What a formwork sheet works with
Film faced plywood — formwork plywood, shuttering ply — is the sheet concrete is poured against. Both faces carry phenolic film, the edges are sealed, the glue is water-resistant phenolic. Its job is hard: on every pour it carries concrete pressure, is stripped, washed, moved and screwed down again. The number of pours it survives is the single figure that sets the cost of the sheet.
And that number is set not by the film but by the veneer under it. The film is paper impregnated with 120–220 g/m² of phenolic resin: it smooths the surface and releases the concrete, but has no mechanical strength of its own. When the sheet wears, holds a screw or bends, it is the wood that works. Birch’s 620–680 kg/m³ density and 120–144 MPa bending strength show here — this is why formwork plywood is identified with birch worldwide.
What the film hides, and what it does not
A common assumption: “the film covers the face anyway, so veneer grade does not matter.” Partly true.
| Defect | Under the film | On the concrete |
|---|---|---|
| Colour variation, mineral streak | invisible | no mark |
| Sound, closed knot | invisible | no mark |
| Open split | shows as a line | printed onto concrete |
| Overlap (two veneers lapped) | ridge | printed onto concrete |
| Hole, fallen knot | film collapses | printed onto concrete |
The film hides colour, not geometry. That is why the face veneer for formwork is grade 1–2 — closed knots allowed, no open defects. Grade 3–4 is enough for the inner plies: their geometry never reaches the concrete. In a mixed 1–4 lot the 22–27% of grade 1–2 goes to the faces; the rest to the inner plies. Grade definitions: grades.
Ply count: 18 and 21 mm
The two most common formwork thicknesses. With 1.5 mm veneer:
| Sheet | Plies | Raw thickness | After pressing |
|---|---|---|---|
| 18 mm | 13 | 19.5 mm | ~18 mm |
| 21 mm | 15 | 22.5 mm | ~21 mm |
The difference is press compression and sanding. The ply count is always odd, adjacent plies cross at 90°, and the two outer plies run the same way — along the long edge of the sheet, so bending strength is highest in that direction. General ply arithmetic: plywood production. The cost calculation by ply count: plywood cost.
Format: why 2600 × 1300 fits formwork
The standard formwork sheet is 2500 × 1250 mm. The 2600 × 1300 veneer format gives it:
- 100 mm trim in length — 50 from each edge,
- 50 mm in width — 25 from each edge.
Enough for edge trimming after pressing; anything more is waste. That makes 2600 × 1300 the natural format for a formwork producer: the trim margin is exact and the loss is minimal. The 1600 × 1600 and 1300 × 1300 formats are for plants making square panels or blanks — not the first choice for formwork. Format page: 2600 × 1300 mm.
How many sheets per container
In an all-birch formwork sheet every ply is birch — this is the product with the highest veneer intake.
| Calculation | |
|---|---|
| Veneer in the container | 33 m³ (2600 × 1300) |
| Veneer per m³ of panel | ~1.1 m³ |
| Panel from the container | ~30 m³ |
| 2500 × 1250 × 21 mm sheet | 0.066 m³ |
| Sheets, 21 mm | ~450 |
| Sheets, 18 mm | ~530 |
In a combi build — birch face, poplar core — the same container covers two to three times as much panel, because birch is only in the outer plies. When combi is enough: combi plywood.
Moisture and phenolic pressing
Phenolic glue and film usually call for veneer moisture in the 6–8% band. Higher moisture means steam pockets in the press and blistering of the film; lower moisture spoils glue absorption. The veneer ships at 5–8%, inside the band. A measurement record travels with every lot. Why moisture is held in this band and what happens to it on a 22–32 day sea leg: the moisture page.
How to start
For a formwork producer the sample test has one criterion: geometry under the film. Press the sample on your own line, film it and shine a light across the surface — any line, ridge or dip. Then thickness: does the sum of 13 or 15 plies land on the target.
Then a single trial container, 33 m³, about 450 sheets. The clauses to fix in the contract — moisture band, thickness tolerance, grade split, the right to measure on arrival — are on the first order page.