅜″, because a raked rail geometrically cannot hold a level rail's spacing without a baluster count nobody would build. And the triangular opening formed by the riser, the tread and the bottom rail is allowed a full 6″, because it is bounded on three sides. Applying the strictest everywhere is safe and costs real money; applying the loosest everywhere fails inspection.
And the limit is on the gap, not on the spacing. What may not pass is the clear opening between balusters — so the on-centre spacing you're allowed is the gap limit plus the stock width. At 4″, a 1½″ baluster may sit 5½″ on centre while a ¾″ one must sit at 4¾″. Which produces the result nobody expects: over ten feet, ¾″ balusters take 25 and 2″ balusters take 20. Thinner stock needs more of it, so picking a slimmer profile to save money often doesn't.
- A guard is triggered by a drop, not a step count. More than 30″ above what's below, measured within 36″ horizontally. One tall step off a deck can require a guard where three short ones don't.
- Handrail height is a band, not a minimum. 34″ to 38″ from the nosing line — set too high it fails as surely as set too low.
- Graspability is a separate test from height, and it's where finished stairs most often fail. A wide flat rail with no finger recess meets neither Type I nor Type II, however handsome it looks.
- On a raked rail, divide the length along the rake. Balusters stay plumb while the rail slopes, so using the level plan distance under-counts them — and the shortfall shows up as one oversized gap at the top.
- Spacing compliance isn't strength. How the guard is fixed to the deck rim is separately regulated, and it's what actually fails under load.
How to use
- Measure the clear run between posts — along the rake if it is a stair.
- Pick your baluster stock and where the guard is, since the sphere limit changes.
- Read the count and the resulting gap, and check the margin before cutting.
- Open the second panel for the guard, handrail and graspability checks.
Frequently asked questions
What is the 4 inch sphere rule?
A required guard must not have any opening that would let a four inch sphere pass through it, from the walking surface up to the guard height. It is written as a sphere rather than a dimension because an opening can be irregular, and what matters is whether a small child’s head could pass. That is also why the test is passage rather than measurement: an opening of exactly four inches fails, because the sphere would go through.
Is the sphere always 4 inches?
No, and this is the part people miss. The code writes two exceptions for stairs. The open side of a stair is allowed a 4⅜ inch sphere, because a raked rail cannot hold a level rail’s spacing without a baluster count nobody would build. And the triangular opening formed by the riser, the tread and the bottom rail is allowed a full 6 inches, because it is bounded on three sides. Three limits, not one.
How far apart should balusters be?
The question is slightly wrong, and that is the useful part. The code limits the GAP between balusters, not the spacing of them — so the on-centre spacing you are allowed is the gap limit plus the width of your stock. At a four inch limit, a one and a half inch baluster may sit five and a half inches on centre, while a three quarter inch one must sit at four and three quarters.
Do thinner balusters mean I need fewer?
No — you need more, which surprises almost everyone. Because the gap limit is fixed, a slim baluster uses up less of each pitch, so more of them are required to close the same distance. Over a ten foot run at a four inch limit, three quarter inch balusters take twenty-five while two inch balusters take twenty. Choosing a slimmer profile to save money frequently does not.
When is a guard required?
Wherever a walking surface is more than thirty inches above what is below it, measured within thirty-six inches horizontally. It is a drop that triggers it, not a step count — so a single tall step off a deck can require a guard while three short ones do not. The horizontal measurement matters too: a surface only slightly raised but adjacent to a stairwell can still need one.
How high should a handrail be?
Between 34 and 38 inches, measured vertically from the sloped plane touching the tread nosings rather than from any individual tread. It is a band rather than a minimum, which catches people out: a rail set at 40 inches fails as surely as one set at 32. A handrail becomes mandatory once a flight has four or more risers.
How high does a guard have to be?
Thirty-six inches under the residential code, and forty-two under the commercial one. That six inch difference catches people converting a house into flats or building a commercial deck to residential habits, and it is not a detail that can be corrected cheaply once the posts are set. Guard height and handrail height are separate requirements, and on a stair you frequently need both.
What makes a handrail graspable?
A round rail passes if it is between 1¼ and 2 inches across. A shaped one passes if its perimeter is between 4 and 6¼ inches and no cross-section dimension exceeds 2¼. Anything larger is a Type II profile, which is only permitted with a graspable finger recess on both sides. A wide flat rail with no recess meets neither type, and it is the commonest failure on an otherwise finished stair.
How do I space balusters on a sloped stair rail?
Measure the run along the rake rather than in plan, because the balusters stay plumb while the rail slopes. The opening that matters is still the horizontal clear distance between them, but the length you are dividing is the longer sloped one. Using the level plan distance under-counts the balusters, and the shortfall always shows up as one oversized gap at the top of the flight.
Can I just space everything at 4 inches to be safe?
You can, and it will pass, but understand what it costs. Four inches on centre with one and a half inch stock leaves a two and a half inch gap — far tighter than any of the three rules requires — and it means roughly a third more balusters than you need across the whole job. On a long deck that is a real amount of material and labour spent on nothing.
Does passing the spacing rule make a guard safe?
No. Spacing stops a child passing through it; it says nothing about whether the guard stays attached when someone falls against it. Load requirements on guards and handrails, post attachment, and how the assembly is fixed to a deck rim are separately regulated, and that is where guards actually fail. A spacing-compliant guard lagged through a rim joist can still come away under load.
Which code do these numbers come from?
The residential code as commonly adopted, with the commercial guard height given alongside for comparison. Editions differ and jurisdictions amend them, so check what yours has actually adopted before building — particularly the guard height, which is one of the more commonly amended figures, and the triangular opening exception, which not every inspector applies the same way.
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