Wood is anisotropic: its linear thermal-expansion coefficient runs roughly 10-15× higher across the grain (tangential, αT ≈ 5.5×10⁻⁵ /°C) than along it (longitudinal, αL ≈ 4×10⁻⁶ /°C), because the stiff cellulose microfibrils that resist length change run parallel to the grain but barely help across it. For a board whose grain sits at angle θ to a reference axis, the effective coefficient along that axis is the standard rotated-lamina formula α(θ) = αLcos²θ + αTsin²θ.
Each floorboard here runs its length along one axis and its narrow width across the other, so as night air cools it barely shortens but visibly narrows. Nails pin the board to the joists beneath at fixed points; the narrowing width shears against each nail shank, building static friction stress at that joint. When the accumulated shear exceeds the joint's slip threshold, the fibers or nail let go in a sudden micro-slip — the classic wood creak — and the local stress resets, only to start rebuilding as cooling continues. Neighboring boards cut with a different grain angle (the mismatch slider) contract by different amounts, so their shared joists desynchronize and creak at different times instead of all at once.
- Temperature drop rate / night low — sets how fast and how far the air cools, driving the total anisotropic contraction available.
- Moisture content — wetter (higher-MC) wood moves more per degree than dry, kiln-seasoned wood; this scales the contraction magnitude.
- Grain angle mismatch — how differently adjoining boards were sawn; larger mismatch desynchronizes creak timing across the floor.
- Joint friction/stiffness — how much shear a nail joint tolerates before slipping; stiffer joints creak less often but release more stress each time.