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A timber high spot is a raised area that prevents a board, frame, floor, or cabinet component from sitting flat. You do not need a powered planer or laser to find one. A reliable straightedge, a sharp pencil, and a few minutes of methodical checking will show where the timber is proud and how much needs removing.
This method works on joists, studs, rafters, worktops, doors, shelves, and rough-sawn boards. It identifies the high spot; removing it is a separate decision based on the required finish and the amount of material involved.
What you need
The straightedge is the critical tool. Use a true, rigid edge at least 600 mm (24 inches) long for small parts and 1.2 to 1.8 metres (4 to 6 feet) for floors, framing, and long boards. A shorter edge can bridge over a dip and make the surrounding timber appear flat.
| Tool | Best use | Main limitation |
|---|---|---|
| Steel rule or short straightedge | Small boards, door edges, and local checks | Too short for long timber |
| Long aluminium straightedge | Joists, worktops, flooring, and cabinet panels | Can be damaged or bent if stored badly |
| Spirit level | Checking flatness while also checking level | May have a curved or damaged working edge |
| Jointing plane or hand plane | Removing a confirmed high spot | Requires setup and controlled cuts |
A dedicated 48-inch aluminium straightedge is usually a better buy than using a random length of timber. Wood can have its own bow, twist, or damaged edge. A spirit level is acceptable if its long edge is straight, but do not assume the bubble matters for this test.
You also need a sharp pencil, a tape measure, a square, and a way to mark the timber without losing the reference. A 2B pencil leaves a visible line on most softwood without making a deep mark. Keep a small block plane or a properly sharpened block plane available if only a small amount needs to come off.
Check the straightedge first
Before trusting the tool, inspect both faces and the working edge. Look for dents, burrs, dried adhesive, or a visible bow. Place it on a known-flat surface, such as a thick sheet of glass or a carefully checked workbench. Press down at one end, then the other. If it rocks, the edge is not reliable.
For a quick reversal test, place the straightedge against the timber and draw a light pencil line along it. Turn the straightedge end for end and place it on the same reference points. If the edge produces a noticeably different line, it may be bent. A small error matters: a 1 mm bow in the straightedge can lead you to remove material that was not actually high.
Find the high spot
Remove dust, loose fibres, and protruding nails or screws. Set the timber on firm supports so it cannot flex while you check it. Flexing is a common source of false readings, particularly with long boards and thin panels.
Place the straightedge across the timber, perpendicular to the suspected hump. Do not press it down hard. Let it rest naturally on the highest points. Shine a light behind the edge if the gap is difficult to see.
Try to slide a pencil or a thin feeler strip under the straightedge. Where the straightedge touches the timber, there is a high point. Where you can see daylight or slide the pencil, there is a low area. The raised section is not always at the centre of the visible gap; it may be one end or one corner.
Mark the timber while the straightedge is still in position. Draw short witness marks on the high area, rather than one large heavy line. Make a cross at the highest point and add arrows showing the direction of the surrounding low areas. Repeat the check every 150 to 300 mm (6 to 12 inches) along the length.
For a board, check in three directions:
- Across the width to find a crown or hollow.
- Along the length to find a hump or dip.
- Diagonally to reveal twist that a straight lengthwise check can miss.
Measure how much is high
Finding a high spot is not the same as knowing how much to remove. Use feeler gauges, thin card of known thickness, or a set of shims. Start with a 0.5 mm (about 0.020 inch) gauge and increase gradually. Keep the straightedge in the same position while testing.
Record both the location and the gap. For example, mark “2 mm high at 420 mm from the left end.” This is useful when checking a run of joists or transferring the correction to a hand plane. If the job has a finish tolerance, measure against that requirement rather than trying to make every piece mathematically perfect.
On flooring or a worktop, a difference of 1 to 2 mm may prevent a rigid covering from sitting properly. In rough framing, a similar difference may be harmless if cladding, packers, or a later surface layer will hide it. Removing material unnecessarily can weaken a joist, reduce a door edge, or create a low spot that is harder to correct.
Confirm before cutting
Check the timber from the opposite face and repeat the test after rotating the straightedge end for end. This helps separate a timber defect from a tool error. Also check adjacent boards or framing members. A single high joist may need planing, but a whole row at different heights may indicate incorrect support or a framing layout problem.
For fine work, use a feeler gauge set rather than folded paper. Paper thickness varies, and compressed paper gives optimistic readings. For ordinary site carpentry, a pencil, straightedge, and a few known-thickness shims are usually sufficient.
Remove the high spot safely
Plane with the grain where possible, taking light cuts and checking frequently. Set the blade for a shaving around 0.25 to 0.5 mm thick on softwood. Work inside the marked area first, then extend the strokes gradually to blend the correction. A sharp iron is more important than an expensive plane; a dull blade encourages force and tears the fibres.
For a broad, flat surface, a jack plane removes material faster than a block plane, but it needs more room and better technique. Sanding is fine for a tiny raised fibre or a correction under about 0.5 mm. It is slow and difficult to control for a substantial hump.
Recheck with the straightedge after every few passes. Stop when the marked high spots no longer contact the edge and the remaining gap is within the job’s tolerance. If the high spot is more than a few millimetres on a structural member, investigate the cause before cutting deeply; the timber may be carrying load or may need packing and adjustment instead of removal.