Quote of the Day
Act as if what you do makes a difference. It does.
— William James.
I am about to begin some construction on my cabin in northern Minnesota, and the design of the stairs has been on my mind. I consider well-designed stairs to be both beautiful and functional. A major factor in making a stairway beautiful is its balustrade – a railing with supporting spindles (aka balusters). The balustrade for a stairs is often referred to as a banister.
While I have discussed stair design in a number of other posts (here, here, and here), I have never discussed the design of the balustrade. Since Fine Homebuilding magazine had a short, but interesting article this month on a formula for designing a balustrade, now is a good time for a post on balustrade design.
My plan here is to present the Fine Homebuilding formula and then to augment the formula for use in my stair-oriented balustrade application. My Mathcad source and its PDF are here.
Figure 2 shows the commonly used terms for the parts of a balustrade. The balustrade can be horizontal or part of a stairway.
Figure 3 shows the variables that I will be using for my balustrade exercise. The Finehomebuilding article uses different variable names (e.g. A, B, C), and I wanted something a little easier for me to remember.
Equation 1 is my form of the formula presented in Fine Homebuilding. I altered the formula to reflect the terms that I normally use in my engineering life (i.e. length, pitch, spacing, width). I also incorporated the ceiling function, which is symbolized using .
- L is the length of the balustrade.
- SMax is the maximum baluster spacing the building code allows for balusters.
- P is the center-to-center spacing of the balusters, which is the actual pitch.
- W is width of the balusters.
Figure 4 shows my derivation of the formula. Note that the code allows a maximum spacing (SMax) between balusters, which drives the minimum number of balusters – you can add more. The variable M represents the number of balusters plus 1. The derivation assumes that the number of balusters is made large enough to ensure that the baluster spacing is less than or equal to the code's maximum allowed value.
Equation 1 applies to both horizontal and stair balustrades. In the case of stairs, the length becomes the length of the horizontal projection of the balustrade length. Figure 5 shows my calculations for the baluster pitch assuming a stair balustrade with the following characteristics:
- L = 50 in (horizontal length)
- θ = 30° stair angle
- W = 2 in (baluster width)
- SMax = 4 in (code specified maximum spacing)
Figure 5 shows the calculations.
Figure 6 shows a scale drawing of the stairs. The scale drawing and the calculations agree. Figure 6 includes the term "phantom block", which is used in the Fine Homebuilding article. The term refers to treating the newel posts like the zeroth and last baluster.
I have never built a balustrade, but I plan on building one next summer. I anticipate using this formula for that build.