We Did Some Weird Math For This Cold Frame Project

cold frame

Last week I decided that we need a cold frame to get through winter. We didn’t want to say goodbye to our little urban garden as the colder weather crept in. And I thought that being able to possibly grow a little winter garden or early spring seeds in a cold cold frame seemed like the best way to stretch out the summer fun.

We looked at pictures of cold frames online and Chef Iggy drew up a plan. We spent two afternoons building it, at a cost of $49.41. Here is our step-by-step guide to building your own.

Cold frames start with windows

Why buy supplies when you can get them for free? We put the word out that we were looking for old windows, and a friend donated a pair of old double-hung sash windows that were lying around in his garage. They weren’t old enough to have lead paint (if they were, we would have scraped them or painted over to seal the lead from leeching into the cold frame soil).

cold frame windows

A friend donated an old set of windows to get them out of his garage.

Pro tip: When you’re planning the cold frame, make sure you are placing the outside of the windows on the outside of your frame. The outside better withstands the elements.

To build a cold frame, start with the windows first, then build the frame to fit them. We noticed that one of our windows was a few inches longer than the other, so we whipped out the circular saw to cut off the excess. Then we measured the two windows side by side to get the length of the frame.

Fancy cold frame maths

Here is the tricky part – the glass lid of a cold frame is slanted toward the sun, so it can maximize the greenhouse effect of the sun’s rays on the plants. We wanted a frame that was three boards tall at the back, and two boards tall on the front. For the sides, we needed two boards and then we would cut the third board corner to corner, diagonally. Each triangle would go on a side to create the slant.

But the slant means you have to figure out how deep the cold frame will be. If you make it exactly as deep as the windows are tall, the long slant makes the windows too short to cover.

This is a critical detail ignored by the cold frame do-it-yourself videos or plans we saw online. I’m going to walk you through it, so bear with me. Fortunately, Chef Iggy remembered a cool part of his high school geometry, which was super helpful.

cold frame building plans

Pythagorean theorem comes in handy.

Our windows are two feet long. The boards we used to build the frame are 5 1/2″ tall. So that’s 24″ squared (576) plus 5.5″ squared (30.25). That’s 606.25, and the square root is 29.5. That makes the depth of the finished cold frame 29.5 inches.

If you don’t want to tie your brain into knots solving for X with a pencil and dusty math brain, click here for a handy online calculator. But I have to say, kudos to Chef Iggy for figuring this one out. If he hadn’t, we’d have built the cold frame and been stuck with windows that didn’t fit.

Once we had the big math out of the way, we made a supply list.

Cold frame supplies

2 recycled glass windows, or one french door

5 boards the width of the cold frame lid

5 boards the depth of the cold frame (fancy math measurement)

2 1″ x 1″ pieces the height of the cold frame in back

2 1″ x 1″ pieces the height of the cold frame in front

6 double-screw piano hinges (for outdoor use)

2 weatherproof handles

Lots of outdoor wood screws

Where to put the cold frame

Since the idea is to capture as much of winter’s weak sun as possible, the cold frame must slant toward the south. We found a spot for ours against our driveway fence. A cold frame is only accessible from three sides, so it can’t be the standard four feet deep that many garden beds are. We made ours 2.5′ deep, which is probably a little too deep for comfort, but we wanted maximum space.

Site the cold frame before building it, because this sucker is a pain to move. You want to build it where it’s going to live.

Getting your DIY on

Honestly, this part was easier than the fancy math. We laid out all the boards, then butted the sides against the front and held them with a few wood screws. We did the same with the back, so that we had the bottom part framed up. Then we built the the next level the same way and put it on top of the first part.

For the back, we put the third board on the cold frame, then butted it up to the tall end of each side triangle. We screwed those into place.

Then we inserted the 1 x 1 pieces into each corner, and screwed those into each board. Our fence was blocking access from the back, so we screwed those in from the inside.

Then we put the windows on to check the fit. It was perfect! Hooray for math!

We screwed in three hinges for each window, because those windows are heavy. The risk to using fewer hinges is they might pull out over time. We installed the handles and then used the leftover bit cut off the longer window as a prop.

finished cold frame

Now that it’s done, I’m all excited to use it. I know a cold frame will help us start seeds two weeks earlier in spring, then we can transplant them into the raised beds. We can also plant fall vegetables and extend them later into the cold weather. I think we can also use this as a regular bed during the summer, propping the windows open. I can’t wait to get some little plant babies into this special project!

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