Dado Case Joints

A dado is simply a rectangular groove that is cut across the grain of a board. The joint formed by placing an intersecting board into that groove.

The dado joint allows the load on the board to rest along the full length of the dado, thereby giving it considerable load-bearing capacity. Because the end of the board is entirely encased by the dado’s sides, the board can not cup or tilt. The dado joint does not offer any protection against the shelf pulling out of the side unless glued or fastened in some manner, and because the joint involves end grain, the gluing strength is limited.

The depth of the dado has only minimal impact on the strength of the joint. The joint is designed to support a load or to control the movement of one board relative to another. If the joint must prevent the shelf from separating, then a sliding dovetail would better suit your purpose. With that in mind, as little as 1/8″ is adequate for hardwood shelves, 1/4″ for manufactured woods (plywood, mdf, particleboard, etc.)

The depth of the dado is how deep the groove is, or the distance from the face of the board to the bottom of the dado. The width of the dado is the distance between the two sides of the dado.

Through Dado

Dados that extend from edge to edge are referred to as through dados. These are the easiest form of dado to construct, however the dado is visible when viewed from the edge. This can be concealed by applying a face frame or other trim after the joint has been constructed.

Stopped Dado and Blind Dado

Stopped dados start at one edge but stop before they reach the other edge. Blind dados both begin and end shy of the edge. The benefit of these options is that the dado itself is not visible from one (stopped) or both (blind) sides. The board that fits into the dado must have one or both ends notched, and the notch should be oversized so the board has a bit of play when it is fitted in the dado to allow you to adjust the board flush with the case edge. Wood movement is not an issue since the grain direction is identical.

Dado-And-Rabbet

This joint has is formed by one board having a narrow dado, and the mating board having a rabbet cut to form a tongue which is sometimes called a barefaced tenon because the tenon is flush with one of the board’s faces.

For shelves it is recommended that the rabbet be cut so the remaining tongue is at the bottom of the board. This will increase the load the shelf can support. If the tongue was at the top of the board, the board would have a tendancy to split if overloaded.

If used to join case sides to case tops or bottoms, you have a choice for orientation. If you have the dado on the top/bottom board and the rabbet on the vertical (tongue flush with inner face for strength), then the joint will resist the vertical board from separating, but the joint will be visible from the side. If you construct the joint with the dado on the vertical board and the rabbet on the horizontal board (tongue flush with the bottom face for strength), the top of the case can support more load, however it does nothing to hold the side, but the joint is only visible from the top.

If you stop the dado before it reaches one edge and trimming the tongue to match, you prevent the joint from being visible from one edge. If you do the same for both edges, it becomes a blind dado-and-rabbet. These variations offer the benefit of hiding shrinkage and gaps.

Dado-And-Tongue or Tongue-And-Dado

Essentially a dado-and-rabbet with the tongue no longer flush with one face, the tongue-and-dado can better resist racking because it has two shoulders.

Dado-And-Spline

This joint is ideal for MDF and particleboard since they do not have any grain, and therefore do not lose anything by having the tongue formed by a spline instead of being an integral part of the board.

This joint is essentially a dado-and-tongue, however the tongue is formed by a spline that is inserted at glue-up time. The spline used should be at least as hard as the wood it is supporting, otherwise the spline will become the weakest part of the joint. The spline should extend into approximately 1/3 of the side’s thickness, and it should extend about twice that distance into the horizontal board. The spline should not extend much further than this into either piece because it risks weakening the boards . If the spline does not extend far enough into the boards, the spline will not provide enough holding strength to support the shelf load. Also, the spline should be located below the center of the shelf board since it can then support a heavier load before the board splits.