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Depth rules are simple tools, but they prevent expensive mistakes. A mortise that is 2 mm too shallow can leave a tenon proud; a rebate that is too deep can weaken an edge or leave a door component loose. The best rule depends on whether you are checking a flat-bottomed recess, a stopped mortise, or a rebate cut along an edge. It also depends on whether you need to check one joint or repeat the measurement across a whole project.
What a depth rule does
A depth rule measures from a reference surface down to the bottom of a mortise, groove, or rebate. Unlike a tape measure, it lets you keep the measuring end square to the work while reading a short, controlled distance. This matters because a small change in angle can make a 20 mm recess appear several millimetres deeper or shallower.
Most useful depth rules have a narrow blade or rod, a shoulder that rests on the surface, and a readable scale. Some are marked steel rules with a sliding stop. Others are dedicated mortise-depth gauges, often with a wide bridge that spans the opening. A basic mortise depth gauge is usually enough for furniture work, while a combination square with a depth rod is more versatile around a general workshop.
Choosing the right depth rule
| Tool type | Best use | Strength | Limitation |
|---|---|---|---|
| Marked steel rule | Quick checks and wide rebates | Cheap, fast, easy to carry | Hard to hold consistently in a narrow mortise |
| Combination square with depth rod | General carpentry and repeated checks | Measures depth and checks square in one tool | Rod can be too wide for small mortises |
| Sliding mortise-depth gauge | Mortises, hinge recesses, and stopped work | Shoulder gives repeatable readings | Less useful for broad, irregular cavities |
| Digital depth gauge | Fine tolerance work and recording measurements | Clear readings in small increments | Needs batteries and careful zeroing |
| Depth caliper | Rebates and open grooves | Quick to use over an edge | Not ideal where access is restricted |
For most carpenters, the best first purchase is a good combination square with a depth attachment or rod. It covers rebates, dados, hinge gains, and general layout. If you cut many mortise-and-tenon joints, add a dedicated sliding gauge. A cheap rule is fine for rough framing and utility shelving, provided the joint does not depend on a close fit.
Checking mortise depth accurately
Start by brushing or blowing chips out of the mortise. A shaving under the rule can add 0.5 to 1 mm, which is enough to make a tenon appear badly sized. Set the rule against a known surface and lower the blade until it touches the bottom. Do not force it. Compressing loose fibres or levering against the bottom gives a false reading.
Check at both ends and in the centre of a long mortise. Chisel-cut mortises commonly have a sloping bottom, especially when the chisel was worked from one direction. For a 60 mm-long mortise, a useful target is often 2 to 3 mm deeper than the tenon length. That clearance gives glue somewhere to escape and prevents the tenon from bottoming out before its shoulders close.
Do not automatically make the clearance excessive. A 10 mm-deep tenon does not need a 5 mm empty cavity beneath it. Too much unsupported space can trap glue, reduce the effective bearing area, and make the joint harder to assemble. For ordinary PVA glue, 1 to 3 mm of extra depth is generally practical; use the lower end when the bottom is clean and flat.
Check the tenon separately with the same reference face used for the mortise. If the shoulder-to-end measurement varies, mark the short side rather than trying to correct everything from the mortise. A depth rule tells you what exists; it does not identify which component caused the error.
Checking rebates and rabbets
A rebate is easier to measure because one side is open, but that does not make it foolproof. Place the rule’s shoulder on the face that will remain visible and measure to the rebate floor. Check near both ends and in the middle. Hand planes and rebate planes can leave a slight hollow or a high spot, particularly in softwood.
For a rebate receiving a panel, glass, or backboard, measure from the actual seating face, not from a nearby edge. A 6 mm backboard usually needs a rebate around 6 to 7 mm deep, depending on whether it is fixed with nails, screws, or a retaining strip. If the rebate is too shallow, the panel stands proud. If it is too deep, the surrounding edge becomes weaker and the panel may sit loose.
When using a combination square with a depth gauge, keep the stock firmly against the reference face. Do not allow the rod to follow a chamfer, rounded corner, or loose fibre. On a wide rebate, make a second check with a straightedge across the floor. A rule may show depth at one point while the rebate still has an uneven bottom.
Tolerances and common reading errors
For rough carpentry, a 1 mm error is rarely important. For furniture joints, aim for readings within about 0.5 mm when checking a mortise or hinge recess. That does not mean every tool must be graduated to 0.5 mm; it means the tool must be held consistently and the work must be cleaned before measuring.
Parallax is a common problem with marked rules. Read the scale directly above the blade rather than from an angle. A bent blade produces another false result, as does a rounded tip that touches the bottom on a single point. Use a narrow, flat end for small mortises and a broader end for softwood rebates.
Dust, bruised fibres, and a tilted shoulder are the main site failures. So is measuring from different faces during the same job. Mark the reference face with a pencil triangle, then use it for layout, planing, and checking. A small steel marking gauge is useful here because it transfers a consistent shoulder line before you cut.
When a digital gauge is worth buying
Digital gauges are helpful when you need to compare many recesses quickly or record exact dimensions. They are less necessary for occasional mortises. Their main risks are poor zeroing, battery failure, and trusting a display without checking the reference surface. Zero the tool on a clean flat surface before each batch of measurements, and confirm it against a known rule.
If you work mainly on site, choose a robust mechanical gauge that fits in a tool belt and can be read with dirty hands. If you build cabinets or fitted furniture at a bench, a sliding gauge with a positive lock is usually the better buy. Spend money on a straight blade, a square shoulder, and a lock that does not creep; fine graduations are secondary.
A practical checking routine
Before final assembly, clean the recess, confirm the reference face, and check depth in at least three places. Compare the reading with the intended tenon, panel, or hardware thickness. Correct high spots with a sharp chisel or plane rather than deepening the whole recess. Finally, dry-fit the parts. A depth rule catches dimensional errors, but only a dry fit reveals a tilted shoulder, trapped glue, or a joint that binds before it closes.