What goes into our 21mm film faced plywood, and when is it the right panel?

Why is 21 mm the common film-faced thickness for European slab formwork?

21 mm is the common film-faced thickness for European slab formwork because many beam-deck and frame systems sold there are drawn around a 21 mm panel, usually 1250 × 2500 mm. The extra 3 mm over 18 mm makes the face about 1.59 times as stiff for the same build, so it spans wider beam spacing and bows less.

That stiffness gain is the whole case for the thickness. Bending stiffness of a panel grows with the cube of its thickness, so going from 18 to 21 mm multiplies it by about 1.59 for the same core, glue and film. On a slab deck, that shows up as flatter soffits between beams, fewer beams per square metre, or both. Formwork system makers design their beam spacing and their frame rebates around one panel, and in much of Europe that panel is 21 mm.

What it costs is weight and volume. A 21 mm sheet is about 17% heavier than the 18 mm sheet of the same core and size, holds about 17% more wood, and fills a container with fewer sheets. Where the system is drawn around 18 mm, those costs buy nothing. Where it is drawn around 21 mm, an 18 mm panel is the wrong part, however cheap.

Your formwork designer sets the panel from the support spacing, the pour height and the finish. We press both thicknesses at our two mills in Chi Linh, Hai Phong; our 18 mm note gives the thinner build line by line.

Our 21 mm build, line by line

Our 21 mm film-faced panel has seven lines on its technical sheet: phenolic film on both faces, smooth hardwood face veneers, a crosswise Acacia or Eucalyptus core, MUF or phenolic glue, water-resistant paint on all four edges, ±0.5 mm thickness tolerance and a sheet size of 1250 × 2500 mm or 1220 × 2440 mm.

A 21 mm panel is pressed the same way as every thickness we make: veneer intake, drying, core lay-up, glue and press, sizing and sanding, film press, edge sealing, then marking and packing. Hot pressing for phenolic bonds runs at about 130–145 °C. The thicker the panel, the more the first press and the calibration matter, because a 21 mm core has more veneer layers in which a gap or an overlap can hide. The pressed core is sanded to an even thickness before the film goes on, so the film sits on a flat panel.

Which sheet sizes are available in 21 mm, 1220 × 2440 or 1250 × 2500?

Both. We press 21 mm film-faced plywood in 1250 × 2500 mm, the common size for European beam decks and frame systems, and in 1220 × 2440 mm (8 × 4 ft), the standard in the UK merchant trade, the Gulf, Africa and much of Asia. Cut sizes are made to a cutting list, with the cut edges sealed.

Nothing is pressed larger than 1250 × 2500 mm, so systems drawn around larger panels are faced with cut sheets. The larger European sheet covers about 5 per cent more area than the 8 × 4 ft sheet and weighs about 5 per cent more, so a 40HC carries fewer sheets of it but about the same area. Our 21 mm EU-format panel page shows the 1250 × 2500 mm build as a product, and cut-to-size formwork panels covers cutting lists.

What does one 21 mm sheet weigh with an Acacia or Eucalyptus core?

A 21 mm sheet of 1220 × 2440 mm weighs about 36.3 kg with an Acacia core and about 40.6 kg with a Eucalyptus core, at our planning densities of 580 and 650 kg/m³. In 1250 × 2500 mm the same sheets are about 38.1 kg and 42.7 kg. Weigh real sheets before final loading.

The formula is density times volume. A 21 mm sheet of 1220 × 2440 mm holds 0.0625 m³ of panel, and a 1250 × 2500 mm sheet 0.0656 m³:

Eucalyptus is measured at 650–750 kg/m³, so a dense Eucalyptus run weighs more than the planning figure. SGS measured 642 kg/m³ on one submitted 21 mm Acacia-core phenolic specimen; at that density a 1220 × 2440 mm sheet would weigh about 40.1 kg. Planning densities are for planning; the scale is for loading.

Weight matters on site as much as in the container. A 1250 × 2500 mm Eucalyptus-core sheet at about 42.7 kg is a two-person lift on most sites, and crane cycles on a deck are counted in sheets. That is one reason buyers who order 21 mm for stiffness often choose the lighter Acacia core. Our plywood sheet weight calculator gives the weight for other sizes and cores.

How many sheets of 21 mm film-faced plywood fit in a 40HC?

A 40HC holds about 752 sheets of 21 mm film-faced plywood in 1220 × 2440 mm with an Acacia core (16 packs of 47, about 47.0 m³), or about 675 sheets with a Eucalyptus core (15 packs of 45, about 42.2 m³), where weight stops the load first. These are planning figures until the packing list is made.

The planning model stacks packs about 1,000 mm high: 16 packs for Acacia, 15 for Eucalyptus, with a planning payload of 28,000 kg of panels that the carrier and route confirm. Sheets per pack are 1,000 mm divided by the thickness, rounded down, then capped by weight. At 21 mm, weight caps three of the four loads below.

The area column is the one a contractor plans with: about 2,250 m² of Acacia-core 1250 × 2500 mm deck per container. Every figure here is a planning figure: real sheets are weighed before final loading, and the packing list is the number that counts. The 40HC loading page shows how the packs go in.

Is 21 mm film-faced plywood covered by your CE marking to EN 13986?

Yes. Our CE factory production control certificate 0407-CPR-2146 from Istituto Giordano, notified body 0407, covers plywood for structural use in dry, humid and external conditions from 3.6–30 mm with MUF or phenolic glue, under EN 13986 and AVCP System 2+. A 21 mm panel ordered as structural CE plywood sits inside that scope.

What the CE route gives a 21 mm order is a declaration of performance for the thickness, with the values a designer needs, and a mill whose production control is checked by a notified body. The harmonised standard behind it is EN 13986, which covers characteristics, evaluation of conformity and marking of wood-based panels for use in construction. In the Construction Products Regulation, factory production control is the documented, permanent and internal control of production in a factory.

Not every 21 mm order needs CE. A contractor buying deck panels for a temporary works system usually orders by glue, film and pour target; a buyer whose market asks for CE on all construction plywood, or whose panel becomes part of a permanent structure, orders it as structural CE. Say which on the enquiry. The CE marking page shows the certificate and the structural CE plywood page the product.

What did the SGS test show on your 21 mm phenolic specimen?

SGS Vietnam tested one submitted 21 mm dark-brown film-faced specimen with an Acacia core and phenolic glue, in a report dated 23 December 2025. It measured a density of 642 kg/m³, passed EN 314 bonding Class 3 after a 24 h water soak, boil-dry-boil and 72 h boiling, and passed E1, with formaldehyde not detected at the 0.04 mg/m³ reporting limit.

The test methods are published standards. EN 314-1 sets the test methods for plywood bonding quality. EN 314-2 specifies requirements for bonding classes of veneer plywood according to their end uses. EN 717-1 is the BSI standard for wood-based panels, determination of formaldehyde release, formaldehyde emission by the chamber method.

Three limits on what the result says. It is one specimen, not a guarantee for every panel in an order. “Not detected” means below the 0.04 mg/m³ reporting limit, not zero, and it is an E1 result, not an E0 claim. And it shows that the phenolic glue line keeps its bond through boiling, not a pour count. Our SGS specimen note reads the whole report.

21 mm or 18 mm: when the extra 3 mm pays

The extra 3 mm pays when the formwork system is drawn for 21 mm, when the support spacing is wider than an 18 mm panel can span flat, or when a fair-faced soffit must stay flat between beams. It does not pay on a system drawn for 18 mm, on close joist spacing, or where crews move every sheet by hand all day.

Two numbers frame the decision. Per sheet, 21 mm carries about 17% more wood and weight, so at the same price per cubic metre it costs about 17% more. Per container, 752 Acacia-core sheets cover about 2,239 m² against about 2,620 m² for 880 sheets of 18 mm. The pours are the same for the same build, so the extra cost is only worth paying for stiffness the job uses. Our cost-per-pour note works through the money.

How many pours does a 21 mm film-faced panel give?

A 21 mm film-faced panel gives about 12–15 pours with MUF glue and up to about 25 pours with phenolic glue, with proper handling, release agent on every pour and sealed edges, including every cut edge. Budget panels typically give about 4–10. The thickness does not set the count: glue, film and site practice do.

A thicker panel helps the count in one indirect way. A panel too thin for its supports bends on every pour, and repeated bending fatigues glue lines and cracks film over the beams; a 21 mm panel on wide spacing avoids that. On close spacing it adds nothing to the count. For a European slab programme planned beyond about 15 pours, the usual order is 21 mm, phenolic glue and a film weight to match. Our note on how many pours a panel gives sets out the site routine.

Black or dark brown film, and which film weight, on 21 mm

Choose the colour your site prefers and the film weight your programme needs: black and dark brown are both phenolic films and colour is not a grade. On our orders film runs from 120–125 g/m², with about 130–135 g/m² most common now and 165, 185 or over 200 g/m² for long programmes.

A 21 mm panel is often bought for fair-faced work, where the soffit is seen. There the film earns its weight: a heavier face keeps its smoothness through more strikes, and a smooth face leaves a cleaner surface. Pair it with phenolic glue, because a heavy film on a MUF glue line still has the bond life of MUF. Write the film weight beside the colour on every order.

Thickness tolerance and flatness on 21 mm panels

Our 21 mm film-faced panels are held to ±0.5 mm, so every sheet measures between 20.5 and 21.5 mm; length and width are held to ±2 mm. The tolerance for each order is written on its technical sheet, and the core is calibrated by sanding before the film press.

On a beam deck, two neighbouring sheets that differ by a millimetre leave a step that prints on the soffit and lets paste leak through the joint. How a buyer checks it on arrival:

1. Measure each sampled sheet with a caliper at the four corners, in from the edge, and at the middle of each long side.
2. Record the highest and lowest reading per sheet; both should sit between 20.5 and 21.5 mm.
3. Lay two sheets edge to edge on a flat floor and look at the joint.

A tighter tolerance can be written for a project that needs it, for example a fair-faced wall with visible joints. Say so on the enquiry, because it changes the sanding and the grading of every pack.

How 21 mm panels fit EN 636 and the order line

A European order for 21 mm film-faced plywood usually names an EN 636 service class, a bonding class and, where needed, CE to EN 13986. Formwork faces wet concrete, so the order names the exterior class and the matching bonding class 3, which our phenolic specimen passed.

EN 636 specifies requirements for plywood for general purposes or structural application in dry, humid or exterior conditions. The dry, humid and exterior classes line up with bonding classes 1, 2 and 3 of EN 314-2. For a formwork order that gives three lines to write:

1. Service class. EN 636-3, exterior conditions, for panels that meet wet concrete and weather.
2. Bonding class. Class 3 to EN 314-2, with the test report that shows it.
3. CE. Structural plywood to EN 13986 with a declaration of performance, if the market or the use asks for it.

A buyer who writes only “21 mm film-faced, WBP” has named a thickness and a promise; the three lines above name a build that can be checked. Our structural CE plywood and film-faced plywood pages carry the build behind each line.

21 mm in inches and the names buyers use

21 mm has no exact inch twin: it sits between 13/16 inch (about 20.6 mm) and 7/8 inch (about 22.2 mm). Buyers in the UK order “21mm shuttering plywood”, in Europe “film faced plywood 21mm 1250 x 2500”, in the Gulf “21mm phenolic film faced plywood”, and in Australia the order names formwork plywood to AS 6669.

The inch arithmetic is fixed by law. Schedule 1 of the Weights and Measures Act 1985 defines the yard as 0.9144 metre and the inch as 1/36 yard. That makes the inch 25.4 mm, 7/8 inch 22.225 mm and 13/16 inch about 20.6 mm, so a metric 21 mm panel will not drop into a frame rebated for 7/8 inch without checking.

In Australia, our formwork plywood is certified to AS 6669:2016 by CMI Certification, a JAS-ANZ-accredited body; no stress grade is claimed from that certificate, so a project that specifies one gets a quote against that requirement separately.

Ordering 21 mm film-faced plywood: the lines to send

A 21 mm order needs eight lines: application and pour target, core, glue, film weight and colour, thickness and tolerance, sheet size, edge sealing and quantity, plus the port of discharge. With those, we return a technical sheet for your 21 mm panel and an FOB Hai Phong offer per cubic metre.

Our terms are the same for every thickness: one 40HC minimum, carrying one or two products and up to six by agreement; production in 15–45 days after the 30% deposit; the 70% balance against the scanned bill of lading by T/T or irrevocable L/C at sight; FOB Hai Phong, CFR or CIF. Typical days at sea from Hai Phong are 30–40 to Northern Europe and 25–35 to the Mediterranean, varying by carrier and route. Free samples of the 21 mm build go out on request, with new buyers booking their own courier. Send the eight lines through the specification form or to [email protected].