How to Build a Cold Frame
There are very few places in the world where you can grow fresh vegetables year-round. That is—without the help of a greenhouse. However, greenhouses aren't a practical purchase for most homeowners, so many experienced gardeners instead opt for a smaller but effective alternative called a cold frame. This simple structure sits over the top of your plants during the offseason and protects them from the elements while trapping warm air inside, creating ideal growing conditions no matter the climate.
What Is a Cold Frame?
A cold frame is a simple structure, most commonly constructed out of wood, that sits over the top of plants during colder seasons. A clear, sloped lid allows light to enter the frame and holds the thermal energy to allow plants to continually grow in the off-season.
Before You Begin
A cold frame can be built out of anything. While ours will be constructed out of pressure-treated lumber and feature an acrylic top, you can easily use scrap wood you already have and build tops out of reclaimed windows. Additionally, a cold frame can be a universal fit like ours, or you can build it to fit perfectly over your specific planters or raised beds. Plan your cold frame accordingly and adjust the provided materials list and steps as needed.
What You'll Need
Equipment / Tools
- Circular saw
- Miter saw
- Drill bits
- Step bit
- Tape measure
- Sealing and staining supplies
- Trigger clamps
- Straight edge
- 1 2x4 x 8-foot pressure treated board
- 1 5/4-inch x 6-inch x 12-foot pressure treated deck board
- 2 2x2 x 8-foot furring strips
- 1 2-foot x 4-foot plexiglass sheet
- 2-inch self-tapping exterior wood screws
- 1-1/4-inch self-tapping exterior wood screws
- 3/16-inch washers
- Exterior-grade penetrating oil
- Wood stain (optional)
How to Build a Cold Frame
Follow the steps below to build your very own DIY cold frame. These steps will yield a cold frame that is 2 feet by 4 feet, but you can easily adjust the dimensions to accommodate your specific garden.
Cut Outside Boards
Cut three 12-foot 5/4 deck boards into five pieces measuring 4 feet and five pieces measuring 22 inches. Set the remainder aside for later use.
Cut Inside Supports
Cut a pressure-treated 2x4 into two pieces measuring 15 inches long and two pieces measuring 11 inches long.
Assemble Front and Back
To assemble the back, lay three 4-foot deck boards flat on a work surface with the sides pushed tightly together. Slide a 15-inch 2x4 beneath the boards at each end, spaced 1 inch from the ends, and flush at the bottom.
Screw through the deck boards into the 2x4s with 2-inch self-tapping exterior wood screws, placing at least two screws per board.
To assemble the front, repeat the process with two 4-foot deck boards and the 11-inch 2x4s.
To attach the sides and assemble the frame, screw two 22-inch boards into the sides of the 2x4 supports on each side. Position the boards in line with the front of the frame, so that the top board on the back is exposed. Once assembled, the full frame should measure 2 feet by 4 feet.
Cut Angled Side Boards
To mark the angle for the top side rails, place a straight-edge spanning from corner to corner on the remaining 2-foot board and trace the line. Clamp the board to a work surface and carefully cut along the line with a circular saw.
Mount the Angled Side Boards
Set the side rails in place with the high side against the exposed upper on the back of the frame. Screw the side rails into the back 2x4 supports. To secure the front, drill a pilot hole through the top of the rail, 4 inches from the end. Screw through the rail into the board beneath.
The tapered end of the side rail can easily split. Avoid this by drilling proper pilot holes and refrain from overdriving your screws.
Cut Wood for Lid
To build the lid, cut a 2x2 furring strip into two 4-foot pieces and two pieces measuring 21 inches.
Assemble Lid Frame
The top will consist of a wooden frame with a piece of plexiglass screwed into the top. Start by assembling the frame into a 2-foot by 4-foot rectangle. Screw through the 4-foot 2x2s into the ends of the 21-inch pieces. To avoid splitting, pre-drill the screw holes.
If you make a cold frame larger than 2 feet by 4 feet, add additional 2x2 stringers in the middle to strengthen the lid frame.
Place the sheet of plexiglass on the frame and adjust until all sides are even. Use a marker to mark screw holes in each corner, then one more on each short side and two more on each long side. With the plexiglass sitting on the wood frame and the protective film intact, drill through the plexiglass and stop once you hit the wood. Use a step bit and make the holes slightly larger than the screw's threads.
Screw the plexiglass to the frame using 1-1/4-inch wood screws and washers. To prevent cracking, hand tighten the screws. Remove the protective film once the cold frame is fully assembled.
Place the lid on the frame with the plexiglass side facing up. Adjust the lid until it's even on all sides. On the back of the cold frame, mount two door hinges connecting the lower frame and the lid frame using the provided screws.
Cut and Mount Lid Arms
To aid in holding the lid open while working or venting the cold frame, two lid support arms are needed. Cut one 2x2 long enough to hold the door open at a height that allows you to comfortably tend to the plants and one more that's only a few inches long to hold the lid open just enough to let excess heat out when necessary.
Mount each arm on opposing sides of the interior of the cold frame. Pre-drill the screw holes and refrain from over-tightening the screws to allow the supports to easily swing up when needed.
Seal the Cold Frame
Seal the total wood surface using an exterior-grade penetrating oil, following the manufacturer's instructions for proper application. Remove the plexiglass top to ensure all wood gets coated or simply seal before attaching the top.
If you desire a darker finish, you can also opt to stain the cold frame, but do this before the sealing process.
How to Use Your Cold Frame
Putting your newly built cold frame to work is as easy as placing it over top of your plants whenever the cold weather hits. While cold frame usage is mostly passive, there is one trick to using it effectively. If there is ever an abundance of heat inside the frame, such as after an abnormally warm day, use the small lid support strut to vent the heat.
While the sloped lid should prevent debris from sitting on top of the cold frame and blocking the light, you may occasionally need to clean the top.
How to Maintain Your DIY Cold Frame
Keeping your cold frame in great condition for years of use is as simple as keeping the wood sealed. Because of the exposure to the elements, it's a good idea to reseal the wood annually. If the plexiglass gets cloudy over time, you can easily replace it. However, the cloudiness shouldn't interfere with the cold frame's effectiveness, so only do so if you desire the look of the crystal clear plexiglass.