Deck railing installation
Railing fails at the post, not at the rail. Everything else in this guide is straightforward carpentry; the post anchorage is the part that decides whether the railing is a guard or decoration.
Post anchorage
A guard post has to resist 200 lb applied at its top. With the deck surface as the pivot, a 36″ post converts that into roughly 600 lb of pull at the base — on two fasteners, in a 1½″ thick rim joist.
Three ways to make that work:
- Blocking inside the rim. A solid block between the rim and the next joist, sized to the post, gives the fasteners something to bite into that is not a single 1½″ board.
- Through-bolts. Two ⅜″ or ½″ bolts through the post and the blocking, with washers on both sides. Bolts carry the load in shear; lag screws rely on thread engagement in wood that shrinks.
- Manufactured post base. A steel bracket with a specified screw pattern, screwed into blocking. Fastest, and the load rating comes with the product.
What does not work: two lag screws through the rim only, or a post bolted to the decking. Both are common and both move within a season.
Post layout
Divide the run into equal bays, then add one for the posts — eight bays means nine posts. Corners get their own post, and each gate opening needs a post on both sides.
Mark the post centres, not the edges. A post that is set ½″ off centre shifts every baluster in the bay.
Rails
The top rail caps the posts and the bottom rail sits 2–4″ above the deck. Both have to be fastened to the posts, not just to each other.
- Top rail: if it is a cap, it covers the post ends and is screwed down through the top. If it is between posts, it needs a bracket or a housed joint at each end.
- Bottom rail: the gap under it is part of the 4″ sphere rule. If the rail is 3″ above the deck, the gap is compliant; if it is 5″, it is not, and you need a kick board or a lower rail.
Baluster sequence
Balusters are installed after both rails, and the spacing is solved per bay rather than measured from one end:
- Measure the clear span between posts.
- Solve the number of balusters for a gap of 3.75–3.9″ using the spacing calculator.
- Mark the centre of each baluster, starting from the middle of the bay and working outward, so any fractional error is split between the two ends rather than accumulated at one.
- Fasten through the bottom rail first, check plumb, then fasten the top.
Working from the centre outward is the difference between a bay that looks even and one with a visibly wider gap at one end.
Stair railing
Stair rail is a separate assembly with its own posts at the top and bottom and a rake rail between them. The rail height is measured vertically from the nosing, not perpendicular to the rail, and the rail has to be graspable — a 2×4 on edge is not.
Checking the work
- Push hard on the top of every post, in both directions. Any movement is a failure, not a nuisance.
- Test every opening with a 4″ sphere — a baseball, a purpose-made gauge, or a tape measure across the widest gap.
- Check the height at the highest point of the deck, not the lowest.
What this guide will not decide
- It does not design the guard — see guard requirements.
- It does not cover cable or glass systems, which have manufacturer-specific framing and tensioning.
- It does not cover metal railing, which uses different post bases and rail brackets.
- It does not check local amendments, which sometimes change the height requirement.
Questions people actually ask
How do I anchor deck railing posts?
With blocking inside the rim joist and through-bolts or a manufactured post base. Two lag screws into the rim alone will not resist the 200-pound load the guard must take.
Should railing posts go on top of the decking or through it?
Through it, notched over the rim and bolted to blocking. A post sitting on the decking is fastened only to the boards.
How do I keep baluster spacing even?
Solve the gap for the bay, then mark from the centre outward so any remainder is split between the two ends rather than accumulating on one side.
What height should deck railing be?
36 inches minimum measured from the deck surface, and 34 to 38 inches for a stair handrail measured vertically from the nosing.