Balusters are the vertical supports that form the inner structure of the railings that typically surround some staircases, balconies or porches. Balusters may be turned on a lathe to make spindles or cut from milled timber. Accurate measurement and calculation is necessary to build a safe and attractive structure with evenly-spaced balusters. Sketching the railing and balusters with dimensions is a good way to organise and double-check your figures.
Sketch the rails -- horizontal top and bottom members -- on a piece of paper. Measure the length of the rail, using a tape measure. Label the sketch with dimensions in centimetres. Mark the approximate centre of the rails.
Check local building safety regulations to determine the maximum distance between the narrowest parts of the balusters. Place two balusters parallel on a flat surface with the maximum allowable space between them. Mark the centres of the tops or bottoms of the balusters. Measure the distance between the centres of the balusters to determine the maximum allowable distances between the centres of the balusters.
Measure the width of the balusters. Add the width of the balusters to the desired space width between balusters. Spacing of the balusters depends not only on building safety regulations, but also the desired appearance of the finished project. Plain sawn balusters, cut from 5 cm (2 inch) wood, are often installed with 5 cm (2 inch) spacing. Spindles are typically spaced farther apart to draw attention to the fine detail of the individual spindle and to give a lighter, more airy overall feel to the architectural design. Decorative cut-out sawn balusters are often spaced very closely.
Divide the length of the rails by the width of the balusters plus the desired space between the balusters. Round this to the nearest lower whole number. This number will tell you how many balusters you need to fill the space. If the number is even, the middle of the rail will have a space between balusters. If the number is odd, there will be a baluster centred to the length of the rail. When installing balusters, always start from the centre of the rail and work outwards so the spaces between the end balusters and posts are uniform.
Divide the length of the rails by the width of the balusters plus space. Round this to the nearest lower whole number. This is how many balusters you need. Multiply the number of balusters by the width of the baluster and subtract this number from the rail's length. The difference is the area occupied by the spaces between the balusters. Divide this difference by the number of balusters plus one. The answer you get is the spacing width needed to make all spaces, including the end spaces, uniform.
For faster and easier installation, cut a wood block that is the same width as the desired space between balusters. As you install the balusters, place this spacing tool against the last baluster, then buttress the next baluster against the block.