Building a garden cold frame is a great additional to your garden. Cold frames allow you to extend your garden season all the way through the winter!
Writing a post on building a garden cold frame has been on my to do list for two years! It took my guest post on the Bakerette today, to finally get me to get it done. So today’s post is a tutorial on how to build a 4 x 8 cold frame with four 2 x 4 plexi glass lids. The original design for this cold frame came from Eliot Coleman’s book Four Season harvests.
Over the last 5 years I have built 9 of these and each time I tweaked the design a bit. I’m pretty happy with the design as I’m going to show it to you today. The biggest improvement I made was adding a bolt and nut to each of the 4 corners of the box so that when you are not using the cold frames they will easily break down for storage. So let’s get started!
Tools & Materials
My suggestion is to get your hands on a couple of tools that will make your job of building a garden cold frame much easier. The first is a chop saw. There are a lot of repetitive cuts in this project and a chop saw makes quick work of those cuts.
Second a table saw. Using a table saw allows you to buy 2 x 4 lumber and cut it to the 2 x 2 pieces you need. 2 x 4’s are always much straighter than 2 x 2’s so your finished product is much better.
You will also need an electric skill saw and some type of electric drill.
Here’s a list of materials you will need:
4 – 2″ x 4″ eight foot boards
2 – 2″ x 12″ eight foot boards
1 – 2″ x 8″ eight foot board
3 – 1/2′ x 2″ eight foot trim pieces (these are basically scrap)
4 – 2′ x 4′ pieces of plexi glass
8 – 3/8″ bolts and nuts. The bolts should be 4 inches long
15 – 3″ deck screws
40 – 1 1/2″ deck screws
8- hooks and eye bolts (if your cold frames are going to be in a high wind area)
I choose to make my cold frames from Douglas fir, so far they have lasted 7 years but are reaching the end of their life. Fir is 1/3 the cost of cedar or redwood so it just depends on how cost sensitive you are. Cedar & Redwood would probably last twice as long.
A few shop safety notes for you when building a garden cold frame. Please be sure to wear eye and ear protection while you are working and please be sure you understand how to use your power tools, a new cold frame isn’t worth a finger or an eye!! This project is pretty basic and I think most people with a good understanding of carpentry can pull it off. For me the work from start to finish took between 3 to 4 hours. When my son helps, we can get one done in maybe 2 1/2. But if you are a beginner at building a garden cold frame you may want to plan a whole Saturday.
Step By Step Instructions for Building a Garden Cold Frame
Cut one of the 2″ x 12″ boards exactly in half with the skill saw. Then again using the skill saw cut each of those resulting 4 foot boards at an angle starting at 12 inches at the back and down to 8 inches in front. For some reason when I did this project I forgot to take a picture of this step. But you can see the end result in the picture above. The top of each piece angles down from back to front.
Using your table saw rip all of the 2 x 4’s into 1 1/2 wide pieces. So you will end up with eight- 8 foot boards that are 1 1/2 inches wide.
Again on the table saw turn two of these boards on their sides and rip them so that they are 1 inch tall. The resulting boards will be 1 1/2 inches wide and 1 inch tall. This cut also leaves you with a couple of long 1/2 inch strips of scrap. Hang on to these.
Using your chop saw cut the two remaining 1 1/2 inch boards exactly in half (48 inches). This gives you 8 four foot pieces. These are the long sides of your lids.
Again on your chop saw cut the 1 inch by 1 1/2 inch boards in to 22 7/8 inch pieces. The resulting 8 pieces are the short sides of your tops.
This is the tricky part of building a garden cold frame. You need to use your table saw to cut a 1/4 inch channel into all 8 of the 4 foot side boards from Step 4. The bottom of the channel needs to be exactly 1 inch from the bottom of the board. This channel is for your plexi glass to set in. You will probably need to run each piece through the saw twice moving the saw rail a little on the second cut to widen the channel. Check that your cut is wide enough by trying to run your plexi glass through the channel. The cut only needs to be about a half inch deep.
I’m all about pre-drilling!! There is nothing more frustrating than splitting a piece while your assembling. So pre-drill two holes in each end of the side pieces. I like to use a 3/8 inch counter sink bit for this so that the screw heads will be recessed.
Assemble the lids by attaching the long pieces to the shorter. Remember that the shorter pieces are assembled so that the glass will sit on top of them in the channel.
Cut the plexi glass to length and width. Your plexi glass pieces will be a little too wide and tall to fit in the frames. Take measurements of each frame and then cut the pieces using your table saw. An alternative to this would be to build your frames before you buy the plexi glass and then have the plexi glass cut to the proper length and width by the store where you purchase the glass. Both Lowe’s and Home Depot will do it for you. I cut mine to 46 1/2″ x 22 3/4″ but I would really suggest you wait to cut until you have the lids assembled. That way you don’t mess it up, that glass is expensive so you don’t want to cut a $28 piece of plexi glass too short!!
Add a stop Block to hold the glass in. This can be as fancy or simple as you like. I usually run a piece of scrap through the table saw to create a small notch for the glass. Then I cut that piece into 8 smaller pieces and attach with a screw. But on some of my lids I simply put a screw at each end to hold the glass in place. The important part here is to keep the glass from sliding out either end.
Slide the glass into the lids. The glass should slide easily into the channel you created in Step 6. The glass should overlap about 3/4 of an inch at each end. Add the second stop block and your tops are finished.
Cut 4 pieces of scrap 2 x 4. Two pieces are 8″ long and two are 12″ long. This step is the change I made that allows the frame to come apart easily. Set the 2 x 4 pieces flush in the corner and then drill 2 holes, one towards the top the other towards the bottom. The hole should be wide enough for the bolts you bought and goes through both the frame piece and the scrap.
Once the holes are drilled install the bolts.
Then put three 3″ deck screws in from the side. The screws go into the scrap piece. Repeat at all 4 corners. Now when you are ready to take the frame apart at the end of the season all you need to do is undo and remove the bolts at each corner and the frame will come apart!!
Next cut a notch in the center of the front and back of the frame.
Install your last 1 1/2″ x 4 foot piece as a stretcher and stiffener for the frame. This piece is also handy for lifting and carrying the frame.
This step is optional but I like to take some of the 1/2 inch scrap pieces that are left over and attach them to the bottom edges of the frame. This is where the wood comes in contact with the ground the most. This piece of scrap adds some separation between the frame and the ground and helps to keep the wood of the frame from rotting as fast. You can even replace this scrap piece every few years.
This is also an optional step. You can add some type of hook or fastener to each of the cold frame lids. Do this if you frames will be in a windy area and you are worried about them blowing off in the wind. Only one of my frames has this option added and I have really never had a problem. The frames are very low profile and usually don’t get caught in the wind.
Well there you have it! Building a garden cold frame is as easy as one, two, three . . . fifteen! Okay so it’s not easy, but it really isn’t that difficult either. For around $130.00 you can have your own 4 x 8 cold frame! If you live in Utah and don’t feel like making one of these yourself I’d be happy to build one for you. I usually charge materials plus labor so the cost would be right around $300, contact me at email@example.com for more info!!
Learn how to use your new cold frame by purchasing a copy of my Year Round Gardening Course: