Spacing for Balusters
Balusters are the vertical pieces running between the top rail and the riser on stairs, or the porch and a railing. Whereas the posts are the structural element of a railing, balusters usually serve a more decorative purpose, although they do provide support for the top rail.
The proper amount of space between balusters on a railing will depend upon a number of factors. The thickness of the balusters will influence their spacing, as will the purpose they serve. If they are purely ornamental, you can let your eye be the judge of the right amount of space; however, if they provide structural support, or are designed to prevent pets or children from slipping through, those needs will determine their spacing.
Some code requirements stipulate that balusters must be 4-inches apart for stair building. Porches may be different, and there may be some deviation on certain construction. The width of the balusters will partly influence how far apart they sit. 6-inch balusters may be placed 6-inches or more apart, while 2-inch balusters may be only 4-inches apart. Much will depend upon how it looks and where they are located. If they are intended to provide support and prevent small children from going in between them, 4-inches apart is a good general guideline. They serve dual purposes on a stairway.
If you have obtained the permits necessary to rebuild stairs or a porch railing, check with the regulatory agency in your state for exact spacing requirements.