What thickness plywood is best for slab formwork?
18 mm film-faced on 400–500 mm joist centres is the commercial soffit default for pours around 150–200 mm deep. Use 21 mm when centres push past 500 mm, the pour is deeper than 200 mm, or live load is unusual. 15 mm needs closer centres. Match thickness to span and deflection limit, not habit.
How is slab formwork different from wall formwork?
Wall formwork takes lateral pressure that peaks at the base. Slab formwork sits flat and carries the full dead weight of wet concrete (about 24 kN/m³) plus crew and bar live load. Sag becomes a permanent wave in the soffit, so stiffness across the joists matters more than on a buried wall.
How many times can slab formwork plywood be reused?
Reuse is up to N on the mill card. Eco Form Plus up to 8, Form Basic up to 10, Form Extra up to 15 (MUF, EN 636-2, not Class 3), Pro Form Lite up to 15 (PF, Class 3), Pro Form up to 20. Floor count is not a cycle count. Same-day clean, edge sealer, and flat stacking protect the card.
Should slab formwork plywood get a release agent?
Yes. Coat the face before the sheet goes down. Once rebar is on the deck you cannot coat evenly. A thin, even film helps strip and protects the phenolic overlay. Miss a patch and the next pour can take film with it.
Which way should plywood face grain run on a slab deck?
Lay face grain perpendicular to the supporting joists. Plywood spans better across the outer veneer. Flip that orientation and you donate deflection you did not need to buy. Stagger joints between rows and support every cut edge.
Is Form Extra Class 3 for slab soffits?
No. Form Extra is Superior Glue MUF under EN 636-2. The extra cycles versus Form Basic come from glue chemistry, not a Class 3 upgrade. If the engineer named EN 636-3, spec Pro Form Lite or Pro Form instead.
