Biscuit joinery, also known as plate joinery, is a woodworking technique in which pieces of wood are joined by means of football-shaped disks of wood (biscuits), usually made of beechwood, that are reinforced with glue. The technique is similar to dowel joints, but the flat surfaces of the biscuits make for a more secure joint, especially where edge-glued stock needs to be carefully aligned. A special tool known as a biscuit joiner, which cuts the oval-shaped slots for inserting biscuits, makes this technique very easy. When the slots are fitted with biscuits glued in place, the glue causes the biscuits to swell, making for one of the strongest joints possible.
But biscuit joinery can be tricky, since there are several different sizes of biscuits available, and it is important to get install enough biscuits to make the joint secure and strong, but not so many that you compromise the wood by cutting too many slots.
Joinery biscuits are all 5/32 inch thick, and are available in four width-and-length sizes:
#FF: 1/2 x 1 3/8 inches
#0: 5/8 x 1 7/8 inches
#10: 3/4 x 2 1/8 inches
#20: 15/16 x 2 1/4 inches
- #FF biscuits are used only for very small workpieces and require a special biscuit joiner tool since standard biscuit joiners will not adjust to this small slot size. Most standard biscuit joiners should be adjustable to accommodate the other three common biscuit sizes: #0, #10, and #20.
- #0 biscuit are typically used to connect small pieces of wood or are used in areas where not a lot of stress is expected. Picture frames are a common place where #0 biscuits are used, and #0 biscuits work well in narrow applications, such as joining narrow rails and stiles on cabinet doors.
- #10 biscuits are a standard size that works well for most framing projects. Hardware stores with limited woodworking supplies may stock #10 biscuits only.
- #20 biscuits are made for projects that will undergo considerable stress or that must bear a lot of weight. These will work well for plywood and particleboard. The long length makes them ideal for edge-gluing stock for tabletops.
When choosing biscuits for plater joinery, you should always use the largest size that is practical. In most cases, you'll use a #20 biscuit if you can, but if this is too large, you can try a #10 or even #0 biscuits for the smallest joints and thinnest wood stock.
As far as how far apart to space the biscuits, this is mostly a matter of preference, although there are some basic guidelines you can follow. The plate joints should be positioned so that the edge of the biscuit is 2 to 3 inches from the edge of the wood stock. Closer than that, and you risk splitting the wood; further away and you compromise some of the holding strength at the ends. Once you've determined the positions for the edge biscuits, you can calculate even spacing for the biscuits between. Anywhere from 6 to 12 inches apart, measured on-center is usually sufficient.
Biscuit Joinery Tips
- Double up biscuits on thick stock. For example, when gluing up heavy stock for a thick tabletop, cutting double-wide slots and inserting pairs of biscuits will double-reinforce the edge-glued joints and make separation almost impossible.
- On narrow face frames, run biscuits the long direction into the slot, then cut it off flush with the edge of the frame. This will allow you to get as close as possible to the end of the frame.