The spacing between balusters on a deck railing is governed by building regulations. In the UK a 100 mm wide sphere must not be able to pass between two spindles. Thus, the maximum gap in a balustrade is 99 mm. This rule applies equally to stair and terrace balustrades. This method determines the baluster spacing for each section of railing. Using this method, the spacing between balusters and rail posts will be the same for each section but the spacing will vary from section to section.
Measure the distance between two rail posts and record the measurement. Measure the thickness of one baluster.
Add the baluster thickness to the maximum space allowed between balusters to obtain the baluster allowance. Example: 5 cm x 5 cm (2 x 2 inch) balusters have a width of 5 cm. Assuming a maximum space of 9.9 cm (4 inches) then 5 + 9.9 = 14.9 cm (5 55/64 inches).
Divide the post spacing by the baluster allowance, rounding the result up for the number of balusters required. Example: Post Spacing = 1.8 m = 180 cm (72 inches). 180 / 14.9 = 12.08. Round this up for 13 balusters.
Multiply the number of balusters by the baluster width and subtract the result from the space between posts to obtain the total baluster spacing. Example: 180 - (13 x 5) = 180 - 65 = 115 cm (45 9/32 inches).
Divide the total baluster spacing by the number of balusters plus one for the space between each baluster. Example: 115 / (13+1) = 8.21 cm (3 15/64 inches) of space between each baluster and between each post and baluster for that rail section.
Plan your deck so that most rail sections are the same length. For the remaining sections, keep the baluster spacing the same and let the space between posts be different. This will give the most balanced appearance; just make sure the space between posts and balusters is less than the maximum space allowed for those sections.
Be sure to check planning requirements for baluster spacing and other deck building requirements before starting construction or ordering materials.