Bed slats support a mattress without a box spring or give extra support to a bed with a box spring. They fit perpendicular to the length of the bed, supported by a lip or ledge on both sides of the frame. Bed slats are typically not screwed or fastened to the frame and have equal spacing to promote air flow through the mattress. You can make bed slats with ordinary construction lumber and a miter saw.
Measure the width of the bed frame. Measure above the supporting lips to the sides of the frame. Measure at the head and foot of the frame to ensure that the measurements are the same.
Purchase 12 lengths of one-by-four. Generally, you'll want 40-inch one-by-fours for a twin bed, 55-inch lengths for a full-size bed or 61-inch lengths for a queen size bed. Examine the pieces. Reject any with cracks, splits or obvious defects.
Cut 10 pieces of fir to the measurement you made in Section 1, using a miter saw. If the head and foot of the bed have different measurements, measure and cut pieces individually to fit snug along the length of the frame.
Sand the corners and round the edges of the slats with a sanding block and 100-grit sandpaper. Remove any rough edges or splinters.
Sight along the length of each slat. If you see a crown or warp -- no matter how slight, the bow or crown should face up. Place the slats in the bed frame with one one flush at both ends. Place all remaining slats in the frame with equal spacing of 3½ inches or as close as possible to that interval.