Materials

A materials entry is a named stock, not a species and not a board. from points at a catalogue family and the remaining keys narrow it to one real, purchasable size.

materials:
  - id: rail_stock
    from: lumber.hardwood.s4s
    species: "=species"
    thickness: 38
    width: 190

Parts then say material: rail_stock, and expressions can read rail_stock.thickness, rail_stock.width, rail_stock.density and rail_stock.modulus_of_elasticity.

Key
id	required	The name parts and expressions use.
from	required	A catalogue family id — see the catalogue.
species		Required for families that are milled in more than one species.
thickness		Millimetres, actual, never nominal.
width		Millimetres, actual. Omit for sheet goods.
label		Overrides the id on the cut list.

Declare a stock per role, not per species

The mistake to avoid is one walnut material used for everything. Posts, rails and cleats are different boards; giving each its own entry is what makes the cut list group correctly, lets the rails change thickness without moving the posts, and gives expressions something specific to reference.

materials:
  - { id: post_stock,  from: lumber.hardwood.s4s, species: "=species", thickness: 75, width: 75 }
  - { id: rail_stock,  from: lumber.hardwood.s4s, species: "=species", thickness: 38, width: 190 }
  - { id: cleat_stock, from: lumber.softwood.s4s, species: pine,       thickness: 25, width: 40 }
  - { id: panel_stock, from: sheet.plywood.birch, thickness: 18 }

Note species: "=species" — a text field, so an expression needs the leading =. Three of those stocks follow the design’s species parameter; the cleats are pine because nobody sees them.

The size must be one that is milled

The catalogue lists the thicknesses and widths a family is actually available in. A combination that is not on the list is RES-009, and the finding names the nearest sizes that are.

RES-009  lumber.hardwood.s4s is not milled 35 mm thick.
         Nearest available: 32 mm, 38 mm.

This is a design rule masquerading as a lookup failure. A 35 mm rail is a rail you cannot buy, and finding that out at the timber yard is worse than finding it out here.

What a resolved material carries

Narrowing a family gives the rest of the pipeline more than two numbers. The species properties come with it, and the validator reads them:

• density — for the weight on the BOM and the load checks
• modulus of elasticity — for shelf and rail deflection
• movement, tangential and radial — for the wood-movement checks on panels and tops
• screw withdrawal, face and end — for the fastener checks
• rough allowance — extra on each dimension for the cut list’s rough sizes

A pine cleat and a walnut rail take the same screw and hold it differently, and the report says so because the material knows which it is.

Sheet goods

Sheet materials have a thickness and a sheet size, not a width:

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

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. Plywood barely moves in plane; MDF holds a screw poorly in the edge and very poorly in the end.

sheet.veneered.mdf is the exception that takes a species, because the veneer is directional and the sheet has a show face.

Sheet goods still need a grain direction

Plywood is stiff along the face grain and noticeably less so across it, and veneered panels have a visible direction. Set grain: on the part, not the material — two parts cut from the same sheet can run different ways, and the nesting is worked out from the parts.

Rough sawn versus surfaced

lumber.hardwood.rough states the size you start with; lumber.hardwood.s4s states the size you finish at. The rough family carries larger allowances — 6 mm on thickness against 3 mm — so the cut list asks for enough board to flatten.

If you are unsure, use the surfaced family. It is what a merchant hands over.

The families available

Family	Class
lumber.hardwood.s4s	solid	Nine species, 19–75 mm, 50–290 mm wide
lumber.softwood.s4s	solid	Five species, 18–75 mm, 40–245 mm wide
lumber.hardwood.rough	solid	Rough sawn, larger allowances
sheet.plywood.birch	sheet	4–30 mm, three sheet sizes
sheet.plywood.poplar	sheet	6–25 mm, cheaper and softer
sheet.mdf	engineered	6–30 mm
sheet.veneered.mdf	engineered	18–25 mm, five veneers

Full specification tables, including every milled size and the per-species properties, are in the catalogue section.