These versatile barriers come in a bewildering variety of shapes, sizes and materials, but can be classified generally as accordion, bifold or sliding bypass doors. The accordion and bifold types make handier room dividers than the more cumbersome sliding bypass doors, which are usually used as closet closures, but all three types can be adapted to serve as room partitions.

The accordion door looks like the bellows of an accordion and is usually made of pleated fabric or vinyl stretched over a light metal or plastic skeleton. Closing the door stretches out the pleats into a substantial-looking partition; when the door is opened, the pleats fold compactly to one side. Accordion doors, which are hung on rollers from a single overhead track and attached at one side to a wall, are the easiest of the three types of track-mounted doors to install and once in place require little or no adjustment.

Bifold doors consist of wood, plastic or metal panels up to about 2 feet wide hinged together lengthwise, usually in pairs. Pairs of panels can be linked together to form one continuous surface. A bifold door consisting of one or more pairs can be mounted at one side of an opening and closed by pulling it all the way across, or the doors can be installed at each side of an opening and pulled together in the middle. An overhead track guides the bifold door but the weight of the door rests on a pivot that is attached to the floor on the wall side. A pivot at the top of the door holds the assembly upright.

Sliding bypass doors usually consist of two large wooden panels, each hung by wheels from an overhead track. The panels overlap by about an inch and when closed are kept vertically aligned by a small floor-mounted guide. All overhead tracks - whether they support or merely guide a door - sustain considerable stress when the doors are in use and should be attached to a level, structurally supported surface.

Occasionally a track can be fastened directly to the ceiling. But since folding or sliding doors more than 6 feet 8 inches high are seldom readily available and since most ceilings are 8 feet high, installing such doors usually involves attaching the track for the door to a header suspended from the joists, the structural beams that support the ceiling and the floor above.

The location of the joists helps to determine the position of the door. After locating the joists and marking the proposed position of the door, carefully calculate the vertical space needed for the door and its track. Design and construct a header suitable for the type of ceiling involved to fit in the space between the track and the ceiling.

To calculate the height of header to be suspended from a permanently attached ceiling, measure from floor to ceiling at several points along the proposed line of the door. Subtract from the shortest of these measurements (thus allowing for any unevenness in floor or ceiling) the height of the door and its track plus the thickness of the wallboard or other covering to be applied to the bottom of the header. The result is the height of the header frame; its length is the distance from wall to wall. Attach the header to the ceiling joists, fasten the track to the header and mount the door in its track.

For a door that is hung directly from the ceiling, locate the joists and attach the track directly to them through the ceiling material.