How do I build steps down from a deck?

Working out equal risers from the total drop, cutting or buying stringers, and founding and fixing the flight so it cannot move.

Difficulty: intermediate
Time: 5 hr
Read: 6 min
Safety: warning

Short answer

Measure the total vertical drop from the deck surface to the finished ground, divide by a whole number of risers to get an equal rise of around 150 to 190mm, and set the going to suit — a deeper going with a shallower rise is more comfortable. Every riser identical. Found the bottom on a concrete pad, fix the top to the deck frame, and never let the stringers stand on soil.

Deck steps go wrong in two places: the arithmetic, and the bottom. The arithmetic produces a final step that is a different height from the rest, which is what people trip on. The bottom produces stringers standing on the ground, which rot within a few years and drop the flight out of true. Both are avoidable with an hour's thought before any cutting.

Safety

Steps with unequal risers are a genuine trip hazard — people's feet learn the first riser and expect every one after it to match. Keep every rise identical to within a few millimetres. A flight of steps may need a handrail and a barrier under building regulations depending on the drop and the number of risers; check with building control before you build.

What you'll need

Stringer timber, 225mm or deeper, treated
Tread timber — decking boards or purpose-made treads
Concrete for a bottom pad
Structural screws or coach bolts, exterior rated
Framing square and a tape
Circular saw and a handsaw for corners
Spirit level and a straightedge
Galvanised stringer hangers or angle brackets (optional)

Setting out deck steps — rise, going and the bottom stringer cut

Step by step

Measure the total rise accurately.From the finished deck surface to the finished ground level where the bottom step will land — allowing for the pad you are about to build, not the soil you can see. Use a long straightedge from the deck out to a peg, with a level on it.
Divide the rise into equal risers.Divide the total by 2, 3, 4 and so on until you get a rise in a comfortable range — roughly 150 to 190mm for a domestic garden flight. Take the whole number that lands closest to the middle of that range. Every riser is then that dimension exactly.
Choose a going that suits the rise.A deeper tread with a shallower rise is easier and safer to walk, and garden steps can afford to be more generous than internal stairs. Around 250 to 300mm of going is comfortable. Keep every going the same as well.
Set out the stringer and check before cutting the second one.Mark the rise and going with a framing square along the stringer, then cut one and offer it up dry against the deck. It is far easier to correct a set-out error at this point than after both are cut.
Remember to drop the bottom of the stringer by one tread thickness.The tread sits on top of the stringer's cut, so the bottom notch must be shortened by the tread thickness or the bottom riser ends up too tall. This is the single commonest error in a step flight.
Build the bottom landing pad first.Excavate to firm ground and cast a concrete pad, or bed a slab on compacted sub-base, set level and sized so the stringers land fully on it. This is what stops the flight settling and rotting.
Keep the stringer ends off the pad surface.A strip of DPC under the stringer, or a galvanised stringer shoe, so the end grain is not standing in the water that runs down the steps. End grain in contact with a wet slab is where deck steps rot.
Fix the top of the stringers to the deck frame.To the rim joist or a bearer, with structural screws or bolts through a bracket or a hanger — not toe-screwed into the boards. The top connection carries the whole flight.
Fit the treads with a gap and a small overhang.Two boards per tread with the usual 5 to 6mm gap for drainage, a small nosing overhang at the front, and two fixings per stringer per board. Check each tread is level across, and that water runs off rather than sitting.
Add a handrail if the flight needs one.More than a couple of risers, or a drop alongside, and a handrail and a side barrier become both sensible and often required. Fix the posts to the stringers or the pad, not to the treads.

Tips

Pre-cut stringers are widely sold and they take the arithmetic out of the job — but check the rise they are cut to actually divides your drop before buying.
Three stringers rather than two on anything over about 900mm wide. Two stringers on a wide flight gives bouncy treads.
Make the flight wider than you think. A 600mm-wide garden step is uncomfortable to use with anything in your hands; 900mm to 1m feels right.
Fit anti-slip inserts as you lay the treads. Retrofitting them into a laid tread is fiddly and looks it.

Common mistakes

Unequal risers, especially at the top or the bottom — People walk stairs by rhythm, not by looking. A single riser out by 20mm is what catches a foot, and it will keep catching feet for as long as the steps exist.
Standing the stringers on soil or on loose slabs — The end grain wicks water, the ground settles unevenly, and the flight both rots and twists. The bottom of a step flight is the wettest timber on the whole deck.
Toe-screwing the stringers to the deck boards — The whole load of the flight and everyone on it goes through that connection. It needs to reach the frame.

If it doesn't work

The bottom step is noticeably shorter than the rest

Cause: The tread thickness was not deducted from the bottom of the stringer. — Fix: Either pack the pad up by the tread thickness, or take the stringer bottoms off and re-cut. Packing the pad is usually easier and is a legitimate fix if it is done in concrete rather than with a slab laid loose.

The steps have twisted or dropped at the bottom

Cause: The stringers are standing on soil or on a pad that has settled. — Fix: Jack the flight, cut out any rot, cast a proper pad on firm ground, and reset the stringers on DPC or shoes.

The treads are slippery

Cause: Algae on a shaded flight — steps stay wetter than the deck because water runs down them. — Fix: Anti-slip strips or inserts on every tread nosing. On steps this is worth doing as standard rather than waiting for someone to slip.

Questions people ask

What rise and going should I use?

For a domestic garden flight, a rise of roughly 150 to 190mm and a going of 250 to 300mm walks comfortably. Building regulations set limits for stairs in and around dwellings, and a deck flight forming a main access may fall under them, so confirm with building control if this is the route to a door.

Can I use decking boards as treads?

Yes, and it is the normal approach — two boards per tread with the usual drainage gap. Use the same board as the deck for appearance, and add anti-slip inserts, because steps get considerably more slippery than the flat deck.

Do the steps need a handrail?

Beyond a couple of risers, a handrail is both sensible and frequently required, and a flight alongside a drop also needs a barrier. The thresholds are set in the building regulations and differ across the UK nations — ask building control about your flight.