Things You'll Need
Compound miter saw (optional)
Circular saw or table saw
1/4 inch drill bit
Eight 1/4-inch lag bolts and washers
Wood stain, paint or other finish
10 furniture sliders
Twin mattress and bedding
If you can use a screwdriver, a saw and other basic tools, you can turn a regular bed into a trundle bed. Except for staining and drying time, this project can be assembled in an hour with a helper. It will take a little longer for a person working alone.
Materials preparation to make trundle bed
Measure the space under the regular bed, both the height from the floor to the bed, and the width and length of the space under the bed. You'll need clearance for the trundle bed frame, mattress and bedding. For a standard twin mattress and frame, this will be 9 inches high, 40 inches wide and 78 inches long. If the regular bed is not high enough, put risers under the feet.
Measure and mark the boards to the following lengths: two of the 2-by-6-inch boards should be 75 inches long; the other two 2-by-6-inch boards should be 39 inches long; the plywood for the bottom of the trundle bed frame should be 39 inches by 76 1/2 inches. The extra 1 1/2 inches allow for the headboard and footboard.
Put on protective eyewear. Cut the 2-by-6-inch boards to length with a compound miter saw, circular saw saw or table saw. Cut the plywood backing with a table saw or circular saw. Remove any large splinters.
Place a 2-by-6-inch board with its 6-inch side on the floor. Use a tape measure and a pencil to mark the location of the lag bolts like this: Draw a light line to divide the board from top to bottom as it lies on the floor. Continue this line around the end of the board just a bit, so it will be visible when viewed from another angle. About 3/4 inch of the board should be above the line, and about 3/4 inch below it.
Mark the first hole along that line, 2 inches from the left edge of the board, and the second one 2 inches from the right edge of the board. The marks should be spaced about 2 inches apart and about 2 inches from the left and right sides of the board, and in the middle of the board's height as it lies on the floor right now.
Pre-drill holes where those marks are, holding the drill as close to perfectly level as you can. Prop the board up on something or turn it on its side if you need more room to get the drill level. Repeat with the other end of that board, and with both ends of the other 2-by-6-inch board. You should have eight pre-drilled holes in the long 2-by-6-inch boards.
Pre-drill holes in the 2-by-6-inch head and foot board pieces so that they line up with the holes in the 2-by-6-inch side boards. With the head board piece standing upright as it will be when attached, the placement of the holes is like this: 2 inches from the top, 2 inches from the bottom, and about 3/4 inch from the edge, aligned with the pencil line you made on the 2-by-6-inch board for the side of the trundle bed. Make holes on both sides of the headboard and footboard.
Sand all of the boards smooth, using the sander and sandpaper. Wipe clean with a slightly damp cloth.
Assemble Trundle Bed
Slide a washer onto a lag bolt. Slide the bolt into the hole in the 2-by-6-inch footboard piece. Using a socket wrench or socket adapter on the drill, drive the lag bolt through that board and into the corresponding hole in the 2-by-6-inch side board, but do not tighten the bolt completely. Repeat with another washer and bolt in the other hole in that side of the footboard piece and into the other hole in the 2-by-6-inch side board piece.
Repeat Step one with the other side of the footboard, attaching it to the side board. You should now have a U-shape made out of the two long 2-by-6-inch boards and one of the short 2-by-6-inch boards.
Attach the other short 2-by-6-inch board piece to the other ends of the 2-by-6-inch boards. This completes the outside edges of the trundle bed frame. Use a carpenter's square to make sure the frame is square and the corners are 90 degrees, and tighten all of the bolts. Do not over-tighten.
Turn the frame upside-down, and attach the plywood backing to the frame. Drive 2-inch-long wood screws through the plywood backing and into the 2-by-6-inch boards. Place one screw every 6 inches for maximum durability. Sink the screws slightly into the plywood, to prevent scratching the floor or snagging on carpet. Remove any splinters if needed. Running the sander over the area makes quick work of this.
Stain, paint or seal the wood as desired. Follow manufacturer's instructions for use and drying time.
Place the trundle bed frame upside-down on the floor. Attach the furniture sliders to the bottom of the trundle bed frame. Place one in each corner and at least one in the middle of each edge of the bed.
Return the frame to its upright position, and check that the trundle bed slides smoothly. If not, add more sliders and return the frame to its upright position once again.
Place the mattress in the trundle bed frame, and put the bedding on the mattress. Slide the trundle under the regular bed.
If for use by children, add an extra coat or two of polyurethane or other finish. The added durability is worth the extra time. Allow plenty of drying time after painting or staining the bed, or bedding may stick to the finish. Allow the trundle mattress to air out for about an hour after each use, because the trundle will not get much air circulation under the bed.
Always use eye protection and other precautions when using power tools. Fingers can be pinched when sliding a trundle bed under the regular bed, so be careful when closing the bed.
Jennifer Harshman has been a writer since 1990, writing, ghostwriting and editing both offline and online content. Her online articles focus on at-home projects. She writes in several fields, including health, business and do-it-yourself. Harshman has a Bachelor of Arts in education from Greenville College.