Wood joinery methods are less based on the same concept. Adhering two members at either a parallel or perpendicular angle, either using extraneous mechanical support or designing clever slots and inserts to reinforce a joint. It is easy to mistake one for the other, but you’ll find if you pay a little closer attention, you’ll quickly understand why there are so many subcategories of wood joints.
The dado is a joint which can easily be mistaken for another type of wood joint, but it is rather unique in its straight forward mechanism of functioning.
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What is a Dado Joint?
The dado joint is a simple joint that every woodworker should feel at ease using, as it is a basic element in almost any woodworking practice. In the UK a dado joint is referred to as “housing”, and in Europe, it’s referred to as a “trench”, which is very indicative of its shape.
A dado is a three-sided channel which is carved against the grain of a workpiece. The detail of it being cut against the grain is a significant detail, for if it was cut with the grain it would be called a groove, and should be placed in a specific area. To be clear, the dado itself is the channel, which automatically labels it as a dado joint. The rabbet is the workpiece inserted into the dado channel which becomes, you guessed it, a dado joint.
This channel should have clean-cut shoulders (walls leading from the floor of the slot to the surface of the workpiece) and a perfectly flat bottom. A correspondingly sized workpiece will then be inserted into the slot. This piece is usually called a rabbet (but more on that later).
The dado channel itself should normally be cut no more than 1/2 the width of the workpiece, as any more threatens to weaken the overall structural integrity of the workpiece. We don’t want your entire book collection tumbling to the floor in the middle of the night! Spooky! 1/3 of the overall workpiece is the recommended measurement.
The dado joint is almost always reinforced with glue. Once it’s loaded with books, or your rock collection, or your favorite dishware, the weight of the load is transferred directly to the sides of the bookcase, which is proxy transferred to the floor.
Dado Joints Can Be Used…
Since the dado joint isn’t particularly sturdy, it isn’t normally used in larger construction projects. It’s certainly sturdy enough to handle whatever you can place on a standard size bookshelf, and that is where you’ll find the dado joint in action.
To be clear, the dado is usually going to be placed along the face of a workpiece, multiple times if you’re looking for multiple shelves. This is not a joint that is used on the edge of a workpiece, and simply gluing a joint at that corner edge angle will be very weak. It needs a top shoulder and a bottom shoulder to be supported.
They are excellent for wherever thin (in width) but wide (in surface area). Ensure the workpieces aren’t too long, since less heavy wood is more often used, its vulnerability to bending in the center under ample weight, is rather high.
Three Types of Dado
The Through Dado: this occurs when the channel is cut from end to end of your workpiece, leaving both ends open (this technically becomes a mortise and tenon joint, only longer).
The Stopped/Blind Dado: this occurs when the channel stops just short of the edge of your workpiece. This is used when the craftsman does not wish for the joint to be seen from the front. This ends up being one of the stronger types of dado joint, as an extra notch will be cut into the attaching piece as a slight overlap. This is a significant extra reinforcement.
The Half Dado: this is where the rabbet comes into place. The dado channel is cut in the same way, but the corresponding member instead of just having one flush edge to insert has an extra little shelf or step (it simply looks like two upside-down stair steps, with the top step inserted into the channel). This is a great solution if there are certain imperfections in either workpiece to help camouflage them.
How Does it Compare to Other Joints?
The dado joint is one of the weaker joints, as it relies heavily on glue and even extra nails to be at its sturdiest potential. But due to its simple and cheap manufacturing process, it’s all that is really needed for cabinetry and shelving. It is most similar to the mortise and tenon joint, but tends to be quite a bit longer, and the mortise and tenon is almost always joined at the ends of members.
It is also often mistaken for the butt joint. They are rather similar in function, but there is one main factor that makes it more sturdy than the butt joint. Since the butt joint is most often just joined at two sides, there is less surface area for gluing.
The dado joint is technically 6 glued sides (as the dado channel has 3 sides and the inserted workpiece has 3 sides joining those). Since this joint is housing, there is more opportunity for reinforcement.
How deep should a dado joint be?
The most common depth of a dado channel is 1/3 the width of the entire workpiece. Any more and it may weaken the structural integrity of the workpiece.
How do I cut a dado joint?
There are several sources on how to achieve a dado joint. Here is a reliable one, explaining the methods with a table saw or a router —> The Spruce.
What is the difference between a dado and a rabbet?
There’s a full article on the rabbet here but simply put, the rabbet has an extra notch carved into its edge as extra reinforcement. It looks like two stair steps flipped upside down, with the top step inserted into the dado channel.
What is a blind dado?
This is a dado used when the craftsman does not want the joint to be visible. It is when the dado channel stops just short of the edge of the workpiece, covering the joint location.
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