Start by measuring the total rise, then divide it into equal risers between 150 and 190mm. Every step must be identical — uneven risers are the leading cause of trips on stairs and will fail a building regulations check.
Get that division right and the rest of the job is formwork, concrete and patience.
Measuring the total rise
Total rise is the vertical distance from the finished lower level to the finished upper level. Measure it accurately, because everything else derives from it.
Set a straight edge or a taut string line level from the top landing out over the lower area, then measure vertically down to the finished lower level. Don’t measure along the slope, and don’t measure to bare soil if paving is going down later.
Dividing the rise
Divide the total rise by your target riser height, round to a whole number of steps, then divide again to get the exact riser.
A total rise of 700mm with a target riser of 175mm gives 700 ÷ 175 = 4 steps exactly. Each riser is 175mm.
A total rise of 820mm gives 820 ÷ 175 = 4.69. Round to 5 steps, then 820 ÷ 5 = 164mm per riser. Five equal risers of 164mm, not four of 175 and one of 120.
| Total rise | Steps | Riser height |
|---|---|---|
| 350mm | 2 | 175mm |
| 525mm | 3 | 175mm |
| 700mm | 4 | 175mm |
| 820mm | 5 | 164mm |
| 1,000mm | 6 | 167mm |
| 1,200mm | 7 | 171mm |
Setting the going
The going is the horizontal depth of each tread. UK domestic guidance puts it at 250mm minimum, with 250 to 300mm comfortable for external steps.
The relationship that makes stairs feel right is 2 × rise + going = 550 to 700mm. With a 175mm riser, that gives a going between 200 and 350mm, and 280mm sits comfortably in the middle.
Multiply the going by the number of steps to get the total run, and check you have room for it. A 4-step flight at 280mm needs 1.12m of horizontal space.
Working out the concrete
Solid steps poured against the ground are a stack of boxes: the first step is one riser tall, the second is two risers tall, and so on.
For 4 steps at 175mm rise, 280mm going and 1m wide:
0.28 × 1 × 0.175 × (1+2+3+4) = 0.49 m³
| Steps | 1m wide | 1.2m wide | 1.5m wide |
|---|---|---|---|
| 2 | 0.15 m³ | 0.18 m³ | 0.22 m³ |
| 3 | 0.29 m³ | 0.35 m³ | 0.44 m³ |
| 4 | 0.49 m³ | 0.59 m³ | 0.74 m³ |
| 5 | 0.74 m³ | 0.88 m³ | 1.10 m³ |
| 6 | 1.03 m³ | 1.23 m³ | 1.54 m³ |
Figures assume 175mm rise and 280mm going. The stairs calculator handles any combination, including a top platform.
Sub-base and reinforcement
Excavate to firm ground and lay 100 to 150mm of compacted hardcore under the whole footprint. Steps concentrate load at the bottom, so the base under the lowest step matters most.
Where the steps meet an existing structure, put an isolation joint between them so the two can move independently.
For anything over three steps, put A142 mesh through the body of the steps, and consider dowels drilled into the adjacent structure at the top to tie the flight in. Steps that pull away from a house are a common failure.
Formwork
Two side forms cut to the stepped profile, and a riser board across the front of each step. Use 18mm ply for the sides and 25mm timber for the risers.
Two details make finishing much easier. Bevel the bottom edge of each riser board back at about 15 degrees, so a trowel can reach into the back corner of the tread beneath. And set the risers with a slight backward tilt — around 10mm over the height — which produces a small nosing overhang and helps water run off.
Brace everything heavily. Wet concrete on steps pushes outward and downward at once, and a riser board that bows leaves a permanent bulge.
Pouring and finishing
Pour from the bottom step upwards. Fill and compact each step fully before moving to the next, rodding the concrete to remove air pockets, particularly in the corners behind the riser boards.
Finish each tread as you go rather than waiting until the end. By the time you’ve poured the top step, the bottom one is ready to float.
Give each tread a slight fall forward, around 10mm, so water runs off. Broom-finish across the direction of travel for grip — external steps must be slip resistant, and a smooth trowel finish is dangerous when wet.
Round the nosings with an edging tool. Sharp arrises chip within a season and a chipped nosing is a trip hazard.
Handrails and safety
UK Building Regulations require a handrail on any flight of three or more risers where it forms part of a means of access to a building. Height should be 900mm to 1,100mm above the pitch line.
Plan the fixings before pouring. Handrail posts can be cast in directly, or you can leave a solid area of concrete for resin anchors later — but do not core fill anywhere near a planned fixing position, and do not drill into a step edge where cover is thin.
Where steps are wider than 1m, consider a rail on both sides. Where they are wider than 2m, regulations may require a central rail as well.
Drainage and ice
External steps ice up because water sits on the treads. Two details prevent most of it.
Give every tread a forward fall of about 10mm so water runs off rather than pooling. And where the flight sits in a hollow or against a bank, put a drainage channel at the bottom so runoff from the steps does not collect at the base.
Specify air-entrained concrete for steps in any climate that freezes. Steps take the worst of freeze-thaw because they are exposed on multiple faces and are the first thing to get salted.
Curing and striking
Cover with polythene and keep damp for at least seven days. Strike the riser boards after 24 to 48 hours, carefully — green concrete chips easily at the nosing. Side forms can stay longer.
Keep traffic off for 48 hours, and don’t put heavy loads on the steps for a week.
Common questions
What is the ideal step height?
150 to 190mm for external steps, with 175mm a comfortable target. UK Part K allows up to 220mm for domestic stairs but that feels steep outdoors.
How do I handle an awkward total rise?
Add a step and reduce the riser height. Divide the total rise by the number of steps to get an exact equal figure. Never make one step a different height from the others.
Do concrete steps need reinforcement?
Small garden steps on firm ground often manage without. Anything over three steps, cantilevered, or in a freeze-thaw climate should have mesh through the body and dowels tying it to adjacent structures.
Can I pour steps in more than one go?
Better not to. A cold joint between steps is a plane of weakness right where load concentrates. If you must split, form a proper construction joint with reinforcement continuing through.
How long before I can use new concrete steps?
48 hours for light foot traffic, seven days for normal use. Full strength at 28 days. Keep them covered and damp for the first week.