| name | materials-and-fit |
| description | Materials and fit for the trades — how wood, metal, stone, fabric, and composite materials behave, how they move with humidity and temperature, and how "fit" between parts is achieved in the presence of material behavior. Covers moisture content, thermal expansion, grain and fiber direction, elastic and plastic deformation, corrosion and finish interaction, and the traditional allowances that experienced tradespeople build into their work. Use when specifying materials, diagnosing a failed fit, or teaching a learner why traditional allowances exist. |
| type | skill |
| category | trades |
| status | stable |
| origin | tibsfox |
| modified | false |
| first_seen | "2026-04-12T00:00:00.000Z" |
| first_path | examples/skills/trades/materials-and-fit/SKILL.md |
| superseded_by | null |
Materials and Fit
Materials are alive. Wood moves with humidity, metal moves with temperature, stone settles under load, fabric stretches and relaxes, and modern composites have behaviors that surprise people who expect them to behave like traditional materials. "Fit" between parts — whether a drawer slides smoothly for a century or binds after the first humid summer — depends on understanding this movement and designing for it rather than against it. This skill covers the core physical behaviors that every trade has to respect.
Agent affinity: vitruvius (Roman material discipline), nasmyth (metallurgy and machine tolerances), brunel-tr (material selection in shipwork)
Concept IDs: trades-material-movement, trades-fit-allowance, trades-material-failure
Why Materials Move
Almost every material used in the trades changes dimension in response to its environment. The two dominant mechanisms are thermal expansion and moisture exchange.
Thermal expansion is the dimensional change of a material with temperature. Metals expand and contract by coefficients on the order of tens of parts per million per degree Celsius — small per degree, large across a day or a season. A thirty-foot steel beam changes length by about a quarter-inch between a winter night and a summer afternoon. Wood and stone expand thermally too, but less. Plastics expand much more.
Moisture exchange is the dimensional change of a material with the moisture content of its surroundings. Wood is the champion here: a piece of wood can change width by several percent between a dry winter shop and a humid summer one. Leather, paper, cloth, and concrete all move with humidity to varying degrees. Metals mostly do not, but ferrous metals rust in wet air, which is a different kind of movement — dimensional growth by corrosion product.
A tradesperson who ignores these movements produces work that looks correct at the moment of assembly and fails afterward, often months later when the first seasonal cycle completes. A tradesperson who designs for these movements produces work that is loose at assembly time (by a deliberate margin) and tight after equilibration.
Wood — The Moisture-Dominant Material
Wood shrinks and swells far more than almost any other material in response to humidity. The magnitude depends on the grain direction:
- Longitudinally (along the grain): almost no movement. Shrinkage is often below 0.1% from green to bone-dry. For most purposes it can be ignored.
- Radially (perpendicular to the growth rings, toward the center of the tree): about 4–8% from green to bone-dry.
- Tangentially (perpendicular to the grain, along the growth rings): about 8–14% from green to bone-dry.
The ratio of tangential to radial movement is the reason for wood's tendency to cup, twist, and check. A flat-sawn board (tangential) moves roughly twice as much as a quarter-sawn board (radial) of the same species. Historically, quarter-sawn lumber was preferred for stable work — drawer sides, wide panels, joinery surfaces — for this reason. Quarter-sawn lumber is more expensive to produce and is wasteful of the log, which is why much modern work is done in flat-sawn lumber with allowances built in for the greater movement.
