Square top wood balusters are generally installed using a plowed handrail with fillet system. Plowed handrail installation with fillet and square top wood balusters is a relatively easy method that makes assembly simple with a little skill and the right stair installation tools.
Many straight handrails are available with a plow and fillet system. A handrail plow is basically a channel cut out of the bottom of the handrail and the top of the shoe rail that is the exact width of the stair balusters (or spindles) being used. On floor level balustrades the newel posts, shoe rail and stair handrail are set first. Then, the balusters are laid out, cut to length then installed. This is done by using a finish nailer to nail the top edge of the baluster up into the handrail and down into the shoe rail. Then wood fillet pieces are added to equally space and secure the balusters in place. The fillet fills in the plow in between the balusters in the handrail and shoe rail and covers the nail holes. The balusters can also be laid out and fastened down to a shoe plate or hardwood flooring without the use of the shoe rail and fillet system. Using this stair installation method, the balusters are secured down first then the handrail is placed on top of them and the fillet added afterwards to secure and fill the plow.
When installing stair balustrades, there are two methods depending on whether the stringer is open or closed. In case of an open stringer, where the treads are notched and the balusters sit directly on top of the tread caps, the balusters are laid out, cut to length and dowel screwed into the stair tread first. Then the handrail is placed on top and the fillet pieces are added to the bottom of the handrail in the space between the balusters. If the stringer is closed then the balusters can either be screwed to the stringer or finish shoe plate with a plug in the face and then the handrail placed on top and the fillet added afterwards to secure and fill the plow. Or a shoe rail with a plow can also be used and the newel post, handrail, and shoe rail are set first and then the balusters are secured in place with fillet in between.
As I mentioned this is a fairly easy method of balustrade installation that creates a very strong and durable system. Because of its simplicity and beautiful finished look, it is used by both professional stair builders and do-it-yourselfers with little experience and just a little skill.