How do I cut a housing joint for a shelf?

A housing — sometimes called a dado — is a trench across one board that another board sits in. It is the standard shelf joint because the trench itself carries the load in shear, so the joint holds even before glue is considered, and it locates the shelf perfectly during assembly. Its one difficulty is that sheet material is never the thickness it claims to be.

What you'll need

• A marking knife and square
• A marking gauge
• A tenon saw, or a router with a straight bit
• A chisel
• A router plane or wide chisel for the bottom (optional)

Step by step

1. Measure the actual shelf, not the label.Nominal 18 mm plywood is routinely 17.5 mm or less and varies along the sheet. Take the width of the housing directly from the piece that will sit in it, at the point where it will sit.
2. Mark both walls of the trench with a knife.Lay the shelf itself against a square on the carcass side and knife along it. A knife line gives the saw and the chisel something positive to register against; a pencil line does not.
3. Gauge the depth on the edges.About a third of the board thickness is the usual rule — deep enough to carry load, shallow enough not to weaken the side. Mark it on both edges so you can see when you have arrived.
4. Deepen the knife lines into a small V trench.Pare a shallow chamfer on the waste side of each line. The saw then drops into a groove and cannot wander across the show face.
5. Saw both walls down to the depth line.A tenon saw with a depth stop clamped to the plate, or careful sighting against the gauge line at both edges. If you are routing instead, clamp a straight batten as a fence and take two or three passes rather than one deep one.
6. Remove the waste between the saw cuts.Chisel from both ends towards the middle, bevel down, taking small bites. Working from one end only tears the fibres out at the far side.
7. Flatten the bottom of the trench.A router plane is the tool made for this and gives a consistent depth in seconds. Failing that, pare with a wide chisel, back down, and check with a straightedge across the trench.
8. Stop the housing short if the front edge will be visible.End the trench 10 to 15 mm from the front, and cut a matching notch in the front corner of the shelf. From the front the joint is invisible.
9. Test fit with hand pressure.The shelf should slide in with firm hand pressure and stay put. A housing that needs hammering will split the side when the glue swells the fibres.
10. Glue and cramp across the joint.Glue the walls of the trench rather than the bottom — the bottom is end grain to the shelf and contributes little. Cramp across the carcass with a block so you do not bruise the outside.

Tips

• In a bookcase, a housing at each shelf turns two independent sides into a rigid ladder. It is the single biggest structural upgrade over screwed butt joints.
• A shouldered or barefaced housing — where a small rebate on the shelf end lets a wide trench take a thinner tongue — is the fix when your material thickness varies wildly between shelves.
• If you are routing, a straight bit slightly narrower than the shelf, run twice from opposite fences, gives an exact fit without needing an exact cutter.

Common mistakes

• Cutting the housing to a nominal thickness — Sheet material is manufactured under its stated size and varies. The joint is then loose, the shelf rattles, and glue alone cannot fill it.
• Cutting the housing too deep — Past about half the board thickness you have removed most of the side's strength at exactly the point it is most loaded, and long carcass sides start to break at a housing.

If it doesn't work

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