Floor slab formwork with Doka or Peri systems

A floor slab is a load-bearing structure. Its thickness, reinforcement, concrete strength class and the way it is propped are set by the structural engineer in the design. Building a house usually needs approval under the local building regulations, and the rules differ between countries and sometimes between regions. Check with your local building authority what applies to your project, and do not assume that a simple notification is enough. In a flat or a block of flats, the building management or owners' association also has a say.

Before the formwork goes up, the following should be in place:

• External and internal load-bearing walls built up to the underside of the slab, levelled and properly cured
• A clearly defined level for the underside of the slab, ideally marked on the walls as a datum line or level mark
• A firm, flat base for the props: a concrete slab, blinding concrete or compacted hardcore with timber sole boards
• A slab drawing showing openings for the stairs, chimney, service shafts and penetrations
• An agreement on who forms the slab edges and ring beams, and whether edge blocks will be used as permanent formwork
• Access for the delivery lorry and enough clear space to store the formwork

It is common for the crew to arrive and find pallets of blocks, a heap of sand and a mixer standing on the ground floor. The props then have to go up between obstacles, the deck loses its regular grid and the job drags on. A cleared, swept floor saves time and frustration.

Each system works a little differently, but the principle is the same. The crew follows the manufacturer's assembly instructions and the load tables, which give the spacing of props and beams for a given slab thickness.

1. Measuring and setting out

First the crew measures the clear room dimensions and checks the height from the floor to the planned underside of the slab. This sets the length of the props. For normal house ceiling heights, telescopic steel props are used. They extend in steps and can be fine-tuned with a threaded collar.

2. Props, tripods and heads

The first props stand in tripods that hold them upright until the beams are laid on top. Each prop carries a head at the top, often a fork head, which the beam drops into. Props go closer together along the edges and in the corners, because that is where the structure is least stable.

3. Primary and secondary beams

With a beam system, the primary (lower) beams sit in the prop heads and the secondary (upper) beams run across them. These are usually timber beams with an I-shaped profile. Their spacing depends on the slab thickness: the thicker the slab, the tighter the grid. With a panel system, ready-made frames with the formwork sheet already fitted go straight onto the props instead.

4. The formwork deck

Formwork sheets are laid across the beams. These are usually three-ply panels with a yellow or other coated face, roughly 21 or 27 mm thick. The sheets are butted tightly together, and the strips left along the walls are cut to fit. The gap between sheet and wall should be as small as possible so that cement slurry cannot leak through.

5. Levelling

Once the deck is closed, the crew checks the level with an optical level or a laser and adjusts the props. On longer spans, the structural engineer sometimes specifies a slight upward camber in the middle of the bay, so that the slab does not sag visibly once it takes its load.

6. Edges, openings and edge protection

Edge formwork goes round the perimeter and sets the slab thickness at the edge. Openings for stairs and chimneys are boxed out with side shutters. Guardrails go on all open edges, because steel fixers and concreters will be working on the deck afterwards.

7. Release agent

Finally the sheets get a coat of release oil or another release emulsion recommended by the manufacturer. It makes striking easier, protects the sheets and gives a better soffit finish. It is applied thinly and before the reinforcement goes in, so that no oil ends up on the steel.

Both Doka and Peri offer several slab formwork systems. On houses and smaller blocks of flats, two approaches are common.

Beam formwork

Typical examples are Dokaflex or Peri Multiflex. These systems use props, timber beams and loose formwork sheets. Their strength is flexibility: they cope with irregular floor plans, angled walls, recesses, changing heights and downstand beams. On the other hand, they have more parts and need more hand work cutting sheets.

Panel formwork

Examples are Peri Skydeck or Dokadek. The panels are ready-made frames, often lightweight aluminium, which the crew hooks in and lifts into place quickly. On regular areas, assembly is much faster and the soffit comes out even. Some systems have drop heads for early striking: the panels come down sooner while the props stay under the slab. On a very irregular plan, though, panels have to be combined with infill beam formwork.

What this means for you

• Simple rectangular house with straight walls: both systems work, and panels are usually faster
• Irregular plan, bay windows, angled walls, downstand beams: usually a beam system
• Multi-storey building with repeating floors: panels pay off because they are reused floor after floor
• Tall spaces such as a gallery or a high garage: longer props or shoring towers may be needed

Combining both systems is normal and is not a fault. What matters is that the crew sticks to the manufacturer's load tables for the actual slab thickness.

The price per square metre depends on how much labour and material each metre really takes. Two slabs with the same area can be completely different jobs.

The job goes faster with:

• A regular floor plan without projections or angled walls
• A normal ceiling height where standard props are enough
• A firm, level ground floor
• A larger continuous area where the crew can get into a rhythm
• Good access for a lorry or crane to deliver and collect the formwork
• Accurately built walls without large deviations

The job takes longer with:

• Downstand beams that need their own side shutters and propping
• Cantilevers, balcony slabs and overhanging sections
• Many openings and penetrations that have to be boxed out
• Greater height, needing longer props, scaffolding or shoring towers
• A thicker slab, which needs a tighter grid of props and beams
• An uneven or soft base that has to be firmed up first
• A small area with many corners where a lot of cutting and fitting is needed
• Winter work, where the concrete needs protection and propping stays longer

Also check whether the price covers assembly only, or also delivery, formwork hire, striking, cleaning and return. Hire is charged for as long as the formwork stays on site, so any delay with the reinforcement or the pour shows up here too.

Slab formwork is not a job for a layperson. It is a load-bearing structure, it means working at height, and a normal slab puts several hundred kilograms per square metre on the deck. A mistake does not only mean a crooked ceiling, it can mean a collapse during the pour. That is why there is no DIY guide on this page. As the client, you help most by preparing the area, the access and the drawings. Leave the assembly to an experienced crew.

Slab formwork is the point where several trades meet. The better they agree beforehand, the less drilling into finished concrete later.

Bricklayers and structural engineer

The walls must be finished and level. The structural engineer sets the slab bearing on the walls, the ring beams and, where needed, how long the props must stay under the slab. For downstand beams and cantilevers, the reinforcement drawing has to be on site.

Steel fixers

Reinforcement only goes onto a finished, oiled deck. The formwork crew should know where the spacers will sit and where the heavier areas are, for example around downstand beams.

Electrician, heating engineer and ventilation installer

Light fitting boxes, cable conduits and sometimes heat recovery ventilation ducts go into the slab. The boxes are fixed directly to the formwork sheet. Penetrations for drainage, flues or ventilation are formed with sleeves or side shutters. A forgotten penetration means core drilling later and the risk of cutting through reinforcement.

Concrete supplier and pump

The pour date is only fixed once the formwork has been checked and the reinforcement signed off. The concrete pump needs a place to stand. The pour should run continuously without long breaks.

Site supervision

Before the pour, it makes sense for the site manager or an independent inspector to check the formwork and reinforcement and record the check in the site diary. Once the concrete is in, nobody can check it any more. Depending on local rules, building control may also want to inspect the reinforcement before the pour, so ask in good time.

Most problems only appear after striking, when they can no longer be fixed easily.

• Props on a soft base. On uncompacted hardcore, frozen ground or without sole boards, props sink during the pour. The slab sags and, in the worst case, part of the deck drops.
• Too wide a grid of props and beams. If the manufacturer's table is ignored, the deck deflects between props and the soffit comes out wavy. With thicker slabs, there is a risk of overload.
• Props not plumb. A leaning prop carries less. Missing tripods at the edges make the whole deck unstable.
• Open joints. Cement slurry runs out through gaps, leaving honeycombing and sharp fins on the soffit, which are harder to skim.
• Wrong level. The slab ends up lower or higher than the design, and later the door frames, stairs or floor build-up do not fit.
• No release agent, or too much. Without oil, concrete sticks and pieces of the surface tear away when striking. Too much oil leaves stains and can weaken plaster adhesion.
• Forgotten penetrations and boxes. Drilling into a finished slab afterwards is slow and can damage the reinforcement.
• Striking and removing props too early. Young concrete has not reached full strength. The slab can sag or crack even if it looks fine at first glance.
• Loading a young slab. Stacking pallets of blocks on fresh concrete, or forming the next floor too early without propping the slab below.

On multi-storey buildings, people often forget that a fresh slab carries not only itself but, through the props, the slab above as well. That is why props stay longer under the lower floors, as instructed by the structural engineer.

Assembly

On a typical house with a regular plan, an experienced crew usually forms the slab in one to a few days. A panel system on a flat area tends to be faster, while an irregular plan with downstand beams and cantilevers takes longer. Add time for fixing the reinforcement, the inspection and the pour.

Curing and striking

The time to striking depends on the concrete class, slab thickness, span and, above all, temperature. As a rough guide:

• Edge formwork can usually come off after one to three days in warm weather
• With early striking systems, the sheets and beams come down sooner while the props stay under the slab
• With a standard beam system, the formwork usually comes down after several days up to roughly two weeks, and the slab stays propped
• Supporting props often stay for several weeks, exactly as the structural engineer specifies
• Below roughly 5 °C, curing slows down a lot and all these periods get longer

Concrete usually reaches its design strength after about 28 days. That does not mean everything must stay propped until then, but any earlier striking should be approved by the site manager or structural engineer. In summer, protect the fresh slab from drying out too fast by wetting or covering it. In winter, protect it from frost.

Back-propping

If the formwork comes down early, props stay under the slab or new ones go in, known as back-propping. The key is not to release the whole area at once but to work bay by bay, so that the slab does not take its load suddenly.

Before the reinforcement and then the concrete go onto the deck, it is worth walking it properly. You do not need to be an expert: much of it you can see from below and above. If anything is unclear, ask the site manager.

• Props stand plumb on a firm base, on timber sole boards where needed
• Tripods or other bracing are in place at the edges and corners
• The grid of props and beams matches the slab thickness according to the manufacturer's tables
• The underside level matches the design, checked with a laser or optical level
• The deck is flat with no visible steps between sheets
• Joints are tight, including at the walls and around openings
• Edge formwork has the correct height and is firmly fixed
• Openings for stairs, chimney and shafts are in the right place and the right size
• Light boxes and penetrations are fitted according to the electrical and plumbing drawings
• The deck is clean, free of sawdust, nails and offcuts
• Release agent is applied evenly and thinly
• Guardrails are fitted on all open edges
• It is agreed when and how striking will happen and how long the props stay
• The inspection is recorded in the site diary

After striking, look at the soffit. Faint joint marks are normal and disappear under plaster or skim. Larger honeycombing, a sagging slab or cracks should be shown to the site manager before any plastering starts.