Quick overview of crack types
Woodworkers use several words for cracks, and the terms are not always used the same way. These are the common ones:
| Type | Where it shows up | Usual cause |
|---|---|---|
| Surface check | Face or edge of a board, along the grain | Surface drying faster than the core |
| End check | End grain of a board or log | Rapid moisture loss from the end |
| Split | Runs fully through the thickness of a piece | Drying stress, impact, or fasteners and joints |
| Shake | Separation along or between growth rings, present in the tree | Growth stresses, wind or felling |
| Honeycombing | Hidden cracks inside the board | Harsh drying that stresses the interior |
Why wood cracks
Wood shrinks as it dries and swells as it takes up moisture, and it moves much more across the grain than along it. Cracks form when that movement is restrained or uneven. For example, the outside of a board dries and tries to shrink while the wetter inside resists, and the surface fibers pull apart.
Other causes include growth stresses in the tree, impact, fasteners driven too close to an end, and glued or screwed panels that cannot move with the seasons. Changes in indoor humidity add to this; see ideal humidity for wood furniture.
Checks (surface and end)
A check is a crack that runs along the grain and does not go all the way through the piece. Checks that start at the end of a board are called end checks, and they are among the most common drying cracks because end grain loses moisture quickly.
Small checks are often cosmetic. They can widen and lengthen if the wood keeps drying, so watch them over a season.
Splits
A split is a crack that goes all the way through a piece, from one face to the opposite face. Splits often grow from an end check, or appear when a nail or screw is driven close to an end, or when a joint is put under stress. For causes, grading and fixes, see split wood defect: causes and fixes.
Shakes (ring, heart, star)
A shake is a separation in the wood that originates in the standing tree or during felling, usually along the growth rings.
- Ring shake follows a growth ring.
- Heart shake radiates out from the center.
- Star shake has several cracks radiating from the center, like a star.
Because shakes are part of the lumber, not a result of your home conditions, they will not close up. Boards with shakes are often rejected for structural use or cut around for furniture.
Honeycombing and internal cracks
Honeycombing happens when a board dries too harshly, creating cracks inside that may not be visible from the surface. It is usually found when a board is cut open or planed. Cut or planed surfaces may reveal hidden cracks, so boards that feel unusually light or show odd cracks at the ends deserve a careful look before you commit to a project.
Cracks in finished furniture and joints
In existing furniture, cracks commonly appear:
- Along glue lines in tabletops where boards separate
- In wide solid panels held rigidly in a frame, with no room for movement
- Near hardware, hinges and screws
- In the finish alone, as fine crazing that does not go into the wood
If the crack is only in the finish, the wood underneath may be sound. Test with a light fingernail drag and a close look.
Cosmetic or structural?
A crack is more likely to be cosmetic if it is short, shallow, away from joints and not growing. Treat it as a possible structural problem when:
- It is in a chair leg, rail, shelf support or other load-bearing part
- It runs through a joint
- It grows, or you can see daylight through it
- It is on a piece people sit, climb or hang things on
When in doubt, do not rely on a repaired load-bearing part. Replace it or ask a qualified furniture repairer or, for a building structure, a qualified professional.
How to repair cracked wood
The best method depends on the crack:
- Hairline checks: often left alone, or filled with a wood filler or a finish suited to the piece.
- Small cracks: wood glue worked in and clamped (see wood glue types compared), or thin cyanoacrylate (CA) glue, sometimes with fine sawdust as a filler.
- Wider cracks: epoxy, tinted if you want it to blend, or a close-fitting wood sliver glued in.
- Splits in tabletops or slabs: wooden butterfly keys (bow ties) across the crack, set into recesses.
- Cracks in turned bowls: many turners fill and treat cracks as a design feature.
Safety first
- Wear eye protection and a dust mask or respirator when sanding. Wood dust can irritate the airways, and some species cause allergic reactions.
- Read labels for glue, epoxy and finishes. Work with ventilation, wear gloves, and keep CA glue away from skin and eyes.
- Epoxy can give off fumes while curing. Follow the manufacturer’s instructions.
- Routing or sawing recesses for butterfly keys involves power tools. Clamp the workpiece, use guards and push sticks where appropriate, and unplug before changing bits or blades.
- Rags with oil or solvent finishes can self-heat and ignite. Lay them flat to dry outdoors or store them in a sealed metal container filled with water, per the product label.
If a crack keeps reopening, find out why. Fix the restraint (for example a screw that does not let a panel move) or the humidity swing, or the repair may fail again.
Preventing cracks
- Buy or use wood that has been properly dried for its end use.
- Seal end grain on fresh lumber with a wax-based end sealer or similar product to slow end drying.
- Allow panels to move: use slotted holes, buttons or clips for tabletops rather than gluing them rigidly to a frame.
- Pre-drill near ends and edges when screwing into solid wood.
- Keep indoor humidity from swinging widely.
Common wood defects and what imperfections are called
Imperfections in wood are generally called defects, and they are often sorted into three groups: those that occur in the living tree (knots, shakes, burls), those from sawing and machining (wane, tear-out, chip marks), and those from drying (checks, warping, honeycombing). Five common ones are knots, checks, splits, warp (such as cupping and bowing) and wane.
The question “What are the 7 types of wood finishes?” is about finishing, not cracks, and the answer depends on how you group products. Finishes are covered elsewhere on this site; start with types of hardwood finishes.
FAQ
What is the best way to repair cracked wood? It depends on the size and location. Glue and clamping suit small cracks, epoxy suits wider ones, and keys can stabilize splits in slabs. Do not rely on a repair for load-bearing parts that failed.
What are five common types of wood defects? Knots, checks, splits, warp and wane are commonly listed.
What are imperfections in wood called? Defects, although some, like knots and figure, are valued for appearance in some projects.