How to Install Railings on a Deck

Measure the exact dimension of each railing run. A railing run is a straight section of railing uninterrupted by stairs or other changes in direction. Calculate the number and location of posts and the gap between balusters for each run. Plumb and attach each post with two carriage bolts or lag screws. Stagger the fasteners on both sides of the centre of the post to prevent splitting. Cut notches in post before attaching, if necessary for the railing style. Make notches for rails the same depth as the notches on the rim joist. Fasten each rail to the posts. Drill pilot holes for fasteners at the ends of each rail to prevent splitting.

Mark the slanted top shoulder of a notch for a post that will attach to a stair stringer. Plumb the post against the stringer at its correct height before marking. Cut the notch and install the post. Position one post at each end of a stair run. Install additional posts for long stair runs or to match the railing design. Mark the location for the upper edge of the top rail or for trimming the post top, depending on the railing style. Lay a long straight 2x4 across the stair treads. Measure to the railing height from the bottom edge of the 2x4 at each post to mark the top edge of a stair railing.

Mark the notches for each stair rail in the posts. Hold or clamp the rails in position against the post. Mark cutting lines on the end of the stair rail. End the rail in the centre of the post if it meets a deck rail at the post. Also mark the stair rail notch location on the post.

Mark the notch location for a deck rail that meets a stair rail at a post. The top of a post where stair rail and deck rail meet will be level with other posts for deck railing. Cut the notch in the post. Remove as much of the notch as possible with a circular saw. Then finish removing waste material with a chisel. Mark the intersection angle between the stair rail and deck rail. Place the edge of the deck rail on the decking with one end protruding over the stairs. Lay the stair rail on edge on the treads to cross the deck tail. Mark both edges on both rails where they cross. Connect the marks across the face of each rail. Align a power mitresaw blade with a marked line and cut each rail to length.

Attach the stair rails to the posts. Drill pilot holes at the ends of each rail to prevent splitting. Install each baluster with two fasteners at each end. Clamp several balusters to the rails at the spacing you used earlier to make sure the spacing looks right. If it doesn’t, reclamp the balusters and recalculate their number. Cut a spacer long enough to span between rails and as wide as the gap between balusters. Check plumb every fourth baluster. Attach a 2x6 cap rail across the tops of the posts and balusters. Join sections over a post and with a scarf join. Drive two fasteners into posts. Drive one fastener every 12–16 inches between posts into rail and balusters.

Mitre the ends of the cap rail and drive fasteners both through the corners and into the corner posts. Rough-cut a 2x6 a few inches longer than the finished length of the stair rail and hold or clamp it against the upper stair rail where it meets the post. Mark the bevel in the top end and cut it. Drive fasteners at an angle through the bevelled end of the cap rail and into the post. Drill pilot holes to prevent splitting. 

Attach a graspable handrail inside the stair railing, if necessary. Use brass, galvanized, or other rust-resistant hardware. Sand any sharp edges on the handrail. To fasten a handrail return, mitre the ends of the handrail stock at 45 degrees. Test-fit the pieces before attaching them. 

Install a stair railing post at the top end of an angled stair stringer. Make a space between the post and the deck rail post no wider than the spacing between balusters. Also install the stair railing post at the bottom end of the stringer. Mark the angle for trimming each stair rail. Measure and mark locations for rail height. Temporarily position each stair rail board against the stair rail posts at the marks to mark the trimming line at each end of the board. 

Cut the mitre at both ends of each stair rail. Install the stair rails, balusters and cap rail following the methods just used.