What height and spacing does a deck balustrade need?

Deck balustrades fail in one of two ways. They are built to look right and are too low or too open to do their job, or they are built to the right dimensions and fixed to the deck boards, which pull out under the first real load. The dimensions come from the regulations; the strength comes from the posts.

Safety

Step by step

1. Establish whether you need one at all.Building regulations require a barrier where there is a drop above a threshold height, and the threshold and the required barrier height differ between a domestic deck, a balcony and a stair. Your local building control team will tell you which applies to your deck in a phone call, and that is a far better source than a forum.
2. Set the height from the finished deck surface.Measure from the top of the deck boards, not from the joist. If you intend to add another board layer or a covering later, allow for it — a barrier that becomes too low when the surface is raised is a common and avoidable problem.
3. Understand the gap rule.The limit exists so a small child cannot get their head through, and it is expressed as a sphere that must not pass through any opening in the barrier — including the gap under the bottom rail, which people routinely forget. Apply it everywhere, not just between spindles.
4. Avoid a climbable pattern.Horizontal rails and wide ledges form a ladder for a small child. Vertical spindles are the safe default in a domestic setting. If you want a horizontal wire or rail design, take advice — the tension and spacing requirements are demanding and it is not a DIY guess.
5. Bolt the posts through the frame.Every post through-bolted to a joist, a bearer or a rim board with two bolts vertically separated, or housed and bolted. Screwing a post to the deck boards, or into the end grain of a joist, gives almost no resistance to the lever action of someone falling against the rail.
6. Space the posts closer than looks necessary.Around 1.2m between posts is a normal domestic spacing. Long unsupported rail runs flex, which makes the whole barrier feel and behave weaker than it is.
7. Fix the rails so they cannot be pulled off.Rails housed into the posts, or fixed with structural screws into the post face, not skew-nailed. The bottom rail should not sit on the deck surface — leave a small drainage gap and check the sphere rule still holds under it.
8. Check it by loading it.Push hard against the top rail at mid-span between posts and at the corner. It should feel completely solid. Movement now is failure later.

Tips

• Cap the tops of posts and rails, or chamfer them, so water runs off. Flat end grain facing the sky is where a balustrade rots first.
• Stainless fixings throughout. A balustrade that fails because the fixings corroded looks identical to a sound one right up until it goes.
• If the deck is at ground level with no meaningful drop, you do not need a barrier at all, and adding a low decorative one that people lean on can be worse than none.
• Glass or mesh infill panels satisfy the gap rule easily and keep a view, but the frame and fixing requirements are stricter — buy a system rather than improvising.

Common mistakes

• Fixing posts to the deck boards — The boards are fixed to the joists with two screws each. A post bolted to them transfers a large lever load into those few screws, and they will pull out.
• Forgetting the gap under the bottom rail — The sphere rule applies to every opening in the barrier. A generous drainage gap under the bottom rail can be the largest opening in an otherwise compliant balustrade.
• Building a horizontal-rail balustrade in a family garden — It is a ladder. Children climb it, and the barrier then delivers them over the top rail rather than stopping them.
• Taking a dimension from a website rather than building control — Barrier heights and drop thresholds are set in the building regulations, they differ between the UK nations and between situations, and they change. Confirm the current requirement for your deck.

Questions people ask