Blocking a treated wood deck post is critically important for making sure your wood railing, composite lumber railing or metal deck railing stays secure for the life of the deck. Blocking/bridging is installed into the side framing mainly to control lateral movement. If you have to run handrail in the direction of the joists, use blocking to create your box and bolt through the post and the blocks. The bridge will help keep the outside joist from deflecting when you lean on the handrail.
Installing a post when the railing runs parallel to the joists
First mark the location of the post with a carpenter's pencil on the interior rim joist. The next step in securing your 4x4 or 6x6 lumber deck post is to measure the distance between your outer rim/band joist and your first interior joist beam. Pre-cut two pieces of pressure treated 2x6, 2x8 or 2x10 wood blocking (often cut from scrap) to a length that will fit loosely between the joists. If the cut pieces have to be pounded into place, cut an 1/8- to 1/4-inch to avoid bowing the joists. Each piece will roughly measure 14½ inches long for joists installed 16 inches on center.
Next, place the treated wood blocking on both sides of the deck rail post markings. Nail the blocking in place with two galvanized nails at each end. Measure, cut and install a small piece of wood blocking as a cross member which completes the box. Make sure this small piece fits into place easily then secure with two galvanized nails on each end.
Fit the pre-cut deck rail post into place while making sure it sits flush with the bottom of the outer rim joist. Tack the post into place with two nails on each side (or use a clamp)
Next, drill ½-inch pilot holes and install two ½-inch galvanized carriage bolts (approximately 8 inches long) and matching washers and nuts (do not over-tighten). The pilot holes should be positioned no closer than one inch from the top and bottom of the joist edge for maximum reinforcement.
Installing a post when the railing runs perpendicular to the joists
This method is similar to the parallel install version except you need to install only one cross-member – as seen in image two.
In both cases use a post level to make sure each post is plumb. Insert a shim if needed prior to cinching the carriage bolts.
By installing the rail post into the framing and then blocking around the post, it makes a for a more sturdy and durable connection, which in my opinion works best.
As always, check your local building codes to make sure this deck post installation method is acceptable in your area.