How to Build a Cold Frame

Not all gardeners have the luck to live in temperate climes. Instead of battling frost, install a cold frame to protect your plants like a mini greenhouse. Its natural warmth lets you foster seedlings in early spring and keep veggies going through fall and even into winter.

This Old House senior technical editor Mark Powers shows how to build the box from rot-resistant wood and a salvaged window. Once it’s finished, choose a south-facing spot in your yard where water won’t pool, and enjoy all that extra gardening time.

Overview for Building a Cold Frame

• Project timeline:
• Friday: Size and cut the pieces.
• Saturday: Assemble the frame and attach the window.
• Sunday: Prepare the site and install the cold frame.

Steps for Building a Cold Frame

Step 1: Measure the Window

The window lid will be hinged along its length and sloped to let in maximum sunlight. Measure the window’s long edge to get the length for the front piece and the two back pieces. Sandwiched between, the sides taper back to front, which means they’ll be shorter than the window is wide.

To determine their length without resorting to geometry, stage one end of the frame two-dimensionally: Butt two boards end-to-end, then pair up a third parallel with one of them to mimic the height of the back. Slide the single board away from the pair and open a gap approximately as wide as the window.

Then stand the window on end, angling it from the “back” to the “front,” and adjust the gap until the window just overlaps the boards at either side, as shown. Now measure the gap to determine the length of each side piece.

Step 2: Cut the Pieces to Length

Using a miter saw, cut to length the front board, two back boards, and four side boards.

Step 3: Mark the Angle

The top board of each side needs to be tapered. To get that angle, run a straightedge diagonally from the top corner of one end of a board to the bottom corner of the other end, and mark the line, as shown.

Step 4: Cut the Angle

Clamp the board to the work surface and use a jigsaw to cut the angle. Now you have your two tapered pieces.

Step 5: Mark and Cut the Tall Battens

The tops of all the frame’s battens must follow the side taper so that the lid can close. For the tall ones at the rear of the sides, cheat them in using a board on edge to block out space for a back batten. With the tall batten flush at the bottom, transfer the taper angle to the board, as shown. Using the miter saw, cut that angle. Repeat.

Step 6: Mark and Cut the Short Battens

Cut two more battens at the same angle. Since they taper down to a single board in front, you can mark each batten’s height with the width of a board, as shown. Cut both battens.

Step 7: Glue the Battens

Using construction adhesive, glue the battens in place, making sure to leave space for the back batten.

Step 8: Screw the Battens

Using a drill/driver and four 1⅝-inch screws, secure each batten.

Step 9: Cut the Back Battens

The battens binding the back boards together must be beveled at the top to match the side taper. Using the miter saw, cut a scrap block to the angle of the taper. Then loosen the jigsaw footplate, hold it flat against the side edge, tilt the blade to match the taper, and set the saw, as shown.

Cut the bevel. Now cut the batten to length so that it’s flush with the frame height from the peak of the bevel to the bottom of the board. Repeat for a second batten.

Tip: If the plants you want to grow are taller than what would fit in this frame, you can make your version higher by adding a plank to each side and sizing the battens accordingly.

Step 10: Position the Back Battens

To allow room to sandwich the sides between the front and back of the frame, set a board on edge as a spacer, flush with one end of the back. Position the batten against the spacer, flush at the top and bottom.

Step 11: Glue and Screw the Back Battens

Using a caulk gun and construction adhesive, glue each batten in place. Then sink four 1⅝-inch screws through the battens and into the back boards.

Step 12: Screw the Four Sides Together

Butt a side assembly into the back to form a corner. Drill a countersink, and drive two 2½-inch screws through the side and into the edge of the back batten, as shown. Repeat for the second side. Attach the front board with two screws through its ends and into the edge of the short battens.

Step 13: Attach the Hinges

Screw removable-pin hinges into the battens on the inside corners. This lets you remove the assembly screws, take out the pins, and easily break down the frame for storage.

Step 14: Attach the Hinges

Using a garden rake, loosen and turn the soil, creating a level bed for the frame.

Step 15: Install the Pavers

Set the frame in place and lift each side to slip pavers beneath it to create a sturdy foundation. The bottom edge of the frame should rest on the center line of the pavers, as shown.

Step 16: Attach the Lid

Refurbish the window as necessary, coat it with an exterior paint, and let it dry. Set the window in place so that its edges are flush with the frame. Screw two strap hinges to the back edge of the window and the back of the frame, as shown.

Step 17: Install the Handle

Refurbish the window as necessary, coat it with an exterior paint, and let it dry. Set the window in place so that its edges are flush with the frame. Screw two strap hinges to the back edge of the window and the back of the frame, as shown.

Step 18: Cut the Vent Rods

Cut several pieces of ½-inch dowel at varying lengths. They can be used to prop up the window for ventilation or to keep plants from overheating on sunny days. This also prevents moisture buildup.

Step 19: Position the Frame

Set the assembled cold frame on its paver foundation. With the lid propped up, use a garden spade to dig holes for your seedlings or plants. Then watch them flourish regardless of outside temperatures.

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