Ed is a do-it-yourselfer who is happy to share some of his ideas and experiences in this monthly column.
Sooner or later, most of us will face the challenge of building or rebuilding a handrail, along a walkway, a porch, a deck, or something like that. The question I’m always asked is “How do you space the balusters?” in the railing. Obviously, if you’re replacing an original “whatever” and you have something to start off with, you should replicate the original spacing if it meets code. Normally, you must have the balusters spaced so that a 4” sphere cannot pass between them. This is a requirement to help keep things safe.
Now, if you’re not afraid of a little math, read on. (If you ARE afraid of math, read on anyway, as I’ll make things as easy-to-understand as possible.) There are a lot of ways balusters might be attached, including with tenons at the tops and bottoms, or “let in” through the sides of stairs. I’ll demonstrate the spacing concept using railing for a deck I recently built.
First, think about the general layout. There will always be a post at either end, then a combination of spaces and balusters between them. There will be one more space than balusters if spaces are left next to the posts, or one more baluster than spaces if balusters are next to the posts, the way I like to do things. Study this picture for a second or two, and you’ll understand what I’m talking about.
Now, let me also say that you can purchase materials with pre-cut and spaced notches into which the balusters rest. That puts the spacing exactly the same in the middle of the “run” but it will always be wrong at the ends, and doesn’t account for any variability in the distance between post groups. I never use those. I’ve also seen folks use a marked elastic band, simply stretching it between posts as needed, then marking the rails to match the lines on the band. Very ingenious! My method relies on the simple math I spoke of above.
Since the spacing between posts can’t exceed 4”, I like to be a bit on the safe side, and make the maximum spacing just a hair less than that. Trust me, your code inspector will thank you for it. (You did check if a building permit is needed for your job, didn’t you?) To keep the math simple, it’s easiest to use the metric system, believe it or not!
Many tapes have inches marked on one edge and centimeters on the other, and that’s what I like. 4” is a tiny bit more than 10 cm, so I use 10 cm as the maximum spacing between balusters. Using my deck railing above, with balusters attached to the posts at each end, makes for a nice-looking, strong frame. The upper and lower rails can attach both to the posts and to the end balusters. Where folks commonly make a mistake is measuring the space left between the end balusters. NO! You need to think of a “space-baluster” as a unit. So, measure from the right edge of one end baluster as a starting point, to the right edge of the other end baluster, as the ending point, thus capturing the number of space-baluster units between those 2 points. Let’s take a closer look. In the picture below there are 10 space-baluster units between those 2 points.
Using the 2 endpoints shown above, let’s assume the distance between them is 153.5 centimeters. (Measure both top and bottom and average them if you need to, but they really should be the same if you’ve done things correctly so far! If way off, you should adjust them or run the calculations below separately for the top and the bottom.) Measure the width of a baluster. Let’s assume that it’s 3.5 cm. That means the maximum size of a space-baluster unit is 10 cm + 3.5 cm = 13.5 cm. Let’s take the actual measurement (153.5 cm) and divide it by the maximum space-baluster unit width of 13.5 cm, and we’ll get 11.37 space-baluster units. You can’t have a fraction of a unit, so you must round UP to 12 units. Go back and divide the actual measurement (153.5 cm) by 12 units and you get 12.78 cm per space-baluster unit. Since the width of the baluster was 3.5 cm, subtracting that width from the space-baluster dimension leaves just the width of the space. 12.8 (close enough) – 3.5 = 9.3 cm. Now cut yourself a carefully-measured piece of wood 9.3 cm in length, and as you fit each baluster into place, bump the spacer between it and the next one, both top and bottom. The spacing will be exactly right and uniform across that length of railing! Do these calculations between each section of railing as the post-to-post distances will likely vary. Each section might have slightly different between-baluster spacing, but they will be uniform within each section, and no one will notice any discrepancies!
I hope all your projects go well. Thanks for reading, and happy restoring!