The heart side of a board is the side that faced the heart (the core) of the tree. The opposite side is called the sap side or outer side. On the end grain you can tell them apart by the growth rings: the heart side is the side with the smallest ring arcs.
Why it matters
Wood ‘moves’ (shrinks and swells) mainly in the direction of the growth rings. As a result, a board typically warps cupping towards the heart side as it dries — the rings want to straighten out. Knowing where the heart side is helps you steer that warping:
- Floorboards and decking — usually lay the heart side up (‘heart up’) so the board does not cup upward at the edges and let water sit; different rules of thumb exist, so consider the application
- Worktops and wide panels — glue boards alternately (heart side turned each way) so warping cancels out
Recognising it
- Look at the growth rings on the end grain — the convex side of the rings points to the heart side
- Medullary rays and figure — the pattern on the face shows whether the timber is flat-sawn or quarter-sawn
Points to watch
- Quarter-sawn timber moves less and hardly cups; ideal for dimensionally stable work
- Fix boards well — enough screws or nails limit cupping, but the stress in the wood always remains
Related terms
- Wood
- Growth ring
- Quarter-sawn
- Flat-sawn
- Wooden floors
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