Things You'll Need
Railings protect people from falling off stairs and decks. Building codes require guardrails on stairs and decks that are over 24 to 36 inches off the ground, depending on the municipality. The rails consist of a cap (top) rail and a bottom rail held apart by balusters. The code specifies the maximum space between each baluster, usually 4 inches. The proper spacing prevents a 4 inch sphere from passing between the balusters. Maintaining even spacing of balusters along a length of rail requires simple math calculations and a good tape measure. Check with your local building permit office to verify codes specific to your area.
Measure the distance between the two end posts supporting the top rail.
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Add the width of a baluster to the maximum spacing allowed by your local building code. For example, for a 2 by 2 inch baluster with a maximum 4 inch space, add 1.5 inches to 4 inches, which gives you 5.5 inches. Remember, a 2 by 2 inch picket is actually 1.5 inches wide because the initial measurement is taken before the wood is planed to achieve the ideal surface.
Divide this figure into the distance between the two end posts. For example, if your distance between end posts is 72 inches, divide 72 by 5.5 to get 13.
Round the figure up to get the number of required balusters. In this case the answer is 13 so there is no need to round up, but if you reach an answer such as 14.67, round up to the nearest whole number--in this case 15.
Multiply the number of balusters by the width of one baluster. In this case, 13 multiplied by 1.5 is 19.5 inches.
Subtract the result from the total distance between two posts. For example, 72 minus 19.5 is 52.5.
Divide this result by the number of spaces between balusters (one more than the number of balusters, since you will have space before and after the last one). In this case, divide 52.5 by 14 (13 plus 1) to get 3.75 inches. This is the measurement of the open space between each baluster.
Step back after installing each baluster to make sure the spacing is visually correct.
The most common errors in calculation are to forget to add the extra space before the final division and to forget to use the baluster's actual measurement instead of the raw wood dimension.