Wood Properties
Part of Woodworking
Understanding how wood behaves saves you from fighting it. Every split board, warped door, and cracked joint traces back to ignoring the fundamental properties of the material. Learn them once and they guide every decision you make.
Hardwood vs. Softwood
This is the most misunderstood distinction in woodworking. Hardwood and softwood are botanical categories, not descriptions of physical hardness.
- Hardwoods come from broadleaf, deciduous trees (angiosperms): oak, maple, ash, walnut, birch, cherry
- Softwoods come from needle-bearing, usually evergreen trees (gymnosperms): pine, spruce, fir, cedar, hemlock
The confusion: balsa is technically a hardwood — yet it is one of the softest woods on earth. Yew is a softwood — yet it is harder than many hardwoods.
Practical Differences
| Property | Hardwoods (typical) | Softwoods (typical) |
|---|---|---|
| Growth rate | Slower | Faster |
| Grain complexity | More complex, varied figure | Simpler, more uniform |
| Density | Generally higher | Generally lower |
| Cost/availability | Less available, more valuable | Widely available |
| Workability | Harder to cut, holds detail | Easier to cut, may crush |
| Rot resistance | Varies widely by species | Cedar, redwood excellent; pine poor |
| Primary use | Furniture, tools, flooring | Framing, sheathing, general construction |
Grain Direction and Its Effects
Grain is the alignment of wood fibers along the length of the tree. Everything about working wood depends on understanding grain direction.
Types of Grain
- Straight grain: Fibers run parallel to the long axis. Splits cleanly, planes smoothly, strongest along its length.
- Interlocked grain: Fibers alternate direction in successive layers. Resists splitting (elm, some tropical species). Difficult to plane without tearout.
- Spiral grain: Fibers twist around the trunk. Weak, unpredictable, generally avoided.
- Wavy grain: Fibers undulate. Creates beautiful figure (curly maple, fiddleback) but challenging to plane.
Working With vs. Against the Grain
- With the grain (planing, chiseling in the direction fibers slope away from the surface): Cuts cleanly, minimal effort.
- Against the grain: Fibers catch and tear out, leaving a rough, pitted surface. Reverse your direction or use a steeper cutting angle.
- Across the grain (crosscutting): Requires a different saw tooth geometry than ripping (cutting along the grain).
Finding Grain Direction
Run your hand along the surface. It feels smooth one way and slightly rough the other — like petting a cat. The smooth direction is “with the grain.” Always plane, chisel, and spoke-shave in this direction.
Heartwood vs. Sapwood
Every tree has two distinct zones of wood:
- Sapwood: The outer ring of living wood. Carries water and nutrients. Usually lighter in color. More susceptible to rot and insect attack because it contains sugars and starches that feed organisms.
- Heartwood: The inner core of dead (but structurally sound) wood. The tree has filled it with extractives — tannins, oils, resins — that often make it darker and more rot-resistant.
Practical Implications
- For outdoor use and ground contact: heartwood only. Sapwood rots quickly even in rot-resistant species.
- For indoor furniture: sapwood is fine structurally but may create color contrast you do not want.
- In young trees: heartwood may not have formed yet. The entire cross-section is sapwood.
- White oak heartwood is waterproof (tyloses block the pores). Red oak heartwood is not. This matters for barrels, boats, and outdoor furniture.
Annual Rings
Each ring represents one year of growth. The ring has two parts:
- Earlywood (springwood): Lighter, less dense. Formed during the fast growth of spring.
- Latewood (summerwood): Darker, denser. Formed during the slower growth of summer and fall.
What Rings Tell You
| Ring Pattern | Meaning |
|---|---|
| Wide, even rings | Fast growth, lower density, easier to work |
| Narrow, even rings | Slow growth, higher density, stronger |
| Rings wider on one side | Tree was leaning or crowded on one side |
| Sudden change wide to narrow | Environmental change (drought, crowding, injury) |
| Very dark latewood bands | Dense species, strong contrast grain pattern |
Ring Count and Quality
More rings per inch generally means stronger wood in softwoods. In ring-porous hardwoods (oak, ash), the opposite can be true — faster growth means more dense latewood relative to the large earlywood pores, producing stronger wood. Know your species before judging by ring count alone.
Density and Workability
Density (weight per volume) is the single best predictor of wood’s mechanical properties. Denser wood is:
- Stronger in compression and bending
- Harder to cut, plane, and chisel
- More resistant to denting and wear
- Harder to nail (may need pre-drilling)
- Heavier to transport and handle
Lighter wood is:
- Easier and faster to work with hand tools
- More prone to denting and wear
- Better for applications where weight matters (shelving, boats, aircraft)
- Easier to nail and fasten
A simple field test: pick up two pieces of similar size. The heavier one is denser. If a piece feels surprisingly light for its size, it may have internal decay.
Moisture Content
This is where most beginners get into trouble. Living trees contain enormous amounts of water — some species are over 100% moisture content (meaning the water weighs more than the wood fiber itself).
Why Moisture Matters
Wood shrinks as it dries. A board cut from a green log will:
- Shrink across the grain (width and thickness) significantly
- Shrink along the grain (length) almost not at all
- Potentially warp, cup, twist, or check (crack) as it dries
If you build with green wood and it dries in place, joints open up, doors stick or gap, and boards split.
Equilibrium Moisture Content (EMC)
Wood constantly exchanges moisture with the air around it. It eventually stabilizes at a moisture content determined by the local humidity:
| Environment | Typical EMC |
|---|---|
| Heated indoor space | 6-8% |
| Unheated indoor space | 10-14% |
| Covered outdoor (shed) | 14-18% |
| Open outdoor | 16-22% |
Build furniture from wood at 6-8% MC. Build outdoor structures from wood at 12-15% MC. Using wood far from its target EMC guarantees movement problems.
Measuring Moisture Content
Weight method (no tools needed):
- Weigh a sample piece
- Dry it completely (in an oven at 215degF/100degC until weight stops changing)
- MC% = ((wet weight - dry weight) / dry weight) x 100
Pin meter: Two pins pushed into the wood measure electrical resistance. Higher moisture = lower resistance. Accurate and fast.
Feel test (rough guide): Green wood feels cool and heavy. Air-dried wood feels room temperature and noticeably lighter.
Common Defects
| Defect | Description | Cause | Impact |
|---|---|---|---|
| Knots | Embedded branch bases | Branch growth | Weaken the board, hard to plane, decorative in some uses |
| Checks | Cracks radiating from center | Drying stress | Structural weakness, cosmetic issue |
| Shake | Separation between growth rings | Wind stress, drying | Board may fall apart along the ring |
| Warp (bow) | Board curves along its length | Uneven drying, grain | Will not lie flat — joint or plane it out |
| Cup | Board curves across its width | Uneven drying, ring orientation | Flatten with a plane or flip and weight down |
| Twist | Corners do not lie in the same plane | Spiral grain, uneven drying | Most difficult to correct — often must cut around it |
| Spalting | Dark zone lines in wood | Early fungal decay | Beautiful figure but weakened wood — stop the process by drying |
Structural Defects
Never use wood with shake or large through-checks for load-bearing applications. A knot at the edge of a beam reduces its strength far more than a knot at the center. Reject any structural timber with defects at stress points.
Species Comparison Table
| Species | Type | Density | Hardness | Rot Resistance | Primary Uses | Workability |
|---|---|---|---|---|---|---|
| White Oak | Hardwood | High | Hard | Excellent | Boats, barrels, outdoor furniture, timber framing | Moderate — splits well, planes with care |
| Red Oak | Hardwood | High | Hard | Poor | Indoor furniture, flooring | Moderate — open pores need filling for finish |
| Ash | Hardwood | Medium-High | Medium-Hard | Poor | Tool handles, bending, sports equipment | Good — planes and bends well |
| Hard Maple | Hardwood | High | Very Hard | Poor | Flooring, cutting boards, turning | Difficult — dulls tools, but finishes beautifully |
| White Pine | Softwood | Low | Soft | Poor | Interior trim, carving, shelving, light framing | Excellent — easy to cut and shape |
| Birch | Hardwood | Medium | Medium-Hard | Poor | Plywood, turning, bark craft, utensils | Good — fine grain, even texture |
| Willow | Hardwood | Low | Soft | Poor | Baskets, cricket bats, charcoal | Easy to work green, weak when dry |
| Black Walnut | Hardwood | Medium | Medium | Good | Fine furniture, carving, gunstocks | Excellent — works beautifully in all directions |
| Cedar | Softwood | Low | Soft | Excellent | Shingles, chests, fence posts, closet lining | Easy — but brittle, splits easily |
| Douglas Fir | Softwood | Medium | Medium | Moderate | Structural framing, plywood | Good for sawing, splinters when planed |
When In Doubt
If you do not know the species, test a small piece. Try splitting it, planing it, and bending it. How it behaves in those three tests tells you more about its properties than any chart.
Wood Properties — At a Glance
- Hardwood/softwood is botanical, not about physical hardness
- Always work with the grain, never against it
- Heartwood resists rot; sapwood does not
- Wood shrinks across the grain as it dries — build with properly dried wood
- Density predicts strength, hardness, and workability
- Know your defects: knots weaken, checks propagate, twist is nearly impossible to fix
- Match species properties to the job — no single wood is best for everything