Dadoes and rabbets that fit real plywood

Dadoes and rabbets are the everyday joints of casework. They locate shelves, bottoms and backs, square up an assembly as it goes together, and add glue area where a plain butt joint would have almost none. Cut to the right width, they are quick and strong. Cut from the label on the plywood, they are usually loose.

Three cuts, three names

• A dado runs across a board or panel, like the housing for a shelf in a bookcase side.
• A groove is the same cut running along the grain or the length of a part, like the slot for a drawer bottom.
• A rabbet is an L-shaped recess along an edge or end, like the recess for a cabinet back.

Width: from the sheet, not the label

Plywood sold as 3/4 inch is usually 23/32 inch or 18 mm (0.709 inch), and hardwood plywood can be anywhere from about 0.70 to 0.72 inch. The table in nominal and actual sizes lists the common cases. A dado cut with a 3/4-inch bit or dado stack leaves a gap of 1/32 inch or more.

Ways to cut a matching width:

• Undersized plywood bits. Router bits made for plywood come in 23/32, 15/32 and similar sizes. They suit most sheets labelled with those fractions, but check your sheet first.
• Two passes. Cut with a bit or blade narrower than the plywood, then shift the fence or guide by the difference and cut again. A spacer the thickness of the difference, or a scrap of the actual plywood used as a gauge, makes it repeatable.
• Shimmed dado stack. Build the stack close to the thickness and add paper or plastic shims until an offcut of the shelf slides in.
• Adjustable jigs. A router jig that uses the actual shelf to set its own width removes the measuring altogether.

Always test on an offcut from the same sheet. The target is a snug slide by hand: tight enough that the shelf does not rattle, loose enough that it goes in with glue on it.

Depth: about a third of the thickness

A dado deep enough to support and locate the shelf, but not so deep it weakens the side:

• Shelves and bottoms in 3/4-inch sides: 1/4 inch deep is common, roughly a third of the thickness. Avoid going deeper than about half.
• Grooves for 1/4-inch drawer bottoms and panels: 1/4 inch deep, set 1/4 to 1/2 inch up from the bottom edge.
• Rabbets for case backs: as wide as the back is thick (measured, often about 0.20 to 0.24 inch for “1/4-inch” plywood), and deep enough to screw or nail through, commonly 3/8 to 1/2 inch. For a cabinet that hangs on a wall, make the rabbet deeper, up to 3/4 inch, so you can fit a hanging rail or scribe the back edge to an uneven wall.
• Rabbets at case corners: as wide as the mating panel is thick, and half to two-thirds the thickness of the rabbeted part.

Sizing the shelf

A shelf that sits in dadoes in both sides is longer than the opening between them:

shelf length = inside width between the sides + depth of both dadoes

For a 30-inch opening with 1/4-inch dadoes, cut the shelf 30-1/2 inches, less a hair for fit. If the sides are already assembled, measure the dado bottoms directly with a pair of sticks or a story stick.

Avoiding tearout

Hardwood plywood face veneers are thin, often well under a millimetre. A dull bit or an unsupported cut lifts splinters along the edge of the housing.

• Score the edges of the cut with a sharp knife against a straightedge before routing, or make a very shallow first pass.
• Use sharp bits and blades; a blade intended for plywood or a scoring blade helps on the table saw.
• Put masking tape over the cut line on especially fragile veneers.
• Rout with a guide clamped firmly to the panel so the bit cannot wander.

Glue and assembly

Plywood’s alternating plies put some long-grain veneer on every glued surface, so a well-fitted dado holds with glue alone. Screws or nails through the side strengthen a dado if the outside of the case will be covered. Stopped dadoes, which end short of the front edge, hide the joint on show faces; notch the shelf to match.

Cumulative error still matters. Mark every dado from the same reference edge, the bottom of the case for example, rather than measuring each from the one before it.

In GrainDraft

Dado and rabbet are two of GrainDraft’s eight joint families. A dado is sized to the modelled thickness of the part it houses, so a shelf given its measured thickness gets a housing that fits it; a rabbet shows the reveal and the stock left at the edge. Review the groove depth and remaining thickness in the joint preview before exporting the cut list.

Related: nominal and actual sizes, sizing a mortise and tenon.