Layers, glue, grades and thicknesses. A short guide to reading the markings on a sheet and picking plywood for the job at hand.
Plywood looks simple — a board made of wood. Yet every sheet carries markings that decide whether it belongs in furniture, in a floor or in concrete formwork. Here is what they mean.
How plywood is built
Plywood is glued up from thin layers of veneer. Every layer is turned 90 degrees against the one below it, so the grain runs crosswise. That is what gives plywood two properties solid timber does not have: it is strong in both directions and it moves far less with humidity. The number of layers is almost always odd, which keeps the panel symmetrical so it does not twist.
Birch or softwood
Birch plywood is denser and stronger, with a smooth face and the familiar striped edge, which is why it goes into furniture, interiors and loaded structures. Softwood plywood is lighter and cheaper, and usually ends up in construction where the surface stays hidden under a finish.
The glue: interior or exterior
The glue group decides how much moisture the panel takes.
- Urea glue (FK) — dry rooms only. Furniture, interiors, packaging.
- Phenolic glue (FSF) — moisture resistant. Roofs, sheathing behind cladding, formwork, trailers, outdoor work.
Moisture resistant is not waterproof: a phenolic-bonded sheet survives rain and damp air, but standing water ruins any plywood. For rooms people spend time in, check that the panel carries an E1 emission class.
Grades
The grade describes the surface — knots, patches, splits. It is always given as a pair: the first mark is the face, the second the back. II/III, for example, means one side is cleaner and meant to be seen.
- E and I (B) — virtually free of defects, for clear finishes and visible surfaces.
- II (BB) — patches and sound knots allowed. The everyday grade for furniture.
- III (CP) — more defects, fine underneath a finish.
- IV (C) — a technical grade where only strength matters.
Sanding, thicknesses and sizes
NS means an unsanded sheet, Š1 sanded on one side, Š2 on both. A sanded face is ready for paint and glue; an unsanded one is cheaper and fine where nobody sees it.
Thicknesses run from 4 mm up to 30 mm and beyond, and the common sizes are 1220×2440, 1250×2500 and 1500×3000 mm. In practice the thickness follows the span: the further apart the supports, the thicker the sheet.
Film-faced plywood
This is phenolic-bonded plywood with a film coating. The surface keeps moisture out and stays smooth, so it is used for concrete formwork, cargo floors and anywhere the same sheet has to work many times over.
Choosing for the job
- Furniture and interiors — birch plywood, grade II/III, sanded both sides, interior glue.
- Floor substrate — exterior grade, 12–18 mm depending on the joist spacing.
- Roof and wall sheathing — exterior grade; the surface grade hardly matters.
- Concrete formwork — film-faced, 18 or 21 mm.
- Trailers and cargo bodies — film-faced or exterior grade from 9 mm up.
Practical notes
- Store sheets flat on bearers of equal height, somewhere dry. Leaning them against a wall bows them.
- Let the material acclimatise for a couple of days in the room where it will live before you cut it.
- Cut with a fine-toothed blade and put a sacrificial board underneath so the edge does not chip.
- Keep screws at least two screw diameters away from the edge, or the outer ply will split away.
- Seal cut edges with paint or sealant in damp places: the edge is where plywood drinks water fastest.
