Breadboard ends that let the top move

How a breadboard end keeps a solid top flat without cracking it: the tongue and tenons, where to glue, elongated peg holes, and sizing the gaps from the expected movement.

A breadboard end is a strip of wood fitted across the end of a solid top, its grain running perpendicular to the top’s. It hides the end grain, helps keep a wide top flat, and gives a traditional look to tables and desks. It is also a deliberate cross-grain joint, and it only works if it is built to let the top move underneath it.

The problem it solves, and the one it creates

A wide top is free to cup, and its end grain is exposed. The breadboard holds the ends flat. But the top changes width with the seasons while the breadboard, running along its length, barely changes at all. Glue the two together across the full width and the top will eventually crack or push the breadboard’s joints apart. The coefficients in wood movement show the scale: a 36-inch-wide flatsawn red oak top over a 4-point swing changes by about 36 × 0.00369 × 4 ≈ 0.53 inch.

Anatomy

A tongue runs the full width of the top’s end, fitting a matching groove in the breadboard. It keeps the surfaces aligned and the top flat.
Several longer tenons project from the tongue into mortises in the breadboard. They carry most of the strength.
Pegs (or drawbore pins) pass down through the breadboard and the tenons to hold the breadboard on.

Glue only at the centre

Fix the breadboard to the top at one place so it stays centred, and let everything else slide:

Glue the centre tenon, or the centre few inches of the tongue, only.
Leave the outer tenons and the rest of the tongue dry.
Glue the pegs into the breadboard only, never into the tenons.

Elongated holes and wide mortises

Movement is zero at the glued centre and greatest at the edges. At any point, the top moves by roughly its distance from the centre times the coefficient times the moisture change. For the 36-inch oak top above, a tenon 15 inches from the centre can move about 15 × 0.00369 × 4 ≈ 0.22 inch in total.

Peg holes in the outer tenons are elongated along the width of the top, so the peg can slide. Leave the elongation on the side the tenon will travel: toward the edges if the top was built dry and will grow, toward the centre if it was built in humid weather and will shrink. Many makers simply elongate in both directions.
Mortises for the outer tenons are cut wider than the tenons by at least the expected movement at that point.
The tongue’s groove runs the full length of the breadboard and does not restrict movement.

Proportions

Typical starting points for a 3/4- to 1-inch-thick top:

Breadboard width of 2 to 4 inches, same thickness as the top or a hair thicker so it can be planed flush.
Tongue about 1/3 of the top’s thickness and 3/8 to 1/2 inch long.
Tenons the same thickness as the tongue, 1-1/2 to 2 inches long, 2 to 3 inches wide, spaced across the width with one at the centre.
Pegs of 1/4 to 3/8 inch diameter, one per tenon.

The mortise-and-tenon fit itself follows the same rules as any other; see sizing a mortise and tenon.

The ends of the breadboard

Because the top grows and shrinks while the breadboard does not, the top’s edges will not always line up with the breadboard’s ends. Two common approaches:

Make the breadboard fractionally longer than the top is wide when you build in the dry season, so the top grows toward flush.
Accept a small seasonal step and ease the corners so it is not sharp.

A breadboard glued at full width may look perfect on the day it is made; the step is the sign that it has been built correctly.

Drawboring

For a mechanical joint without clamps, drill the peg hole through the breadboard, insert the tenon, mark the hole on it, remove it and drill the tenon hole about 1/16 inch closer to the shoulder. Driving the peg pulls the breadboard tight to the top. On the outer tenons this still needs an elongated hole, so drawboring is usually applied only to the centre tenon.

When to skip it

Plywood and veneered tops do not move and do not need breadboards for stability; use an edge band instead. For solid tops, a well-attached top with good fasteners stays flat without breadboards; see attaching a solid-wood top.

In GrainDraft

The tenons of a breadboard end can be modelled with GrainDraft’s mortise-and-tenon joint and inspected in the section view. The movement allowances, elongated peg holes and wider outer mortises are shop decisions you add to the drawings; check them against the calculation above before you cut.

Related: edge-gluing a panel, glue-ups.

Numbers here are starting points from common shop practice and published references. Check your own stock, tools and hardware instructions, and cut a test piece before committing project wood. Spotted an error? The contact address is on the About page.