Old balusters in Alta Vista, how do I check them against the 100 mm test?

The 100 mm (about 4 in.) sphere rule comes from the 2024 Ontario Building Code, section 9.8.8.5: openings in a guard must not allow a 100 mm sphere to pass through. To check your balusters, you need a sphere or a round object that is exactly 100 mm in diameter, and you pass it against every opening between balusters, between a baluster and the newel post, and between the bottom baluster and the floor or the base rail.

A few things to check while you are at it. The guard height rule for inside a dwelling unit is at least 900 mm (about 35 1/2 in.), measured from the floor or the stair nosing to the top of the guard. A guard is required wherever the drop is more than 600 mm. Measure from the floor to the top of the guard.

On older guards, the openings that fail the sphere test are not always the obvious baluster-to-baluster gaps. Check the gap between the bottom rail and the floor, the gap at each newel post, and any decorative cutout in a panel or a turned baluster with a narrow waist.

Section 9.8.8.6 adds a climbability rule: nothing between 140 mm and 900 mm above the floor may form a climbable pattern, unless it can be shown not to be a hazard. Point out any horizontal members in that zone when you talk to a carpenter.

If any opening passes the sphere, or the height falls short of 900 mm, the carpenter will assess whether the fix is new balusters, a new bottom rail, or a fuller rebuild. That scope question is worth settling before any work is quoted, because it changes materials and labour significantly.

When you get a quote, ask the carpenter to break out materials, labour, and disposal as separate line items so you can see exactly what each part of the repair covers.