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Cast-in-Place Concrete Staircase: Step-by-Step Guide

18/04/2026 15 min read

A cast-in-place concrete staircase is the champion of strength and longevity — it is also one of the most demanding structures to build as a self-builder. Once cast, there is no going back: every defect remains set in the slab for a century. This guide details the complete method for successfully building a cast concrete staircase, straight or quarter-turn: step calculation, waist slab sizing, formwork, reinforcement, mix design, pouring and striking. We get straight to the point, with measurements and dimensions throughout.

Cast-in-place concrete staircase: when to choose it

A cast-in-place concrete staircase (sometimes called a monolithic concrete stair) differs from a prefabricated staircase (delivered in sections) by being built entirely on site: rebar tied in situ, bespoke formwork, concrete poured as part of the structure.

Advantages

Complete solidity: monolithic with the slab, no joints, no vibration, lifespan equivalent to the house itself
Freedom of form: straight, quarter-turn, helical, with or without intermediate landing
Low material cost: concrete + steel for under £250 in materials for a standard staircase
Excellent sound insulation — nothing like a timber staircase
Universal substrate for all finishes: tiling, solid wood glued, resin, polished concrete

Disadvantages

Complex formwork: 1 to 2 days of precise carpentry
Weight: 2 to 3 tonnes for a standard staircase — the arrival slab must be able to support it
Lead time: 21 to 28 days between casting and full striking before use
Irreversible: any calculation error will be settled by a demolition contractor

Tip — Reserve the cast-in-place concrete staircase for enclosed interior staircases or basement stairs. For a visible staircase that contributes to the interior design (designer balustrade, timber string…), a steel or timber staircase will give a lighter result and be less expensive to finish.

Calculating your staircase: Blondel’s rule and the right dimensions

Before any formwork, you calculate. A poorly dimensioned staircase is uncomfortable and dangerous — and no cladding will ever correct a calculation error.

Question

Blondel’s formula

The universal rule: 2 × rise + going = 60 to 64 cm

Rise (h): ideal 17 cm, acceptable range 16 to 18 cm
Going (g): ideal 28 cm, acceptable range 25 to 32 cm
Headroom (clear height under ceiling above the staircase): 2 m minimum

For a floor-to-floor height of 2.70 m (typical ground floor / first floor with 20 cm floor slab):

2.70 / 0.17 = 15.88 → 16 risers of 16.87 cm
Going: 64 − (2 × 16.87) = 30.26 → g = 30 cm
Plan length (run): 15 treads × 30 cm = 4.50 m

Clear width

Minimum requirement: 80 cm between nosing and wall or balustrade
Comfortable: 90 cm (allows passing)
Generous: 100 cm or more

In a private dwelling, allow 90 cm for a main staircase and 70 cm minimum for a cellar or loft staircase.

Waist slab: the load-bearing structure

The waist slab (or soffit slab) is the inclined reinforced concrete slab that carries the steps. Its thickness depends on the span (distance between supports):

Staircase span	Waist slab thickness
Up to 2.50 m	12 cm
2.50 to 3.50 m	14 cm
3.50 to 4.50 m	16 cm
Over 4.50 m	Structural engineer calculation required

Warning — For a staircase with a span of more than 3 m between supports, or whenever it is a quarter-turn staircase with balanced steps over a void, a structural engineer’s design becomes essential. Allow £250 to £450 for a calculation note and dedicated reinforcement drawing.

Reinforcement: the skeleton that makes concrete strong

Concrete alone resists compression but not bending. The rebar takes up the tensile forces in the bottom of the waist slab.

Standard reinforcement layout

For a waist slab 14 cm thick and 3.50 m span:

Main reinforcement (bottom layer): H10 rebar at 150 mm centres, parallel to the slope, with hooked ends into the supports (anchorage 40 × diameter = 40 cm)
Distribution reinforcement (bottom layer, transverse): H8 rebar at 200 mm centres
Top bars at supports: H10 rebar at 150 mm centres over 1/4 of the span
Step reinforcement: H8 U-bars at the nosing of each step, tied to the waist slab
Cover: minimum 30 mm between rebar and formwork (use plastic spacers / cover blocks)

Ties and connections

All bars are tied with annealed binding wire (1.2 mm diameter), not welded. Each crossing receives a figure-of-eight tie. Laps between bars must be at least 50 cm for H10 rebar.

Best practice — Never order reinforcement blindly. Make a precise cutting and bending schedule with lengths, diameters and shapes, then take it to a local rebar fabricator (there is one in most industrial estates). You leave with bars cut, bent and ready to place. Considerable time saving, and bending quality is incomparable to manual bending on site.

Formwork: the stage that demands the most care

The formwork gives the structure its final shape. Everything not contained by the formwork will leak out — and will have to be broken off with a chisel.

Materials

Soffit formwork: 18 mm marine plywood (WBP/CTBX) or phenolic-faced ply — smooth surface for a clean concrete finish
Side stringers: 63 × 175 mm timber or 27 mm thick boards on edge
Riser boards: 27 mm thick, cut to the exact riser height dimension
Props: 2 per linear metre of waist slab, maximum 800 mm spacing
Screws and fixings: 5 × 80 mm wood screws (preferred over nails, as they are removable)
Release agent: vegetable-based shutter oil (prevents the timber from clogging)

The 6 formwork stages

Mark out on the wall — Using a chalk line, transfer the waist slab slope, the position of each nosing and the walking line
Fix the soffit formwork — Plywood on bearers, propped from the floor with adjustable props tightened with a mallet. Check the slope with a laser level
Fix the side stringers — Against the wall and on the open side, screwed to the soffit formwork. They define the width and carry the risers
Fix the riser boards — Vertical boards between stringers, positioned at the exact riser height. Fix with brackets or external battens
Place the reinforcement — Bottom layer on cover spacers, then top bars, then step bars. Tie progressively from bottom to top
Check — Plumb of riser boards with a plumb bob, width with a tape every 3 steps, slope with a laser level along the full length

Warning — Never forget your cast-in sleeves before pouring: conduit for electrical cables, duct for wall-mounted handrail, balustrade holding-down bolts, fixing sockets for timber string if cladding is planned later. A M12 cast-in socket costs £2; drilling it after casting costs 2 hours and £50 in diamond blades.

Mix design and pouring

Concrete mix

For a reinforced concrete staircase, use C25/30 concrete (characteristic strength 25 MPa in compression, S3 consistence class if pumped).

Mix design for 1 m³:

Component	Quantity
CEM II 32.5R cement	350 kg
0/4 sand	680 kg
6.3/20 gravel	1,180 kg
Water	175 L (w/c = 0.50)
Plasticiser (optional)	1.5 to 3 kg

For a staircase of 3.50 m span × 90 cm wide × 14 cm waist slab + 16 steps:

Waist slab: 3.50 × 0.90 × 0.14 = 0.44 m³
Steps (triangular volume): 16 × 0.5 × 0.17 × 0.30 × 0.90 = 0.37 m³
Total ≈ 0.85 m³, order 1 m³ (safety margin)

Tip — From 0.5 m³ upwards, order ready-mix concrete delivered by truck rather than mixing in a drum mixer. For 1 m³, expect £140 to £180 delivered to the skip, compared to £100 in materials but 6 hours at the mixer with 2 people. The time saving is enormous, and the consistency of the mix guarantees actual design strength.

Pouring

Conseil

Wet the formwork with clean water 1 hour before pouring — dry formwork absorbs water from the concrete and weakens the surface skin
Pour from the bottom working progressively upwards: concrete into the first step, vibrate, then the next
Use a poker vibrator (40 mm diameter) for each 300 mm layer — without touching the reinforcement or the formwork. Vibration drives out trapped air and embeds the rebar
Float the treads: with a wood float immediately after pouring is complete, to produce a flat surface that will key to the future finish
Screed the top of the riser boards with an aluminium straight-edge drawn from support to support
Cover with polythene sheet for 7 days: moisture is essential to curing. In hot weather, spray with water twice a day

Progressive striking

Striking follows a precise schedule:

Element	Minimum delay
Side riser boards	3 days
Side stringers	7 days
Soffit formwork + central props	21 days minimum, 28 days ideal

Warning — Never remove the waist slab props before 21 days. Concrete reaches 70% of its strength at 7 days and 100% at 28 days. A staircase struck too early deflects under its own weight — irreversible cracking on the soffit.

Decision tree: cast concrete staircase, yes or no?

flowchart TD A{Interior or exterior staircase?} -->|Enclosed interior| B{Span between supports?} A -->|Exterior or visible| Z[Prefer steel or timber
lighter appearance] B -->|Less than 3m straight| C[Cast concrete DIY possible
with careful formwork] B -->|Simple quarter-turn| D[Cast concrete with structural
engineer design recommended] B -->|Over 3.5m or helical| E[Structural engineer required
professional installation recommended] C --> F{Competent self-builder with formwork skills?} F -->|Yes formwork mastered| G[GO, follow the guide] F -->|No first time| H[Prefabricated stair to install
or contractor] style A fill:#0F4C81,stroke:#0F4C81,color:#fff style B fill:#0F4C81,stroke:#0F4C81,color:#fff style F fill:#0F4C81,stroke:#0F4C81,color:#fff style Z fill:#FDB813,stroke:#FDB813,color:#0F4C81 style C fill:#56C6A9,stroke:#56C6A9,color:#fff style D fill:#F58220,stroke:#F58220,color:#fff style E fill:#CD212A,stroke:#CD212A,color:#fff style G fill:#56C6A9,stroke:#56C6A9,color:#fff style H fill:#FDB813,stroke:#FDB813,color:#0F4C81

Indicative budget and timescales

For a straight staircase 2.70 m floor-to-floor height × 90 cm wide × 14 cm waist slab:

Item	Self-build	Contractor
Structural engineer calculation (if quarter-turn)	£0 to £450	£250-450
Plywood + timber for formwork	£160-220	Included
H10 + H8 rebar fabricated	£100-160	Included
C25/30 ready-mix (1 m³ delivered)	£140-180	Included
Poker vibrator hire (1 day)	£35-55	—
Props and laser hire (1 week)	£85-130	Included
Labour (3 days × 2 workers)	—	£2,000-3,200
Total	£520-1,195	£2,600-4,000

Total lead time: 4 to 5 weeks from the start of formwork to putting into service, including the non-negotiable 28-day curing period.

Fatal mistakes to avoid

Ignoring Blondel’s rule — a riser height of 19 cm, and the staircase becomes tiring and potentially dangerous
Insufficient rebar cover (< 25 mm) — long-term rebar corrosion, concrete spalling visible on the soffit
Riser boards out of plumb — steps come out “pointed” or “tapered”, no finish can correct this
Concrete too wet (water added at the mixer) — loss of strength, gravel segregation
Vibrating the rebar — moves bars out of position, loss of the composite steel-concrete effect
Premature striking — permanent cracking on the soffit
No wet curing — shrinkage microcracking on the surface, finishes impossible
Failing to allow for cast-in fixings for handrail, lighting, balustrade

Finishes and cladding

Once the concrete staircase has been cast and dried, several finishing options:

Porcelain or ceramic tiles: the most common solution, £25-55/m² in materials, durable and economical
Glued solid timber (oak, beech 20 mm): the warmest finish, £70-110/m² + PU-MS adhesive, requires a dry and flat substrate
Polished concrete: on-trend and elegant, £55-80/m² as self-build, requires a crack-free substrate
Resin: for a modern uniform finish, £35-65/m², applied in a thin layer

For more detail, see our dedicated article cladding a concrete staircase.

Useful links

On the same subject: building a timber staircase to compare with the joinery approach, and calculating a staircase: Blondel’s rule to go deeper into the dimensions
To master reinforced concrete basics: reinforcement rules for foundations and concrete strength classes
Safety and regulations: DTU 21 — execution of concrete works (CSTB), NF P01-012 (AFNOR — balustrade and handrail dimensions), OPPBTP (formwork and working at height safety)