How do I deal with a cupped, bowed or twisted board?
Naming the four kinds of distortion, deciding which are worth correcting, and the machining and fixing tricks that use a bad board anyway.
- Difficulty
- intermediate
- Time
- 30 min
- Read
- 4 min
Short answer
Cup across the width and bow along the length are often correctable or workable. Twist, called wind, is the one to reject — it rarely recovers and it fights you through the whole job. Where the board must be used, cut it into shorter lengths, which removes most of the distortion.
Distortion has four names and knowing which one you have decides whether to fix it, work around it, or take it back.
What you'll need
- straightedge
- winding sticks or two straight battens
- hand plane or planer (optional)
- clamps and weights (optional)
Step by step
- Identify the distortion.Cup is a curve across the width. Bow is a curve along the length in the face. Spring, or crook, is a curve along the length in the edge. Wind, or twist, is the two ends being in different planes.
- Check for wind with winding sticks.Lay two straight battens across the board at each end and sight along them. If they are not parallel, the board is twisted.
- Reject twisted stock if you can.Twist is the hardest to correct, needs the most material removed, and often returns. In a merchant, sight along every board before it goes in the trolley.
- Try re-equalising a cupped board first.Sticker it flat with weight on it in the conditions it will be used in. Cup caused by uneven drying often reduces substantially over a week or two.
- Dampen the concave face if you need to speed it.Lightly dampen the hollow side and leave the board weighted flat. The damp side swells and the cup reduces. It is a temporary correction unless the board then equalises.
- Cut the board shorter.Bow and spring are curves over a length. Cut a bowed 2.4 metre board into three 800mm pieces and each piece is nearly straight. This is the most useful trick there is.
- Machine it flat if you have the thickness to lose.Flatten one face, then thickness to the other. Remember you will lose the full depth of the distortion, so a badly cupped 22mm board may finish at 16mm.
- Use fixings to pull mild distortion out.Cupped floorboards and decking can be pulled flat with fixings at every joist. Do not try this with severe cup — the board will split or the fixings will pull through.
- Orient cup deliberately.On decking and floorboards, opinions differ but the usual advice is to lay boards so water cannot sit in a hollow. Consider which way the board will move as it dries further.
- Use the distortion where it helps.A slightly sprung board fixed against a straight rail can be pulled straight and the tension does no harm. Keep the worst boards for short pieces and hidden work.
Tips
- Cut it shorter. Almost every bow and spring problem disappears when the piece gets shorter.
- Sight along every board in the merchant before buying it. It takes seconds and saves the whole problem.
- Twist is the one to walk away from. It costs the most material to remove and it frequently comes back.
Common mistakes
- Forcing a badly cupped board flat with fixings — The board is under permanent tension and will either split along the grain or pull the fixings through, sometimes months later.
- Machining a distorted board flat before it has acclimatised — If the board is still equalising, it will move again after machining and you will have lost thickness for nothing. Let it settle first.
If it doesn't work
Board rocks on a flat surface
Cause: Twist, or wind — Fix: Check with winding sticks. Machine flat if there is thickness to spare, cut it shorter, or set it aside.
Cupped floorboards
Cause: Moisture difference between the two faces, usually damp underneath — Fix: Investigate the underfloor ventilation and any damp source. Correcting the cause matters more than the boards.
Board bowed along its length
Cause: Uneven drying or growth stresses released in sawing — Fix: Cut into shorter lengths, or fix it against a straight member at close centres if the bow is mild.
Board moved again after machining flat
Cause: Not acclimatised, or internal stresses released by removing material — Fix: Let stock acclimatise fully before machining, and machine in stages, taking equal amounts from both faces.
Questions people ask
Why does timber cup away from the heart?
Growth rings tend to straighten as the wood dries, so a flatsawn board cups away from the side that was nearest the centre of the tree. Reading the end grain lets you predict which way a board will go.
Is quartersawn timber more stable?
Yes, markedly. The growth rings run through the thickness rather than across the width, so it cups very little. It costs more and yields less from a log, which is why it is a premium.
Can a twisted door be straightened?
Sometimes, by clamping it flat with weight over a long period, or by adjusting the hinges and the stop to accommodate the twist. Severe twist usually means replacement.