Wednesday, February 4, 2009

How I Made My Cold Frame

Below are pictures of the cold frame I made. I have a table saw at work, so this made the job easier than if I had to make it at home. But cold frames can be made from a variety of things, like stacked bricks or cement blocks with an old glass window on top. If you are making it out of wood, you could just jigsaw (or even hand saw) some plywood into a similar pattern and throw a piece of plexiglass (or even plastic sheeting) on top.

The pattern can be as simple as a rectilinear box, but I sloped mine so I could let more sun into the box and allow rain or snow-melt to run off the lid. Yours could be set onto or into the ground. I will raise mine a few inches with some screw-on wooden legs because I do not want to smash any underlying plants (upcoming bulbs, particularly).

My cold-frame has quite a small footprint at roughly 28 x 18 inches, but they can be much larger and taller. I designed mine for a city gardener, someone with a small garden and not too many plants to start.

I used scrap wood left from student projects at work. This wood is primed, finger-jointed 5/4 pine. It is 1 and 1/16-inch deep by 5 and 1/4-inch wide. In order to get the height that I wanted, I used a dado to rabbet-joint and glue two pieces together, making for a 10-inch wide board. This gave me a maximum height of 10 inches for the rear plank. The side planks are cut on a diagonal, sloping from 10 inches down to 8 in height. I bevel-cut the top of the rear plank to accommodate the slope of the side planks.

I cut rabbet joints at the four corners to more securely hold the four sides and to help keep out cold air.

I cut the front plank at 4 inches high, leaving another 4 inches for a piece of polycarbonate glazing. I dado cut a groove into the top of this front piece to snugly hold the glazing. After that, I bevel-cut the top of the front piece at 30 degrees to help shed any water.

This view points to the inside-bottom of the cold-frame. I rabbet-cut the bottom to accommodate planking that will be the floor. Exterior water will shed without contacting the interior floor planks with this set-up. However, if you set yours on or into the ground, floor planking isn't necessary.

These views show the polycarbonate glazing on the front. I used glazing here to increase the amount of light reaching the plants inside. You can see how it is held tightly within the groove on the front plank. I cut the glazing 1/4 -inch taller between the side planks so that the roof-glazing would make contact with it.

The roof glazing is a sheet of double-walled polycarbonate set into a dado-cut groove in the wooden frame. The rear of the glazing-frame can be seen below resting on the back planking. Out-door hinges will attach the roof glazing frame to the cold-frame.

This is the cold-frame with the roof glazing on. I left the plastic film on the glazing so that I know which side goes out.

This is a close-up of the roof and front panel glazing. The glazing is held snug in the dado-cut grooves in the wooden frame. The roof glazing overshoots the front plank by 1/2 -inch so that rain drips beyond the frame.

For now, I will use a stick to prop open the cold-frame for venting.

The joints will all be set with waterproof wood glue and out-door quality screws. I will paint the cold-frame to protect it from weather and sun damage. If I had made this out of cedar or redwood I would not bother, but this finger-jointed pine is really meant for interior applications. But with a good couple of coats of paint, it should last long enough. I have some old black barbecue paint that I think will do for the outside. The inside I'll paint with glossy white house paint. The idea is to not spend any money, or more than I have to. The polycarbonate cost 30 dollars at Canal Plastics, and that's about what this whole project is worth to me.

I see that I could buy a really nice one at Johnny's for $325 plus shipping. Maybe in better times. I could also add an automatic roof opener (I actually have a couple of these, but they're in Minnesota, I think). These openers are often wax-filled cylinders. The wax expands as it heats up and pushes a bar which opens your roof. The roof needs to be lightweight for this and the polycarbonate fits the bill.

Tom Chrisptopher at Green Perspectives has some good points on the use of a cold-frame. The kind he describes is much larger, and I like his idea of using the removable-pin hinges as a way of connecting the side planks. His point about "managing" the opening and closing of the roof is well taken. I want to experiment to see how it goes, but will get the auto-open cylinder if it becomes too much hassle.


  1. Her name is Pinky. She must contribute to whatever you are working on.

  2. I think we are going to make a cold frame for this winter. We have lots of extra wood from working in our attic. This looks pretty cool!

  3. Just thought of a question, is polycarbonate easy to cut?


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