Slab Formwork Explained: How Floor Forms Carry Wet Concrete and Where the Plywood Sits

Walk onto a floor deck on pour morning and you can read the whole thing in a minute. Rebar's tied and sitting on chairs. The pump boom is swinging in over the edge. Under your boots there's a skin of plywood, then a grid of beams, then props going down to the floor below like table legs. By lunch that deck will be holding more weight than anything else on the job, and it has to stay flat while it does. That's slab formwork: a temporary floor that carries a real one until the concrete can stand up by itself.

We get asked about this a lot by buyers who know their panels but haven't spent time on a deck. So here's the site view: how the load gets down, which systems you'll run into, where the plywood face fits, and why stripping the deck is its own engineering problem. If you want the commercial side, panel choice and quoting, the Vinawood mill wrote that up separately in slab formwork plywood.

What slab formwork actually does

Wall forms fight sideways pressure. Slab forms don't, mostly. Their problem is weight pushing straight down: wet concrete, the steel inside it, a gang of finishers, the pump line, a pallet someone dropped where they shouldn't have, plus the thump of concrete landing from the hose. Some of that only lasts an hour. All of it still has to get to the ground, or to a floor strong enough to take it.

Site people call this the load path, and honestly it's the most useful idea in the whole subject. Each piece of the deck passes load to the piece under it. Break one link and the deck sags, or worse. So the path gets drawn and checked before anyone stands a prop up, and on a well-run job somebody walks the deck before the pour to confirm it matches the drawing.

The load path, from face to foundation

Top down, it goes like this. Concrete sits on the face, which these days is nearly always plywood with a resin film. The face spans across joists. Joists sit on bearers (some crews say runners or primaries). Bearers rest on props, frames or towers, and those stand on base plates or sole boards over whatever's below: a slab poured last week, compacted fill, maybe a transfer beam.

Load spreads out at every step, and every spacing depends on the member above it. Use a softer face and the joists need to come closer together. Bring the joists closer and each bearer picks up more. Change one thing and it ripples down. You won't find spacing or thickness numbers in this guide, on purpose; they belong to the design calc and the maker's tables for the gear on that particular job.

The paperwork side varies by country, though the basics line up. In the US, OSHA 29 CFR 1926.703 wants formwork drawings kept on site, and shoring inspected before it goes up, then again before, during and right after placement. UK jobs work under BS 5975, the code for temporary works and falsework, which is where the temporary works coordinator role comes from. ACI 347R from the American Concrete Institute is the usual reference for designing the forms themselves.

The main system families

Visit enough sites and you'll keep seeing the same five or so approaches. Which one wins depends on the building. Floor shape matters, so do ceiling height and how often the layout repeats, and crane hours and programme speed usually settle it.

Prop-and-beam is the old reliable. Adjustable steel props, timber or aluminium beams on top, plywood cut to fit over that. It copes with columns, odd edges and steps in level, which is why you'll find a patch of it on almost every job even when the main deck is something fancier. It's slow, though. Loads of loose bits. Lots of cutting. The soffit is only as tidy as the crew.

Panel decks package face and frame together into modules that drop onto props. On a regular floor plate they go up quicker and leave a neater joint pattern. A lot of them hold a replaceable plywood sheet inside a metal frame, so when the face is worn you swap the sheet and keep the frame.

Drop-head systems are built around early stripping. A special prop head lets the beams and panels drop away while the prop stays put under the slab. The young concrete keeps its support; the deck goes up a floor. On fast residential towers that can cut how much kit a contractor needs to own by a fair bit.

Table forms, or flying forms, are whole chunks of deck with beams and legs, lowered after the pour, rolled to the edge and craned up. They love repetitive floors with open edges. At the other end, shoring towers and frames handle tall ceilings, thick transfer slabs and bridge decks, where one prop just isn't going to cut it.

Where the plywood sits in all this

Whatever system's underneath, the face is what touches the concrete, and it's what the client sees when the props come out. It sets how smooth the ceiling is and how loud the joint lines look, and so how much patching the painters end up doing. If the soffit's staying bare, the classes in our fair-faced concrete guide come into play.

It's structural too. The face spans between joists, so its stiffness decides how far the deck dips under wet load. That's why slab decks normally use exterior-bonded structural sheets; see structural plywood and concrete forming plywood. Australians call the same sheets formply. In the UK you'll more often hear shuttering plywood.

And it takes a beating. Walked on, nailed, cut, hosed, struck and stacked, sometimes all in a day. Edges suffer first. Once water gets into an open edge the sheet swells, and you'll see it as a ridge on the ceiling. Good edge sealing, the right release agent and careful stacking buy you more pours. Treat any reuse number on a data sheet as an "up to" figure that assumes that care.

Striking, backpropping and reshoring

Stripping a deck isn't building it backwards. Fresh concrete gets stronger over days and weeks, and it has to carry itself, plus whatever's going on above, before the support comes out. OSHA's line is simple enough: forms and shores stay until the concrete is strong enough, as shown by the plans and specs or by compressive strength tests. Timing is set per job; the mill's note on when to remove concrete formwork runs through the practical factors.

Multistorey work adds a twist. The floor you're pouring sits on shores standing on a floor that might be ten days old. The American Concrete Institute's shoring and reshoring guide points out that construction loads on one floor can be bigger than the load it was designed for in service. So crews leave props in (backpropping) or put them back after stripping (reshoring), and the weight gets shared across several levels.

ACI recommends an engineering check of load sharing and early-age slab strength before any of that starts. It's exactly why drop-head kit is popular, since the original props never move. It also explains the odd sight of a finished-looking floor with a full forest of props still underneath.

Who decides what on a slab pour

A handful of people touch the deck, and each one's asking something different. Job titles shift between countries. The split below is roughly how it falls.

Buying for these systems? The Vietnam formwork plywood guide covers how the sheets get made and sourced, and the mill weighs plywood decks against steel in plywood vs steel formwork.