If you live in the northern half of the US, then you probably have to deal with some pretty cold winters. Instead of having to move your plants indoors, or just waiting until the spring time, build a cold frame and extend your growing season all the way through fall and in some cases into the beginning of winter!
Save on costs by using salvaged or recycled materials, and help keep your grocery bill down at the same time!
Step 1: Finding the Materials
You don't need much for this project, my wife found some antique windows at a flea market and I managed to get some old oak flooring from a friend of mine who was remodeling his house.
If you're in a hurry and don't have time to search for salvaged supplies, you can always buy regular 2x or 1x lumber or decking. Plexiglas with a frame around the edge works well as a window alternative too.
- old windows
- wood boards
- 2 galvanized strap hinges per window
- a handle or pull for each window
- wood glue
- bricks, pavers or gravel (optional)
- string or thin rope
- four 2" sections of dowel
Step 2: You Didn't Say There Was Math Involved!?!?!
Don't worry, before you freak out trying to remember middle school algebra, I've already laid it for you! A few variables will determine the overall size of your cold frame, and the angle of the windows.
First, add the widths of your windows and add about 1-2". This will be your overall length.
Next, determine how tall you want the front of your box to be by estimating what the smallest plant is you'll be growing. I wouldn't recommend going below 5" as anything lower will really limit how much you can grow. We went with 10".
Now that you have the front height and the length, you can move on to the the fun part: figuring out the angle to lay out your window. There's two ways to go about this:
- You can predetermine your width and use some basic trig to get the full height at the back of the frame (refer to pic 1).
- EX: if your window measures 28" and you decide on a width of 25", then using the Pythagorean theorem, you can get the total height of the back.
- A² + B² = C²
- A² + (25)² = (28)²
- A² = 784 - 625
- A² = 159
- √A² = √159
- A = 12.6"
2. You can pick the angle you want for your window and, once again with a little number crunching, let that determine your final height and width (refer to pic 2).
- EX: if you know you want an angle of 20°, and your window measure 28", you first figure out the back height, then use the pythagorean theorem again to figure out the width.
- SIN(20) = A/28
- A = (.34)28 = 9.52
- 90.6 + B² = 784
- B² = 693.4
- B = 26.3
Step 3: Cut Your Pieces to Size
Once you figure out all of your dimensions, you should only have two different lengths to cut to; the total length and width.
If you do end up using tongue and groove flooring like i did, you'll have to notch each end to accommodate for the next piece in order to form a box joint.
Step 4: Start Stacking
Now you can start building the frame one level at a time. Butt a long piece up against a side (it helps if you clamp the long piece in a vise) and put 2 screws through the short piece. On the opposite side you want to butt the short piece up to the long one and screw through the long piece this time. Do the opposite for the remaining long piece. When you finish this first level, you should have 2 screws going through one corner of each piece.
For the next level, you can start in the same corner with the opposite arrangement (instead of the long piece against the short one, you'll have the short against the long).
Keep doing this until your frame has reached the front height. For me this was four levels.
Step 5: Add Your Legs
Before you go any further, you want to attach the legs to each corner. Lift the frame on top of some 2x4's on end (you want it about 3.5" off the ground), place the 2x2 posts in each corner and mark where they intersect the bottom. Also mark where the 2 front posts intersect the top of the frame.
You want to cut off the tops of the 2 front posts so that they are flush with or a little below the front. On the bottom of all the posts, cut an angle on all four sides from the mark you made to the bottom. This should give you a nice point to stick into the ground.
Now you can start screwing the existing levels to the posts, alternating levels to avoid hitting any screws.
The back posts will have more levels covering them. So take your cut offs from the front posts, drill a small hole in the center about 1" deep and a matching hole in tops of the back posts and connect these to with some glue and a small piece of dowel.
Step 6: Angled Cuts and More Stacking
Now you need to start adding the angled side pieces. To do this, stack the remaining side pieces for one side and offset each piece by the thickness of your boards. Then, lay a straight edge from corner to corner, draw a line and cut each piece.
*If you're not using tongue and groove wood you can use both halves of the angled pieces on either side of your frame. If you are using t&g, like I did, you have to repeat the angled cut on another set of boards because only one half will fit the existing frame*
After that you can continue adding levels the same as before except now you don't need to add a front piece. Since the angled side pieces won't reach the front, you'll have to screw them into the piece below it.
Step 7: Add the Windows
Once your frame is complete you can add your windows. Lay them on top of your frame to space them out the way you want. Check for any wobbly spots you may have to level out.
After you get your windows where you want them, install 2 hinges on the inside of the window. Now lay them back on the frame with the back half of the hinge laying on the outside of the frame and install them.
Step 8: Finishing Touches
You're almost done! Now just add a few last odds and ends.
First up, install the handles into the windows. This may vary depending on the type of handle you get. If you want a more rustic look, you can always add a rope handle.
Second, fill the gap between the windows. Since this is meant to stay warm when the temperature drops, you want to try and close it off as best you can. I cut and screwed a strip of wood to the back of one of the windows that extends past it and underneath the second window.
Third, I added a support in the center of the front edge so the windows don't twist or bend inwards when shut.
Lastly, you're going to need a way to hold the windows open. I installed a screw at the front inside corner of the frame, and the lower inside corner of the window. I tied some jute cord to each and tightened the screws.
Step 9: Enjoy!
Now all that's left is to find a nice sunny corner in your yard, preferably facing south. Dig out the area around and beneath the frame, add some soil and if you want, add a border of gravel. You can also add some bricks or pavers along the outside edge to make it easier to access the frame. Make sure you press the legs into the dirt to stabilize the frame.
And after planting a nice selection of vegetables, now just sit back, enjoy, and reap what you sew!