Sheet goods

materials:
  - id: panel_stock
    from: sheet.plywood.birch
    thickness: 18

Sheet materials take a thickness and no width — the width comes from the sheet. They also carry their own properties rather than inheriting from a species, because a panel behaves the way it does because of how it is built, not what tree it came from.

Set grain on the part anyway. Plywood is stiff along the face grain and noticeably less so across it, and a veneered sheet has a visible direction.

sheet.plywood.birch

Birch plywood. The default choice for a structural panel.

Thicknesses	4 · 6 · 9 · 12 · 15 · 18 · 21 · 24 · 30 mm
Sheet sizes	2440 × 1220 · 2500 × 1250 · 3050 × 1525 mm
Rough allowance	length 10 · width 10 · thickness 0 mm
Cost	≈ 95 per sheet

Density	E	MOR	Movement	Screw face / end
680	9500	60	0.15 / 0.15	medium / low

sheet.plywood.poplar

Poplar-core plywood. Lighter, cheaper and softer; fine for a back or a bottom, marginal for a shelf.

Thicknesses	6 · 9 · 12 · 15 · 18 · 22 · 25 mm
Sheet sizes	2440 × 1220 · 2500 × 1250 mm
Cost	≈ 55 per sheet

Density	E	MOR	Movement	Screw face / end
470	6500	40	0.15 / 0.15	low / poor

sheet.mdf

Medium-density fibreboard.

Thicknesses	6 · 9 · 12 · 15 · 18 · 22 · 25 · 30 mm
Sheet sizes	2440 × 1220 · 3050 × 1220 mm
Cost	≈ 38 per sheet

Density	E	MOR	Movement	Screw face / end
750	3600	32	0.25 / 0.25	low / poor

Very poor edge screw holding. Anything structural wants a threaded insert or a through bolt rather than a screw into the edge. It is also the least stiff material here — at E 3600 it is a third of birch ply, so an MDF shelf of the same span sags roughly three times as far.

sheet.veneered.mdf

Veneered MDF. The one sheet material that takes a species.

Species	walnut white_oak ash maple birch
Thicknesses	18 · 22 · 25 mm
Sheet sizes	2440 × 1220 · 2800 × 2070 mm
Cost	≈ 130 per sheet

Density	E	MOR	Movement	Screw face / end
760	3800	34	0.25 / 0.25	low / poor

The veneer is directional and the sheet has a show face, so parts cut from it are grain-constrained in a way plain MDF is not. Set grain: and mean it.

Units: density kg/m³ · E MPa · MOR MPa · movement % per 10% change in moisture content, tangential / radial.

What to notice

Movement is tiny, and equal both ways. Plywood at 0.15% barely moves in plane, against 2.5–3.5% for solid timber. That is the whole reason a panel goes in a groove and a solid top does not: a plywood panel can be captured on all four sides and glued, and a solid one cannot.

Stiffness is the deciding number. Birch ply at E 9500 is close to softwood; MDF at 3600 is not close to anything. For a shelf, the deflection check will make the choice for you and give you the thickness that passes.

Panel stiffness figures are along the face grain. Across it, plywood is considerably less stiff. The shelf-sag rule accounts for that separately, using the part’s grain.

Edge screw holding is the failure mode. Every sheet material here rates low or poor in the edge, which is where people put screws when assembling a carcass. The fastener checks will raise it; the fix is an insert, a dowel, or a through bolt.

Sheet sizes and nesting

The cut list nests parts into sheets and tells you how many you need. Two things follow:

• A part longer than the sheet is an error, not a warning. Sheets have sheet sizes rather than a list of lengths, so the check is against both dimensions of the sheet — and it is the compiler’s job, not yours.
• Sheet choice matters. sheet.plywood.birch at 3050 × 1525 will yield a 1600 mm panel in one piece where the 2440 × 1220 sheet will not.