The majority of bifold doors are readily available in sizes from 24 inches to 72 inches in width, graduating in increments of 2, 6 or 12 inches. If you have an odd-sized opening that manufactured doors won't fit, you can build a custom bifold in an afternoon.
Bifolds are aptly named because they fold. Common builder lingo refers to bifolds with a type of code. For example, a 3'0" bifold -- pronounced "three-oh" -- is 36 inches wide. The height is almost always standardized at 80 inches. When bifolds are referred to by size, it's in reference to the accumulated width of two or more panels hinged together.
Bifold panels are generally hollow-core doors, just like the majority of interior doors in homes. They consist of thin plywood glued to a wood frame. Wider interior hollow-core doors have a honey-comb filler of cardboard. Bifold panels don't need fillers because they're more narrow.
Make a Bifold
Lots of bifold doors employ 1/8-inch-thick luann plywood for the face and back. More substantial bifolds use 1/4-inch-thick plywood. Use hardwood plywood for an elegant-looking door. Use fir plywood for an economy door. The frame is typically made with fir lumber.
Step 1 Measure the Opening
Measure the width of the bifold opening. If it's a rough opening, add 1 inch for drywall, or 1 1/2-inch for 3/4-inch-thick lumber. Measure the height. Subtract 1/2 inch for drywall at the top and the thickness of the floor covering if you plan on adding floor covering. Write down the width and height of the opening.
Step 2 Divide the Measurement
Divide the width measurement into equal panels. Bifolds up to 36 inches wide are composed of two, equal-width panels. Bifolds over 36 inches in width should have four panels. Divide the opening for the size of the panels needed.
Step 3 Subtract for Hinges
Subtract 1/16 inch from each panel where they hinge together to allow for a 1/8-inch gap between panels. Subtract 1 3/4 inches from the height, to allow for the track and hardware at the top.
Step 4 Cut the Frame
Cut enough 3/4-by-1 1/2-inch fir, using a miter saw, to build a rectangular frame to the measurement for each panel. Apply glue to the joints and assemble the frame using a brad nailer, with 2 1/2-inch brad nails. The common butt-join is fine, or miter the joints if desired.
Step 5 Add the Plywood
Cut two pieces of 1/4-inch plywood to the same size as the frames. Apply glue liberally to both sides of the frame, sandwich it between the two pieces of plywood and square it. Place the bifold door across two sawhorses. Apply clamps every 6 inches, using long scrap pieces of wood to pad the plywood, and apply equal pressure to the panel.
Step 6 Sand and Finish
Sand each panel smooth with 100-grit sandpaper. Apply your choice of stain or paint. If you choose lacquer, use two coats, sanding between the coats with 180-grit sandpaper. Allow the finish to dry overnight.
Install the Hardware
Hardware instructions vary per manufacturer, but most of them have only a top track, with corresponding wheels and pins. Remove the parts from the kit and identify them. Parts can differ per manufacturer but there should be a wheel assembly for the doors, with matching nylon coated pins. Springs and or bushing for the track, and 90-degree brackets that fit on the floor to anchor the door in the corners. Screws and hinges should also be included. Follow the instructions for your specific model. The majority install similarly.
Step 1 Install the Jamb Bracket
Center two jamb brackets, one at each side of the opening where the vertical jambs of the opening meet the floor. The jamb bracket is L-shaped. The short side of the L is vertical. Place it against the jamb. Screw it to the jamb with a drill/driver, using the drilled holes in the bracket, with 1 1/4-inch screws provided with the kit.
Step 2 Install the Hinges
Place the panels side by side on a flat surface. Install one bifold hinge 8 inches from the top, and another 8 inches from the bottom, using screws provided with the hinges. Install a third hinge centered between the two.
Step 3 Drill for Pins
Stand a pair of the hinged bifolds on their edges, with the hinges on the top. Drill 1/2-inch holes, centered, 1 inch up from the bottom edge opposite the hinges, to a depth of at least 2 inches. Do the top and bottom the same way. Repeat for each pair of bifolds. If you're doing sets with more than two pairs, drill only the two pairs on the outer side, skipping the middle pair.
Step 4 Install a Pin and Wheel on Top
Hammer one nylon bushing and one smooth, top-pivot pin into one hole at the top of the bifold. Hammer a bushing and wheel into the matching hole at the top.
Step 5 Install the Bottom Pivot Pin
Hammer a bushing, and the knurled bottom-pivot pin into one hole on the bottom, directly opposite the smooth pin at the top.
Step 6 Install the Track
Measure the width along the top of the closet. Use a hacksaw to cut the track to the measurement. Follow the manufacture's instructions if required, to install springs and bushings into the track. Install the track, centered front to back, in the top of the hole using 1 1/2-inch screws.
Step 7 Hang the Doors
Pick up the door, and allow it to unfold. Insert the smooth pin and the wheel assembly into the track at the same time; the wheel should be on the inside, with the pin nearest the wall. Apply upward pressure, tilt the bottom of the door inward, and allow the bottom knurled knob to drop into the serrated jaws of the jamb bracket.
Step 8 Operate and Adjust
Operate the door by opening and closing it. If it sticks, rubs or doesn't move cleanly, twist the knurled bottom knob -- it's also an adjusting knob -- up or down as needed until the door operates smoothly.