Cutting diagram

Step-by-Step

1. Using a saber saw or circular saw, follow the cutting diagram to make 9-x-48-inch front; a 15-x-48-inch back, and two 48-inch-long sides that slope from 15 to 9 inches wide.

 

Steel angles

2. Join the side pieces (with the angled cut facing up) to the front and back pieces using the 18-gauge perforated steel angle. (Buy a 3-foot piece and cut it with a hacksaw.) Use the 1/4-inch hex bolts to attach the angles to the plywood. Always use a washer between the head of each bolt and the plywood. The finished box is 48 inches wide by 49 inches long (48 inches for the length of each side plus 1/2-inch thickness for the front and back pieces).

 

L-brackets

3. To make the lid frame, cut two 8-foot clear pine 1x3s into four pieces: two 46 1/2 inches long; two 47 1/2 inches long. These four pieces can be arranged to form a frame that is 49 inches by 50 inches. Join these pieces using 4-inch steel L-brackets on the underside. Expert tip: Before you cut, verify that these measurements will produce a lid frame that will overlap the sides and front of your box approximately one inch. To accommodate the hinges, the lid frame will be flush with the back of the box.

 

Plywood corner braces

Flip the lid frame over. Cut triangles from the 1/4-inch exterior plywood scraps and attach them to the top side of the lid frame using 4-3/4-inch-long No. 8 wood screws in each brace.

 

Hinges, from inside

4. To hinge the lid to the box, use the three 3-1/2-inch hinges. Attach the hinges to the underside of the lid frame first, using 3/4-inch-long No. 8 wood screws.

 

Hinges, from outside

Next, lay the lid frame atop the box and used the hinges to mark the location for drilling 1/4-inch holes on the outside of the back of the box. Use 1/4-inch hex bolts to fasten the hinges to the plywood. Use washers between the nuts and the plywood.

 

If the lid and/or the box is not quite square, you can push on the corners of the box to adjust the shape.

5. To complete the project, remove the hinge bolts from the box, then unscrew the hinges from the lid frame. (To make it easier to get everything back together, number the hinges and their location on the lid frame. Also, mark the location of the screw holes in the lid frame by circling them with a bold marker.

Wrap the lid frame with a single layer of 6-mil poly plastic sheeting. (Expert tip: See the next page for a tip involving rigid foam insulation material.) Start with a piece of poly sheeting about 4 1/2 feet wide by about 10 feet long. Use the staple gun or roofing nails to secure the poly sheeting to the lid frame. To reduce tearing, use strips of thin cardboard between the poly and the fasteners. Try to get the covering as taut as possible, and avoid getting multiple layers of poly on the hinge side of the lid.

 

6. Place the covered lid atop the box, and reattach the hinges to the box with the bolts.

Your cold frame is now ready to use and will protect plants from cool weather. A very important note: Your cold frame will warm up very quickly when the sun is shining directly on it. Always prop open the lid a little on bright sunny days, even if the air temperature is cool. On some days, you may want to open the lid completely. Always close the lid before nightfall to preserve the day's warmth.

To get the most use out of the frame, you should plan to add the insulation and heating options described on the next page.