To build deck railings, cut the length of one 4×4 post about 30 inches. On some decks, you can use 4″x4″s or larger posts running continuous from railing to footing, supporting both the structure of the deck and railing.
Attaching both upper and lower railings with the 2×4 posts, as shown in Figure [D], uses two three-inch deck screws on each end. Measure up both posts to height of the rails, less 1 1/2 inches, and mark the posts. Make the 2 by 4s flush to the post faces on the railing side of the posts.
Measure up from the marks on the upper posts side closest to the deck, and the opposite side from the lower posts. Measure from the top and bottom posts to your handrail height, then mark it.
Use a 2 by 12 board as your reference for marking a line on top and bottom of your stairs. Repeat this process to mark a cut-rail and bottom trim rail assembly where a lower section of rail overlaps with a joist on the bottom rung of the staircase. Mark the angle and length of the end of the rail where it intersects with the face of the post.
With the lower section of rail still cranked into place at the bottom of the tread nosing, read the angle of rake using the Speed Square. Line up the rail segment against the top of the tread nosings to determine the angle. Find the angle and length of the lower and upper rails by placing both pieces together at the tops of each tread, marking where they meet each rung.
Make sure that the bottom rail is at least 1 inch above the tip of the staircase nosing, and the rail is parallel to the sloping line created by the tread nosings point. Note that the gaps between the lower railing sections of the deck and the stairs should not exceed 6 inches. Assuming the deck rails are already installed in the deck surface, measure the gap between the lower deck stair rail posts and those on the deck surface that are near the stairs.
I like to drill pre-drilled pilot holes into stair railing posts so they do not split open while installing the 3-inch deck screws. Sturdy deck railings are a crucial safety feature for raised decks, and are required by code on any 30 in. Building code requirements for railings and stairs on outdoor decks are particularly strict, as a well-built deck can help prevent serious injuries.
The International Residential Code (IRC) and International Building Code (IBC) both state that you must have handrails installed on any stairs used for entering or exiting the home. Whether you are building a composite deck, wooden deck, or deck made from another type of decking material, it is generally required that a grab-resistant secondary handrail should be installed at the side of a stairway rail in the deck if it has four or more risers. You do not require handrails on either side, but if you have more than 30 stairs, then a handrail is likely necessary.
Secure railing posts to other railings, and run the posts side-to-side and front-to-back flush, using a post level or 4-foot-long hose. Secure a crown joist via the bottom of the 2×4 or 2×6 into the side of the lower stair railing post. Note the top cap ends in a 4×4 rail post that is at the same height as your deck rail. Transfer the angle of rake onto the scrap rail piece to work out where the cap stands, when the cap overlaps the lower post.
Position the post such that the basecap also sits well on the staircase, approximately 1.5 inches away from the edge of the concrete. Mark the base of your railing mount The railing mount is marked by lining up the square on the post face, where it crosses over the top edge of the lower railing piece. Points (in Figure [C]) at the front and rear of the lower posts, as well as at the front of the upper posts, where the posts intersect with the top of 2×4 railing material (Figure [C]).
A common detail is the toe nails from the top of the rails and from the 2×4 rails on both sides of the upper posts. I used 2x stock for hanger boards, and where possible, attached hanger boards to the backs of deck posts, which increases strength in the railings, as well as helping to hold up the hanger boards. This strangely shaped stock helps to make sure that you are making your angle cuts correctly, and gives you a guide to where you should position your upper and lower rails to ensure that they are in the correct location, which allows you to get a safe, solid railing.
Attach your railing to your banisters using the staircase brackets. Lay down the lower railing over the top of the stairs to establish proper angles. If you are running the vertical line upwards of that sloped line, then the top rail should be at least 34 inches, and at most 38 inches, above the line connecting the tips of the staircases treads.
The lower posts are the hardest posts to secure securely when building an outdoor staircase railing. Family Handyman Decks and outdoor stairs can develop wobbly railings, usually because of the shaky bottom post. Few older deck railings meet this requirement, but building inspectors and professional deck builders are starting to take greater care with this crucial detail. Carpenter Nathan Gilbert built his deck staircase railings from various sizes of cedar lumber, including 2x4s for top and bottom rails, 4x4s for posts, and 2x2s for balusters.