Butt joints are one of the weakest forms of case joints, but they are the easiest and quickest to make. A butt joint is achieved whenever you join the straight and square end grain of one board to face grain of another board. End grain is notoriously difficult to glue or fasten, and butt-joints present the smallest amount of surface area to be used for gluing. In order to achieve any significant strength and sturdiness, you must reinforce the joint.
Glued Butt Joint
As its name implies, the glued butt joint is held together by the force of the glue adhering to the end grain of one board and the face grain of another. The face grain of most wood species providing a very good glue bonding surface. The end grain, however, is notoriously weak and is prone to failure.
Butt Joint with Glue Block
Glue blocks are square or triangular wedges of wood that are glued to the hidden side of the joint. These blocks can run the full length of the joint, or only be used in strategic locations. One problem with case joint glue blocks, though, is that their constructions usually has the glue block’s grain oriented perpendicular to the case boards’ grain, resulting in a cross-grain construction. This can cause problems when wood movement is involved.
Butt Joint with Fasteners
Nailed Butt Joint and Screwed Butt Joint
Nails or screws can be used to fasten a butt joint. Inserting the nails or screws at alternating angles strengthens the joint and prevents the boards from separating. In softer woods that have trouble holding screws or nails in the end grain, you can reinforce the end grain portion with a strategically placed dowel which provides enough cross-grain to increase the holding power of the screws.
Dowelled Butt Joint
Dowels are a decorative replacement for nails or screws. Again, they can be inserted in alternating angles, increasing their holding power. Blind dowels can be created by using stopped holes on the face grain board, however these holes are difficult to produce without the aid of specialized tools, since doing them by hand or even a drill-press is prone to uneven hole depths, or mis-aligned holes.
Using a contrasting wood type for the dowel can produce an attractive decorative element to the workpiece.
Biscuited Butt Joint
Biscuits serve two purposes in case joints: They provide positive registration between boards during assembly, and they provide a mechanical brace against shifting in the finished product. Biscuits are essentially thin oval wafers that are inserted and glued into a matching pair of slots that are cut into the two pieces to be joined.
When the biscuits are used at the top of an upright board, offset the location of the slots slightly by 1/4 to 1/8 of the upright’s thickness towards the hidden side of the vertical. This is because if the joint should fail, the damage done to the wood will be in a hidden location, and more easily repaired invisibly. If the biscuit were placed in the center, there is an equal risk that the damage would be exposed.
When the biscuits are used to secure a horizontal shelf to an upright member, the biscuits should be offset slightly by 1/4 to 1/8th of the shelf’s thickness towards the bottom of the shelf. When the shelf is loaded with weight, the biscuit will be applying force towards the top of the shelf. By shifting the biscuit’s location downwards, you thicken the amount of wood that is resisting that force.