Wood Materials, Lumber Grades, and Moisture Behavior
Wood Materials, Lumber Grades, and Moisture Behavior
This domain covers species, grain, defects, grading, nominal versus actual sizes, and how moisture moves wood. Exam items often ask why lumber shrinks, warps, or changes dimension after installation.
Core rules
- Softwoods come from conifers; hardwoods from broadleaf trees. “Hardwood” is botanical, not a hardness guarantee.
- Grade stamps indicate strength or appearance class under a grading rule, not species or dryness by themselves.
- Nominal sizes are larger than dressed sizes: a 2x4 is 1-1/2 by 3-1/2 in; a 2x10 is 1-1/2 by 9-1/4 in.
- Wood shrinks most tangentially, less radially, least longitudinally. Flat-sawn boards cup and move more than quarter-sawn boards.
- Equilibrium moisture content follows temperature and relative humidity. Install interior trim near indoor conditions and keep framing dry.
Worked example
A 12-ft flat-sawn board is installed at 19% moisture content and later reaches 9% indoors. It will shrink across the width, so a tight fit can open at one edge. Allow for movement at ends and edges, and do not assume the length will shorten by the same percentage: longitudinal shrinkage is small.
Traps
Confusing nominal with actual size, treating a grade stamp as a species label, and sealing green wood on all sides are frequent errors. Another trap is expecting paint to stop moisture movement; it slows it but does not eliminate it.