The plywood butt joint is two pieces of plywood butted together with glue. It's the simplest technique to join plywood, but without some type of fastener or modification, the plywood butt joint is too weak in most instances. There are a number of other woodworking joints such as the rabbet and spline that, although technically not true butt joints, are sometimes referred to as a butt joints.
Nails and Screws
Screws and nails are the most common fasteners used to reinforce glued plywood butt joints. Cabinetmakers use adhesive-coated pin nails and staples to butt-joint jambs, shelves and drawer bottoms together at 90 degrees to each other. Pin nails leave only small holes that are easily hidden with putty or other filler. Screws add strength for heavy-duty applications, or when visible screw holes aren't an issue. Cabinet or woodworking screws are sharp and penetrate plywood with ease. Use a straightedge to center a line over the contact point of the butt joint. For 3/4-inch plywood, drive 1 1/2-inch screws or nails along the line, spaced about 6 inches apart.
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Pocket-hole joinery is used to join two pieces of plywood butted end-to-end or side-to-side. This type of butt joint relies on 1 5/8-inch screws drilled through the face on one piece at a controlled angle, to penetrate into the end or side of the adjoining piece. Cabinetmakers use this technique to butt-join plywood sheets to fabricate panels that exceed 48 inches in width, such as for a kitchen island or peninsula back. The pocket-hole butt joint is accomplished using a simple pocket-hole jig, which consists of a small bracket with an integral sleeve that's set at a prescribed angle. Fit the jig over the end of one piece of plywood to position the sleeve. Insert a drill bit into the sleeve and drill the hole at an angle through the end to penetrate through the face. After adding glue, the two pieces are screwed together with glue and, optionally, bar clamps. The screw holes should be on the bottom.
The biscuit butt joint is another technique for butt-joining plywood edge-to-edge. This type of joint is unobtrusive, and often used on hardwood plywood or furniture. The biscuit joint is durable and rivals any other joint for strength. The biscuit butt joint is accomplished using a hand-held power tool dedicated to the task. Use the biscuit joiner to cut corresponding, curved slots along the edges of both pieces of plywood. Add glue and football-shaped wooden discs called "biscuits" to the slots and clamp the pieces together. If you're planning on doing any production work that involves butt-joining plywood, the investment of a biscuit joiner can be beneficial.
Dowels are sometimes used to reinforce end-to-end or side-to-side butt joints. Dowels are as effective as the biscuit joiner when joining 3/4-inch-thick plywood with a butt joint. The doweled plywood joint is similar to the biscuit joint: Use a hand-held doweling jig to drill corresponding, holes in both pieces of plywood, spaced about 6 inches apart. Add glue, dowels and clamps. Doweled butt joints can be a bit complicated to assemble because glue begins to set quickly. Practice the routine without glue before attempting it.
Other types of joinery, sometimes referred to as a butt joint, include the half-lap or rabbet. The half-lap or rabbet involves cutting a lip on the mating edges of two pieces of plywood. The lip on one piece is on the top. The corresponding lip is on the bottom. When the two pieces of plywood butt together, the lips fit together like puzzle pieces that, when glued, form a strong half-lap or rabbet joint.
The spline joint is similar to the biscuit joint, except that the spline runs full-length along the edges of both pieces of plywood. The spline joint is created by cutting a slot or channel, centered down the edges of both pieces with a table saw or hand-held router. The spline -- a strip of wood sized to fit the slot -- is glued into the slot. When the two pieces are clamped together, the spline becomes part of the plywood on both pieces. The spline is one of the strongest of all the butt-joint reinforcement techniques.