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It isn’t hard to learn how to build a cold frame.
I built this cold frame in an attempt to prolong the growing season. And, it is easy to build this season extending tool. Plus, it is fairly inexpensive. And I’ve found this cold frame is a good size that’s 37.5″ long. And that’s plenty big to start several plants in spring and over winter some too. 
Your shopping list to build this project:
The most expensive part of the cold frame is the glazing. So, for my project I just used acrylic glazing. And I was able to purchase it at my local home store. But you could also use old windows, glass panels or polycarbonate greenhouse panels.
Along with some kind of glazing, all you need for this cold frame is:
- a sheet of 1/2″ plywood that’s 4′ x 8′
- 3 – 2x6x10s
- 2 – 2x2x8s
- 3″ corner brackets
- 5″ T-hinges,
- plenty of 2 1/2″ #8 wood screws
- one 3/4″ #6 Round head screws
Begin building by cutting your plywood:
To begin building your cold frame, start by cutting the plywood to make the sides. In the end, you will need 2 pieces to form the sides. So start by turning the plywood on its long edge. Then cut a section off that is 37 1/2″ long and nearly square. This 37 1/2″ length will be plenty to make the two sides. But they need to be cut again to make two pieces that are angled so the top will have a slope.
To create your angled sides, measure up 11 1/2 inches on one side of the piece and make a mark. Then measure up 23 inches on the other end of the same side and make another mark. Now, draw the diagonal line from each mark point. This marks the angle you will cut. Make this cut.
Then return to the original piece. And mark this one at the 11 1/2 point and the 23″ point. Draw the diagonal line. And make the cut.
Now you have your two sides of the frame.
Next measure & trim your boards:
Next, I took the 2x6x10s and cut them in half. This makes 6, 5-foot pieces.
You will use 2 of these 5-foot pieces to make the front of the frame. And use the remaining 4 to build the back of the frame.
Now start building your cold frame!
Start by lining up one of the 2×6 with the front (or short end) of your trimmed plywood. Then screw them together with the #8 wood screws. Next, repeat the same on the other side. Next stack another 2×6 in the front, screwing it to the trimmed plywood on either side. Then, stack and screw a total of four 2×6’s in the same manner to the back. Now your main box is built!
How to reinforce your cold frame box:
To strengthen everything, cut two pieces of 2x2s. You will install these to run at the base of the plywood on the inside of the box. Then cut another set and install these to run along the inside of the plywood on the top edge.
Bonus! These pieces not only strengthen things, but will also give the cold frame lids something to rest on.
Cold Frame Filled With Veggie Starts
How to craft the wooden frames for your box lids:
You’ll be making two lids fo your box. And each lid will need to be long enough to go from the top of the back of the box to the front of the box. This is about 39″ long. And each lid will need to be just under 3o” wide to cover half of the cold frame.
So, to make your lids, begin by by cutting the 2x2s so you have four 30″ lengths and four 39″ lengths. Fit together two rectangles, each using two 30″ lengths for top and bottom with two 39″ lengths for the sides. Fasten each corner of each lid frame with 3″ corner brackets.
How to prepare the glazing tops for your box lids:
Next cut the acrylic glazing to size of each frame lid. (I made it about 1/4 inch shorter and thiner then the frame itself.) Attach the glazing to the frames with #6 wood screws in the four corners of each lid. Extra tip: Pre drill the holes through the glazing before screwing them to the lid itself. Then use rounded head screws every few inches to tighten the glazing to the frame.
How to attach your cold frame lids to the frame:
Now it’s time to attach the lids to the frame. Do this using your T-hinges. Start by putting both lids on the cold frame and positioned them between the sides so each can open individually. Then attached the T-hinges first to the back of the cold frame and then to the back of the lid.
Now your cold frame is basically finished and ready to use.
A few finishing touches to try:
In this model I added perforated straps from my local hardware store to help hold up the lids for venting. To do this attach a screw to the front of each lid in the center and allow it to stick out 3/8th of an inch. Then half way down the top front and in-line with the screw in the lid attach a L-hook. Now, hook one end of the strap to the L-hook. Then, to prop up the lid, open it and connect on hole on the other end of the strap around the screw in the lid. But if you do this, take care with those straps. Sometimes they like to “bounce” out of place in the wind.
Since Bob built this, we have modified the box a bit. I didn’t like the metal poles that hold the box open. They’re great for venting, but they were a bit of a hazard for holding the box open when working in it. To modify, Bob added rope holds. Now the entire lid can be pushed back and held open by the rope. Also, to help with keeping the box warm, Bob lined the “floor” with scraps of rigid foam insulation. Since our box is on a cold patio, the extra ground insulation helps keep the soil and roots warm and protected. And, don’t forget to put a thermometer inside the box. You might be surprised how hot they get even on a cold day!
What did you use to cut the glazing without cracking it or chipping the edges?
The glazing was cut on my table saw with a fine toothed blade. I also fed it through at a slower rate. Personally I don’t think this was the best technique and I understand it can be cut with a plexiglass tool similar to a glass cutter. I would check with the store you buy it from on the best way to cut the material.
Why did you opt to build a rigid cold frame (which I will grant is the style used for decades in U.S. gardens) versus the style that uses plastic sheeting spread over hoops (as was done by the gardener on Mother Earth’s web site)?
It looks great. I’d love to have some that would fit over our raised beds.
We’ve come up with some design ideas to do just this. Get in touch if you’d like a gardenmentors consultation to help you with yours 🙂
Good question. We actually have both. The hoop house is over raised beds in one part of the garden, and we’ve been using that for several years. The cold frame, which stays a bit warmer, is great for potted seedlings that need some time hardening off after being started indoors. It allows us to start hardening tender seedlings but not put them into larger beds/early where they may struggle in the beginning. West of the Cascade Mtns we have lots of soil microbes that can take down tender seedlings rapidly. This just gives us an extra insurance policy.
By-the-way, the cold frame was almost 100F when I went out to vent it this morning. That was from sunlight. The air temp outside the frame was hovering around freezing and frozen rain was covering the exterior of the cold frame.
How about some closeups of the straps used to keep the lights open as I don’t know where the L shaped bit goes and also of the modification for keeping the lights open.
How about some closeups of the straps used to keep the lights open as I don’t know where the L shaped bit goes and also of the modification for keeping the lights open.
Jim, Sorry. We don’t have those straps readily available to add to the post at this time. Good luck!