What thickness plywood for formwork?
18 mm (3/4") is the US default for slabs, walls, and most columns at ordinary joist or stud spacing. 12 mm for light work and tight curves with close backing. 15 mm for residential footings, low walls, or a gang frame that already carries the load. 21 mm for wide spans, tall pours, or a face that has to stay dead flat.
Can you use 1/2 inch plywood for concrete forms?
Only for light work and tight curves, and only with close backing. Skip 12 mm (~1/2") on a 16" joist grid. It is not a slab or wall sheet at ordinary spacing.
Which is thicker, 3/4 or 5/8 plywood?
18 mm (3/4") is thicker than 15 mm (~5/8"). 18 mm is the US workhorse. 15 mm needs tighter joists, or a steel gang that already takes the load.
Does thicker formwork plywood give more reuses?
No. Reuse is overlay plus glue class plus how the crew strips and stores the sheet. Millimeters help with dents and flex. They do not mint extra cycles. 18 mm HDO Basic is still up to 10. 18 mm HDO Premium is up to 20. Same thickness, different face.
When should I step up to 21 mm formwork plywood?
Wide span, tall pour, fast rate. Slabs: 21 mm for about 500 mm (20") joist spacing, or slabs thicker than 200 mm (8"). Walls: above 3 m, or architectural walls where a 2 mm belly fails the pour. Columns: faces wider than 600 mm (24"). Faster than 1 m/hour, or taller than 3 m: step up one thickness. Overlay does not cancel that step.
Is formply stronger than plywood?
Formply is overlay plywood, not a different species. Thickness sets stiffness; overlay and glue class are separate calls. A thin HDO Premium sheet can still dish on a wide joist grid. A thick Form Extra sheet can still scuff: that panel is WBP MUF, EN 636-2, not phenolic Class 3.
