What do the letters A, B, C and D mean on plywood?
They grade the look of each face, with A the cleanest and D allowing the most open knots and splits. A takes neat repairs only, B allows solid patches and tight knots, and C allows larger knots and some splits. The pair reads face first and back second, so a B-C sheet has a B face and a C back. The letters describe appearance only and tell you nothing about the glue or the strength.
What does the X in CDX plywood mean?
The X means the veneers are bonded with exterior-type glue, not that the sheet is rated for permanent outdoor service. CDX is usually stamped Exposure 1, so the bond can handle rain and damp during construction, but the panel is meant to be covered once the building is closed in. For a sheet that stays exposed, look for an Exterior bond rating on the stamp.
How do EN 636 bond classes relate to plywood grades?
Bond class is a separate rating from the face grade, and it comes from the glue line. EN 636-1 covers dry interior use, EN 636-2 covers humid conditions, and EN 636-3 covers exterior exposure. MUF glue lines usually reach EN 636-2, while phenolic (PF or WBP) glue lines reach EN 636-3. Two sheets with the same face letters can therefore behave very differently once they get wet.
Which plywood grade is used for concrete formwork?
In North America the usual choice is APA Plyform, which is normally a B-B panel sold as Class I or Class II depending on the wood species on the faces. It can be bought bare, or with a high-density or medium-density overlay for a smoother finish and more reuses. Outside North America, film-faced plywood rated by EN 636 bond class is the norm, and the film plus the glue decide how many pours a sheet will see.
How many pours can a formwork plywood panel take?
It depends mostly on the glue and the face, and good site care matters as much as the panel. In our range, Form Basic is rated up to 10 reuses and Form Extra up to 15, both with MUF glue at EN 636-2. Pro Form and HDO Standard or HDO Premium are rated up to 20, with phenolic glue at EN 636-3. Treat those as ceilings, because release agent, edge sealing and careful stripping all move the real number.
How do APA grades compare with EN 635 and GOST grades?
They line up only roughly. APA A sits close to EN 635 E or I and GOST E or B, APA B to EN 635 II and GOST BB, APA C to EN 635 III and GOST CP, and APA D to EN 635 IV and GOST C. A "B/BB" quote from an Asian mill is usually near EN 635 II or III. Always ask for the supplier's written tolerance sheet, plus the glue type and EN 636 class, rather than trusting the letters alone.
