UK building regulations for decks dictate a maximum gap between spindles of 99 mm (3 57/64 inches). So, builders must calculate the spacing of spindles precisely. Maximum gap size codes, designed to prevent children from getting their heads stuck between spindles, also dictate the number of spindles to be used in a length of deck rail. The challenge begins because the width of the spindles must be considered in the spacing. Using a mathematical formula will help streamline the process of spacing spindles evenly while adhering to building codes.
Number of spindles
Measure the length of the deck rail between posts.
Subtract the maximum gap size -- 99 mm (3 57/64 inches) -- from the length, and divide the length of the rail by this number.
Round the quotient up to the nearest whole number. This number is how many spindles you will need to conform to the building code. For example, if the rail is 2.65 m (104 inches) long, the gap space code is 99 mm (3 57/64 inches), and the spindle width is 4 cm (1.5 inches), the formula would be (265 - 9.9) / (9.9 + 4) = 255.1 / 13.9 = (104 - 4) / (4 + 1.5) = 18.35. Round up to 19; you will need 19 spindles.
Calculate the space the spindles will occupy by multiplying the spindle width by the number of spindles. Subtract this answer from the rail length to get the amount of gap space. In this example, the formula would be 19 x 4 = 76; 255.1 - 76 = 179.1 cm.
Calculate the spacing between spindles by dividing the amount of gap space by the number of spindles. For example, 179.1 / 19 = 9.4 cm.
Starting at the end post of the rail, measure the gap space and attach the first spindle there. Measure the same distance from the first spindle for the placement of the second spindle. Repeat this process until all the spindles are installed.
Use a number for your gap space that is slightly less than the building standard (100 mm). That way, if any shrinking occurs in the spindles because of cold weather or ageing, the code is maintained.