Wall formwork vs slab formwork: how they differ and when you need which
Wall formwork and slab formwork both give fresh concrete its shape, yet they deal with opposite forces. A wall form must resist concrete pushing sideways against its faces. A slab form must carry weight: concrete, reinforcement, workers and equipment, all bearing down on props or towers until the new floor can support itself. That single difference decides which components you need, what you check and how a used lot should be put together.
Wall formwork: resisting fresh concrete pressure
Most wall formwork on today's sites is framed panel formwork. Each panel is a steel or aluminium frame carrying a plywood or plastic facing. Panels are set up in pairs, one on each face of the wall, joined along their edges with clamps and held together through the wall by tie rods. The ties do the real work: the push on one face travels through the rod to the opposite face, so the form needs no external bracing to resist the concrete.
Until it begins to stiffen, fresh concrete behaves like a dense liquid. Pressure on the form therefore grows with depth and peaks towards the bottom of the pour. How high it climbs depends on several factors:
- Pour rate: filling the wall quickly gives the lower concrete less time to stiffen, so pressure rises.
- Pour height: a tall wall cast in one lift builds up more pressure than a short one.
- Temperature: cold concrete sets slowly and keeps its pressure for longer.
- Consistency: soft and self-compacting mixes can act like a liquid over almost the full height, and retarders have a similar effect.
- Compaction: vibrating deep into concrete that has already begun to set makes it fluid again.
Each panel system has a permissible fresh concrete pressure published by its manufacturer. Site teams stay within it by controlling the pour rate, which is why the pump schedule and the formwork design have to be planned together. Light, hand-set systems suit lower pressures and smaller pours; heavy, crane-handled systems take higher pressures with fewer ties per square metre.
Ties are not the only accessory. Walers, steel profiles fixed across the panels, keep long runs straight and bridge filler zones. Push-pull props set the form plumb and hold it against wind, but they are not meant to carry concrete pressure. Concreting brackets with guard rails give the crew a working platform along the top.
Slab formwork: carrying the load down
A slab form is a temporary floor. Reinforced concrete weighs in the region of 25 kN per cubic metre, so a 25 cm slab puts roughly 6 kN on every square metre before anyone steps onto it. Construction loads from workers, heaped concrete and equipment come on top. The deck, its supports and whatever lies beneath, ground or an older slab, must take all of this. In Europe such temporary works fall under EN 12812 for falsework, while adjustable telescopic steel props are covered by EN 1065.
Beam and panel systems
Primary and secondary beams, in timber or aluminium, rest in fork heads on top of props, and plywood sheets are laid across the secondary beams. This approach adapts to nearly any floor plan, beam spacing can follow the slab thickness, and most parts are fairly generic. The trade-off is labour: many loose pieces per square metre and more time for erection and striking.
Panel slab systems
Framed panels, often aluminium, sit directly on prop heads or on light main beams. On regular plans with repeating bays they go up quickly and with few pieces per square metre. Edges, column zones and odd shapes are closed with timber or with the system's own infill parts.
Dropheads and early striking
Many panel and beam systems use dropheads, prop heads with a release mechanism. When the concrete has gained the strength specified by the engineer, the panels or beams are lowered a few centimetres and taken out while the props remain under the slab. The costly deck material moves on to the next bay early and only the props stay behind. Striking times belong in the method statement and depend on strength gain, in line with EN 13670.
Props, shoring towers and back-propping
Telescopic steel props cover ordinary storey heights. For greater heights or heavier loads, such as thick transfer slabs or bridge decks, shoring towers built from frames and braces take over. A young slab also rarely carries the next floor's construction load alone: back-props are left beneath it, often through two or more storeys below the floor being cast. Back-propping is easily forgotten when quantities are worked out, and it can add a large number of props to an order.
Which projects need which
| Project | Wall formwork | Slab formwork and shoring |
|---|---|---|
| Basements and foundation walls | the main share; single-sided walls against an existing face need support frames instead of ties | mostly the ground floor slab |
| Residential blocks in in-situ concrete | party walls, lift and stair cores | a slab on every storey, plus back-props |
| Car parks and office frames with columns | cores and a few shear walls | large, repetitive slab areas suited to panel or drophead systems |
| Water tanks and treatment plants | high walls with watertight tie details | cover slabs, sometimes heavy shoring |
| Bridge decks, transfer slabs, tall halls | abutments and piers | shoring towers and heavy load-bearing systems |
Many contractors keep a fleet of wall panels and buy slab material per project, because props and beams are easier to source in matching numbers. Others do the reverse. The table shows where the bulk of the material is likely to go on a given job.
Combining used lots
Used formwork seldom arrives as a complete, project-ready set. Putting lots together works when a few rules are followed:
- One panel system per wall: clamps, ties and hole positions are specific to each system. Panels from two systems cannot be clamped into one face, and mixing them would also invalidate the load data.
- A shared grid: panels that are stacked or set side by side must follow the same module of heights and widths.
- Accessories from the same system: corners, fillers, walers and brackets must fit the panels. Panels without their accessories are only half a set.
- Props are more forgiving: telescopic props made to EN 1065 are widely usable as individual supports, but fork heads, dropheads and tripods must suit the props and beams they are combined with.
- Beams by depth: beams of one depth belong in one deck; mixed depths leave steps in the formwork.
Plan by pour cycle rather than by building: how much wall or slab you form per cycle, times the number of sets running in parallel. The guide to buying used formwork covers this cycle planning in more depth.
What to check in the lot list
A good lot list lets you rebuild the set on paper before it arrives. Look for:
- Panel quantities by height and width, so a wall or bay can be laid out.
- Inside, outside and hinged corners in a sensible ratio to the panels.
- Ties, wing nuts, clamps and walers in numbers that match the panel area and the number of joints.
- For slab lots: props grouped by length range, dropheads in proportion to panels, beams by length, plus fork heads and tripods.
- The state of the facing, stated as used, repaired or refurbished, with photos of typical panels.
- Shoring towers listed as complete towers with frames, braces, base jacks and head jacks, not as loose parts.
Wall sets are listed under wall formwork, deck systems under slab formwork, and props under floor props. If you send us the wall lengths and slab areas of one cycle via the contact page, we can propose a way to combine lots.