Creating a butt joint is the most basic method there is for connecting two pieces of wood. While it is not as strong as other methods, such as dovetails, finger joinery, or lap joints, the butt joint is still very useful in some situations. Because it does not have the kind of finished look of other woodworking joints, butt joints are more often used in more utilitarian projects, such as outdoor furniture or casual pieces made from softwoods.
To make your butt joints as strong as possible, use proper technique, as described below.
First, the definition: as shown in the picture, a butt joint is where one piece of stock is butted against another and affixed with glue of some kind. The joint is usually strengthened by wood screws or nails driven through one of the pieces of stock and into the end grain of the other. Or, the inside corners of the joint may be reinforced with some kind of metal brackets or braces.
This article discusses the square butt joint here, but there is a slightly butt joint, the mitered butt joint, in which the end grains will be hidden entirely.
Square, Smooth Cuts are Key
The key to a quality butt joint is to make certain that the ends of the two boards are cut as square as possible, using a saw blade that gives the smoothest possible cut. This is easiest using a miter saw with a fine-tooth woodworking blade, although quality results can be obtained by using a circular saw and a layout square or other straightedge guide, provided that saw bevel angle can be set very precisely at 0°.
Inexpensive circular saws are notoriously imprecise in their ability to set the bevel angle of the saw foot, so unless you have a good quality circular saw, a power miter saw or table saw is a better choice.
Glue Provides the Strength
The strength of a butt joint comes from the glue in the joint. Glues come in several types, from traditional yellow woodworking glue to polyurethane glues, such as Gorilla glue.
However, there are two problems with using glue as the only means of holding the connection.
First, when glue is applied to the end grain of a board, it tends to soak into the stock far more than glue applied to the face of the board. The end grain is the most porous part of the wood, so you may need to apply a bit more glue than normal. And polyurethane glues tend to expand slightly as they set up, which can leave you with hardened glue to clean up after you are finished.
Also, glue alone does not provide much in the way of lateral strength. Because of this, butt joints should be reinforced with screws or nails to strengthen the joint. Typically, this reinforcement is done by driving the fasteners through one piece of stock near the end and into the end grain of the adjoining piece of stock.
If using hardwood for your project, be certain to drill pilot holes before driving the screws or nails. Hardwoods can often split when driving fasteners near the ends of the boards, unless you first drill pilot holes. The pilot holes should be just slightly smaller in diameter than the thickness of your fasteners. If the holes are too large, the fasteners will lose their holding power and your joint will be weak.
Holding pieces of wood stock in place while the glue sets up and the nails or screws are driven is critical, so plan on using clamps of some kind to tightly hold your pieces together as you join them.