Q: What is the rule of thumb for laying out balusters?
A: Jed Dixon, a master stair builder and presenter at JLC Live, from Foster, R.I., responds: In my stair building and repair work, I have looked at hundreds of traditional balustrades, built over the last 300 years in New England. I can tell you 99% of them use the same simple rules for spacing balusters, and those rules probably originated much earlier than colonial times—in the Renaissance, or maybe 2,000 years ago in classical Greece. Modern building codes have added a few rules, but these mesh well with the traditional ones.
The first rule is that the balusters should be spaced evenly: The on-center distance between balusters should be the same for the whole stairway (see Baluster Spacing, below). This distance is dictated by the unit run of the stair (the horizontal distance between the face of each riser and the one above or below), which is typically between 10 and 11 inches on residential stairs. The IRC requires a minimum 10-inch run per tread, but Massachusetts and several other states allow runs down to 9 inches, which will make for a safe and comfortable stair if it’s designed correctly.
The distance between the baluster centers must be an even fraction of the run: 1/2, 1/3, or 1/4, depending on the number of balusters per tread. Generally, it looks better to have several balusters closer together, and code requires that no space between balusters be wider than 4 inches. For a 10 1/2-inch run, the balusters would be spaced 3 1/2 inches on-center (one-third of 10 1/2 inches). For a 10-inch run, the spacing would be 3 1/3 inches (don’t panic, your framing square has a 12th scale and 4/12 inch = 1/3 inch).
The location of the shortest baluster on each tread determines the location of the others. This baluster is always located with its front (downhill) face directly over the riser below and its outer face over the face of the face skirt—that is, the outer corner of the baluster should always be lined up directly over the point of the miter joint between the face skirt and the riser. This will always look right. If laid out properly, the longest (or rear) baluster on each tread will have the same spacing between it and the short baluster on the next tread up. It will look like you meant to do it that way. And, the centerline of the handrail will be one-half of a baluster in from the face skirt.
Balusters on the landings (where the railing is level) should have close to the same spacing as the balusters on the stairs. But here you need to use your carpenter’s eye as we think about negative space. In this instance, negative space is the shape made in the air between two turned balusters—like that famous optical illusion where two faces create the shape of a vase. You see it even if you don’t notice it right away. Balusters should be close enough to each other that the negative space between them is apparent. At the top of a staircase, the stair rail and the landing rail usually meet at a newel post, which is much larger than a baluster. The negative space between the newel and the first baluster in the landing rail, and the space between the last landing baluster and the wall must be adjusted to look right. Here’s how you do this using your eye.
Hold two landing balusters the same distance apart as the balusters on the stair (3 1/2 inches, plus or minus) and move them towards and away from the newel post until the negative space between the newel and the baluster closest to it is the same as (or a little smaller than) the space between the stair balusters. Mark the center of the baluster closest to the stair, and mark the same distance at the other end of the landing. Now divide the distance between these two marks by the on-center distance between two stair balusters to find the number of landing balusters. Use a construction calculator to divide the landing distance by the number of balusters for the exact on-center spacing.
