The Quest for the Holy Cold Frame - Extended Edition

This is the long, detailed version of an article that I contributed to my wife's blog (Loving Our Guts).

A while back, I was cleaning out some old lumber under our back deck and ran across a nice big sheet of 1/4 inch thick glass.  In the spirit of The Big Clean-Up  (a favorite of mine from my childhood and arguably a factor in my hoarding instinct), I decided to build a cold frame so that we could garden into early winter and start again in the very early spring (actually, I'm giving January a shot for a starting point this year).  In this post, I'll describe the process with lots of pictures and lessons learned along the way.

Since I like flashback, I'll start with the final result:

First off, I figured I should attempt a bit of research.  I procured Building & Using Cold Frames: Garden Way Publishing Bulletin A-39  (the Kindle Edition will save you 79 cents off the budget busting $3.95 list price) and skimmed an article on Mother Earth News on the topic.  I'm enough of a guy that I wanted to forge my own path, so I decided to make my own plans.  So armed with a decent recollection of my high school wood shop class, I set out to design my cold frame.  You may wonder how I arrived at my design.  I used the following design parameters:

1. The frame should be designed to fit the sheet of glass I happened to have on hand.
2. I skipped the suggested angle from the above e-book and decided to draw inspiration from articles that I've read on planning solar heating or electrical systems - namely, the angle of surface should be either exactly, or at least generally, perpendicular to the sun for your growing seasons.  The more precise method involves taking your latitude into consideration but I remember reading that a 30% slope is a good guideline for the U.S.   So I did a bit of math and came up with height and width required to get that angle.  30% has the advantage of being twice as deep as tall so that kept the math simple.  I then added an extra layer of boards along the bottom so that the plants in the very front wouldn't bump their heads as soon as they emerged.  This also allows me to slightly inset the frame into the ground.
3. I wanted to use standard dimensional lumber, so I decided on 1x8s and 1x2s.  

In the below drawing, you can see a top view (upper left), side view (lower left), front view (lower right) and finally a view of the lid on the upper right (laying flat, not on the slope).  The squares inside the top view are 1x2s that run vertically.  I used these to tie the horizontal pieces together.  If anyone wants the plans as a Visio to adapt to their needs, let me know and I'd be glad to share them.

I also wanted to limit the potential from anything untoward leaching into the soil from wood preservatives, so I decided to try to find a source of wood that is naturally rot resistant and then stain with the lest toxic stain possible it to increase the rot resistance.  Since I wasn't going to use the most effective chemical treatment, using a naturally rot resistant wood was of the essence.  I did a bit of searching and found that MWANZI would sell me local, sustainable wood.  They didn't have cypress, which is the best option for rot resistance, but we eventually ended on walnut which should fit the bill and it's very pretty.  I found the owner to be very nice to work with.  Their main business is furniture, cabinets, flooring, etc. but he was open to selling plain lumber.  He even cut it to order.  So our local produce will be grown in a locally sourced cold frame made with local labor. How's that for cool? It's a shame I can't get local hardware.  For finish, I eventually found BioShield Aqua Resin Stain - Clear .75 Liter from Home Eco (a local store). Not only does this stain include an ingredient list, but it's pleasantly simple and non-toxic.

Here's some of the lumber that I started with:

I actually built the lid last summer.  Unfortunately, I didn't take any pictures during the process.  The process was simple if a bit slow due to lack of practice.  I borrowed a friend's table saw and cut channels down the narrow side of each board.  I measured to make sure that the depth was sufficient to allow the glass to nest inside when the four sides are to be butted together.  Since the blade wasn't 1/4 inch wide, I cut multiple channels to allow the glass to fit.  I made the same cut in every board before moving to the next board to ensure that they came out even when assembled (the channels didn't end up being quite centered).  I probably made it more work than necessary by attempting to not cut channel into the very ends of the boards.  My theory was that would help keep water out and reduce rot (and look better).  To facilitate that, I also ended up purchasing a 1/4 inch wood chisel.  I don't know that it was necessary, but it was enjoyable and it's always nice to have an excuse to acquire a new tool.  The four boards were placed around the glass and fastened together with flat brackets and thick wood screws that were just short enough to not protrude through the wood.  I pre-drilled all of the holes for this project before attempting to install the screws.  I also used screws instead of nails for extra strength.  Here's a glimpse of one of the brackets in the corner of the lid.

I started on the main body by attaching the 1x2 tie pieces to the front panel and the back panel. In the case of the back panel, I then proceeded to attach the 2nd and 3rd piece to complete it.  Here's one of the front boards after affixing the tie piece.  The girls' sand bucket made a decent stand.

 For the back panel, I caulked between each board to limit air leaks.

Walnut is strong. I predrilled each screw hole and I still broke a couple of screws in the final tightening (and stripped the heads of a few). I'm wondering if I should have used something stronger than zinc screws. If I remember right, zinc is considered to be a soft metal.

Finally, I attached the bottom layer of side boards to rough out the final box form before attempting to saw the two sloped side pieces.

I got to use most of my tools on this project. Clamps are a most wonderful invention. At times, I wished I had 4 of them (I have two but was too lazy to run upstairs to dig up the one stashed up in the office).

Speaking of tools, I was so happy that I had broke down a few months previously and acquired a good corded drill. Walnut is pretty hard and even with pre-drilling the screw holes it took some oomph to drive in the screws. I ended up using this drill both to pre-drill the holes and drive the screws and the other drill with a bigger bit to drive the counter sink wider parts. I found myself wishing that I had a third drill so that I didn't have to incessantly switch the bit and driver. I may eventually break down and have this upgraded to a keyless chuck.

This drill also has an adjustable depth gauge that I used extensively to enable me to consistently drill holes that were the exact depth that I wanted them.

Next, I sawed the two angled boards for each side.  Walnut is hard.  The wood was smoking from the effort to cut through it and you'll see char from the heat coming up.  I'm grateful to a friend for the use of his saw.

See the shiny part on the side? That was daughter W's idea. It's a screw to tie one side board to another. I had planned on attaching strap iron pieces on the inside to tie them together but this seems to be working.

Here's a closeup of the verticle 1x2s that I used to tie the sides to the front and back.

And here's the screws coming in from the sides.  I tied into both the tie pieces and the front/back directly.  The only trick was avoidng the screws that ran perpendicular to these.

It turned out to be very strong.  I was afraid that the corners would flex and that I'd have to handle it with kid gloves but they don't flex at all. I could have hidden the screws on the inside like I did with the front and back screws by only screwing into the 1x2s but this was stronger.

I really want to weigh this. It's very substantial. I'm even considering adding handles on the sides to make it easier to move it around.

The total size is nearly 5 feet wide by 2 1/2 feet deep and about 2 feet tall.

 I ended up beveling the back with my reciprocating saw. I probably should have used my friend's bench saw but it turned out pretty nice for free hand.

I hadn't intended to bevel the edge but I did need to trim the tops of the 1x2s that I used to tie the sides to the back and one thing led to another.  It ended up being the right decision for the lid to close properly.

Now, it's time to see how the lid works.  Here's one of the hinges. I had to add a washer to one hole on each hinge to keep from hitting the glass. The glass is in a channel that I cut into the lid frame. The lid took an entire day all by itself.

Do you think I might need to clean the glass?

And here it is attached!  Thank you lovely helper Patty for holding the lid while I attached it.

You'll also notice the grey strip to the right of the hinge.  That is a strip of weather sealing tape that I installed all around the inside of the lid.  This prevents the warm air from leaking out overnight (at least to some degree).

Hinges from the back. I wasn't able to use the matching screws that came with them since they would have been slightly too long. But that gave me an excuse to use slightly larger screws with deeper threads so they should be stronger despite not being as deep.

The lid is heavy. A handle helps a lot.

I put it kind of low to avoid running into the glass inside the frame. The center of the holes is 15 mm from the edge of the wood, but I think it'll be strong enough.

A little vinegar, water and judicious application of elbow grease and voila, you can see through the glass. More importantly, the sun can do its magic on plants.

To prepare for the stain, I needed to remove some excess caulk that had seeped out.

The addition of a black lab is optional.

Next, it needed some sanding. I did a bit by hand and borrowed a friend's belt sander for some larger sections (such as removing the burns from the sloped sides.

No better way to spend lunch hour than staining the cold frame.  It requires three coats of stain.

Note the piece of scrap unfinished wood in front of the stained.  The stain is untinted, but it still imparts a gleam and brightens the wood. 

I can't forget the bottom - it will bear the brunt of contact with the elements.

Finally, we just needed to prepare a bed, firstly be removing the sod...

... and then mixing up some quality soil.  We used peat moss (10%), sifted soil from the location (20%) and the remainder finished compost.

We planted some radish seeds immediately since we had them on hand and placed an order with The Seed Saver's Exchange.  I also purchased a Wireless Thermometer with an external sensor (two actually) so that I could compare the temp inside the cold frame with that outside.  I was surprised to discover just how humid it stays in there.  These also keep track of high and low readings for both temperature and humidity for each sensor (including indoor  reading).

Yes, that's right.  It's 94% humidity inside.  I find that at least at this time of year, the maximum temperature rise (compared to the outside temp) is a bit over 20 degrees.

Besides radishes, we planted arugula, lettuce, turnips and dinosaur kale.  So far, at least some of everything is up and a couple of rows look like we need to start thinking about thinning. I think we're all having to go outside and check on them daily out of sheer enjoyment.  Planting this early, there's still a chance that we'll lose everything, but seed is cheap and it's worth the gamble if it pays off.  Everything we planted is cold weather tolerant.  I've also got a couple of ideas on how to further tweak the overnight heat loss.  I already place an old blanket on it on cold nights but I'm going to get a piece of rigid insulation.  That will also protect the glass from hail.  I've also placed old bricks in the back of the cold frame to absorb heat during the day to be released overnight.

It does get quite beautiful when it frosts:

And here's what it's all about.  We can literally see them grow day by day, even noticing differences from morning to evening.