Concreting stairs including formwork

Cast-in-place staircase between floors

This is a reinforced concrete slab with steps, cast on site into formwork. The slab, called the waist, bears on the floor slabs, landings or loadbearing walls. It is a structural element, so a structural engineer decides the slab thickness, the reinforcement and how the flight is supported. The waist is usually roughly 12 to 18 cm thick, depending on span and load. Reinforcement is never a matter of guesswork or "how it is usually done".

External steps on the ground

Entrance steps and garden steps sit on compacted hardcore or on strip foundations. They are not suspended, so they are structurally simpler. Outdoors, however, the concrete has to withstand frost, water and de-icing salt. The right concrete, a slight fall on each tread and separation from the house wall are what matter here.

Steps on dwarf walls or on a sloping slab

For taller external steps, side walls are often built in blockwork, or a sloping slab is cast first and the steps are formed on top of it. This leaves a hollow space underneath, saves concrete and avoids loading the ground with a solid mass.

Most problems with stairs are not caused during the pour but during preparation. Before the contractor starts, these points should be clear:

• Drawings and structural design: for an internal staircase, a drawing showing the number of steps, rise and going, the position of any landing and the reinforcement specified by the engineer.
• Finished floor levels: this is the most common source of mistakes. Stairs are measured from the finished floor at the bottom to the finished floor at the top. You need to know the build-up of both floors (screed, insulation, tiles or timber) and the thickness of any future tread and riser finish.
• Final finish: whether the treads will get stone, tiles or timber, or stay as exposed concrete. This decides how much lower the concrete has to be cast.
• Balustrade fixings and services: whether the balustrade is fixed to the side or the top of the steps, and whether there will be step lighting. Cable ducts have to go in before the pour.
• Access for concrete: whether a truck mixer can reach the house, whether a pump is needed or whether the concrete will be carried in by bucket and barrow.
• Water and power on site for the mixer, the poker vibrator and curing.

In a new build, the staircase is part of the approved design. If you are adding steps to an existing house, changing the entrance or altering loadbearing structures, check with your local building authority what your local building regulations require. Do not assume that no approval or only a simple notification is needed. In a block of flats, any steps in shared areas or at the entrance must always be agreed with the building management or the owners' association.

Calculating and setting out the steps

First, the total height between finished floors is divided so that every step comes out the same. In houses the rise is usually around 15 to 18 cm. The going is worked out using the well-known rule: twice the rise plus the going should give roughly 60 to 65 cm, which matches the length of a normal stride. The stair profile is then drawn directly on the wall or on the side forms. An experienced tradesperson checks each step once more against the finished levels, not only against the bare structure.

Formwork

For a staircase between floors, the sloping soffit formwork goes up first. It is propped on props and bearers that must carry the weight of wet concrete without sagging. Then the side forms with their stepped profile are fixed, followed by the riser boards. The bottom edge of each riser board is usually bevelled slightly so the tread can be finished underneath it and trapped air can escape. The formwork must be tight, otherwise cement paste leaks through the joints and leaves honeycombing on the surface. Before the pour it is coated with release agent.

Reinforcement

The reinforcement goes into the formwork as the engineer specifies: main bars along the flight, distribution bars across it and, on larger stairs, bars in the steps themselves. The steel must sit on spacers so that concrete surrounds it on all sides. Cover to the steel is usually roughly 2 to 3 cm indoors and more outdoors. The connection to the floor slab or landing is made with starter bars. If that connection is left out, the flight is not properly supported.

Pouring

Concrete goes in from the bottom up, one step at a time. It is compacted with a poker vibrator or, on small steps, by tapping the formwork and rodding the concrete. Very wet concrete flows easily, but it runs out under the riser boards and loses strength. Very stiff concrete does not fill the corners. The treads are screeded off with a straightedge and finished with a float. Outdoors, each tread is given a slight fall towards the front so that water does not stand on it.

Curing and striking the formwork

Fresh concrete must be protected from sun, wind and frost and kept moist. It is usually sprayed with water or covered with plastic sheeting for roughly 5 to 7 days. Riser boards and side forms can come off earlier, often after one to three days. The soffit formwork and props under a suspended flight stay longer, usually two to four weeks depending on temperature and the engineer's instructions. Concrete reaches its design strength after roughly 28 days.

How long it takes

Formwork and reinforcement for a straight flight in a house usually take one to three days. Winding stairs or stairs with a landing take longer. The pour itself is a matter of hours. The waiting takes longest: you can usually walk carefully on the stairs after a few days, but loading them with materials and removing the props comes later. Tiles or stone go on only once the concrete has dried enough, as the adhesive manufacturer specifies.

Types of formwork

• Sawn timber boards: inexpensive and adaptable to any shape, but the concrete surface is rougher and shows board marks. Suitable when the stairs will be clad.
• Film-faced plywood: smooth surface and crisp arrises. Chosen when the concrete will stay visible or when you want to save on skim coating.
• System formwork and props: faster, stiffer support, especially for greater heights and longer flights.
• Permanent formwork: on external steps, the sides are sometimes built from hollow formwork blocks or kerbs that stay as part of the steps.

Concrete

For a structural staircase the concrete strength class follows the design. In houses you will often see C25/30. External steps need concrete that resists frost and de-icing salts, which is specified through the exposure class under EN 206. When ordering, it is enough to say the concrete is for external steps exposed to frost and salt, and the supplier will propose a suitable mix. Ready-mixed concrete from a truck has consistent quality. Mixing on site makes sense for small external steps, but keep to the mix proportions and do not add water by eye.

Reinforcement

A structural staircase is reinforced with steel bars according to the reinforcement drawing. Steps on the ground often only need mesh or a few bars against cracking, but the design should suit the actual situation. Only very small steps on firm ground are cast without reinforcement.

The effort involved, and therefore the cost, can vary a lot. The job is easier with:

• A straight flight without a landing and with simple supports
• Good access for a truck mixer or pump
• Exact finished floor levels and a completed drawing
• Stairs that will be clad later, so the surface need not be perfect

The job is harder with:

• Winding, curved or dog-leg stairs with tapered steps, where every step is different
• Tall propping and cramped space in the shell of the building
• Carrying concrete by hand through the house or up scaffolding
• Exposed concrete, where the formwork must be spotless and tight and the arrises accurate
• Pouring in cold or hot weather, which needs protective measures
• Adding stairs to an existing house, where anchors must be drilled into old structure and the connection detailed

Stairs sit at a crossroads of several trades. If these are not coordinated early, finished steps often end up being broken out again.

• Bricklayers and floor slabs: the connection of the flight to the slab and landing has to be prepared when the slab is cast, for example with projecting starter bars.
• Acoustics: if the staircase should be quiet, it is separated from the walls and slabs with resilient bearings or a break. This must be decided before the pour, as it cannot be added later.
• Electrician: ducts for step lights or switches go into the formwork. The wiring and connections themselves must be done only by a qualified electrician.
• Metalworker or joiner: balustrade anchors or sleeves are cast in, or the areas where drilling is not allowed because of reinforcement are marked.
• Tiler, stonemason, floor layer: they need to know the exact step heights and flatness. For timber treads, the joiner usually measures the finished concrete.
• Insulation and waterproofing: external steps next to the house are separated from the external insulation and the plinth waterproofing so that water does not run towards the wall.

• Uneven steps: usually the first or last step, because the floor build-up was forgotten in the calculation. People trip on such a step, and fixing it afterwards is laborious.
• Sagging formwork: weak props deflect under the concrete, the soffit ends up wavy and the treads slope.
• Reinforcement without cover: steel lying on the formwork rusts later, the concrete spalls and rust stripes appear on the soffit.
• Poor compaction: honeycombing on arrises and risers that has to be patched.
• Props removed too early: cracks in the flight or deflection. A structural engineer should assess this.
• No fall outdoors: water stands on the treads, turns to ice in winter and frost breaks down the concrete.
• External steps rigidly tied to the house: when the ground settles or moves, the steps tear away from the wall and water gets to the plinth.
• Poor curing: concrete dries too fast in the sun, the surface dusts and hairline cracks appear.

A structural staircase between floors is not a job for a layperson. It is a loadbearing structure and belongs in the hands of a contractor working to a design. You can, however, manage two to four steps in the garden or up to a terrace on firm ground, as long as they support nothing else. The procedure is roughly as follows:

1. Measure the total height difference and divide it into equal rises, then work out the going with the stride rule.
2. Dig down to frost-free depth, or at least remove the topsoil, and lay a hardcore bed compacted in layers.
3. Build side forms from boards or plywood with the step profile drawn on, and brace them well with stakes.
4. Fix the riser boards, check their height and leave a slight fall towards the front of each tread.
5. Coat the formwork with release oil and lay mesh on spacers.
6. Separate the steps from the house wall with a movement joint strip.
7. Mix or order frost-resistant concrete, keep to the mix proportions and do not add unnecessary water.
8. Pour from the lowest step upwards, rod the concrete and tap the formwork to release air.
9. Screed the treads with a straightedge, finish them with a float and round the arrises with an edging trowel.
10. Cover the concrete with sheeting and keep it damp for several days, protecting it from frost and direct sun.
11. Remove the side forms and riser boards usually after two to three days, and only put heavy loads on the steps after a few weeks.

Wear gloves and eye protection when pouring, as wet cement irritates the skin.

At handover, bring a tape measure, a spirit level and, for an internal staircase, the drawing. Go over every step, not just the overall impression. Check:

• Equal height of all steps once the future floor and finish are allowed for
• Equal going of all treads and level treads, with a slight fall to the front outdoors
• A straight soffit with no sag and no visible reinforcement
• Intact arrises and risers without honeycombing or larger voids
• No cracks, especially at the connection to the slab and landing
• Ducts, balustrade anchors and movement joints placed as agreed
• External steps separated from the wall and plinth
• Written guidance from the contractor on when props may be removed and when the finish can be laid