Building the box and setting the slope
Okay. You've got your window, or windows, hopefully free off a classifieds listing like mine was. You've got scrap lumber. Last lesson was about finding that stuff. This lesson is about actually putting it together, so let's get to it.
A cold frame is just a bottomless box with a lid that lets light in. That's it. Don't let anyone dress it up as more complicated than that. The box sits over your bed, the lid is your window, and the whole thing works because it traps solar heat during the day and slows down heat loss at night. Simple wins. I said that a couple lessons back about cold frames versus hoop houses and I'll keep saying it, because it's true of the build too.
Sizing the box to your window
Start with the window, not the bed. The glass (or the storm window, or whatever you scrounged) is the one piece you can't easily cut down or extend, so build the box to match it, not the other way around.
Measure your window's outer frame. Your box's top opening should be just slightly smaller than that, maybe a quarter inch on each side, so the lid rests on the frame instead of sliding around. Too loose and it'll shift in the wind. Too tight and you'll be fighting it every time you open it.
The slope, and why it matters
Here's what actually happens if you build a cold frame with a flat top: rain and snowmelt sit on the glass, and in winter that water can freeze right onto the lid. Then you're chipping ice off something you're trying to open every morning. Not fun, and it's hard on the glass.
So you slope it. Back of the box taller, front shorter, angled toward the sun (south-facing here in the valley, generally). I build mine at roughly a 30 to 45 degree slope depending on the window I've got to work with. A good rule of thumb: back wall around 18 inches, front wall around 12 inches, and let the lid bridge the difference. Water runs off, snow slides off, and you get a better sun angle in winter when the sun's already low in the sky.
Run the numbers on that for your own window before you cut anything. I've eyeballed it before and ended up an inch off on one side, which doesn't sound like much until the lid won't sit flat.
Building the box itself
Basic steps, and none of this needs to be fancy:
- Cut your side pieces first. Two side panels, cut at an angle so they slope from your back height down to your front height. This is the only tricky cut in the whole project. Mark it, double check it, then cut.
- Cut the back and front panels to match the width of your window opening.
- Screw it together, don't nail it. Screws let you take it apart and adjust if something's off, which it might be the first time.
- Attach hinges to the lid along the back (the tall side), so it opens up and away from you.
- Set the box in place over your bed before you do anything else permanent. Check that it sits level side to side, even though it's sloped front to back.
Honestly, most of the mistakes I see people make happen because they skip step 5 and build the thing somewhere convenient, like a garage floor, and then discover the ground where it's actually going isn't flat. Dig or shim as needed before you call it done.
The prop arm, no exceptions
This is the one part I won't soften. Put a prop arm on the lid. Every time, no matter how small the frame is.
My oldest, Bryce, helped me build my very first cold frame out of a free window years ago. We hadn't thought about a prop, figured we'd just prop it with a stick when we needed to. The lid was heavier than we'd guessed, the stick slipped, and it came down right on his fingers. Nobody got hurt bad. But it scared both of us, and I've built a proper prop arm into every cold frame lid since, no exceptions. A simple notched piece of wood attached to the side of the box that holds the lid open at a few different heights does the job. It costs you almost nothing and it's the difference between venting your frame calmly and doing it nervously with one hand.
Venting, and why you'll be doing it constantly
Speaking of opening the lid: you'll be doing that a lot, even in the dead of winter. More cold frame crops die from cooking on a sunny 45-degree day than from actual cold. It seems backwards until it happens to you, and then you believe it forever. A closed cold frame on a bright winter afternoon can hit 90 degrees inside even with snow on the ground outside. So the prop arm isn't just for safety, it's a tool you'll use constantly to manage that heat.
My kids Maggie and Mia have their own bed and mostly grow carrots and one very stubborn pumpkin most years. Half of what they plant doesn't come up, and that's fine, they're learning the same way I did. I mention it here because building your first cold frame is the same kind of learning. Your first slope might be off by an inch. Your first lid might not sit quite flat. Build it anyway, use it, and fix version two.
Before next time: get your box built and set over the bed, level and sloped, with the prop arm attached before you install the lid. Next lesson we'll cover sealing gaps and insulating the sides so you're not just leaking heat back out the seams you just built.