Thursday, December 13, 2012

Cold Frame/Mini-Greenhouse

I’ve been thinking about building an outdoor garden structure this winter, something for storing tools, growing seedlings and cold hardy greens.  I made a rough design of a shed, about 6 feet on a side, with a south facing window and a bench below the window opening.  From my years spent (or misspent) working as a carpenter and the books I've read on passive solar I had convinced myself that I could design something that would not only give plants good sun exposure but retain some of that solar heat gain on cold nights. 

I came up with some rough sketches but finally set the plans aside.  To build in the performance that I wanted the structure was going to cost more than I was willing to spend.  There was also no good level spot with full sun to place a shed of that size. 

I checked out some garden blogs for information on cold frames, but an in-ground cold frame wasn’t quite what I was after.  There was also a plan I found for salad tables.  Elevating the growing surface to countertop height definitely had an appeal.
Then I came across a plan for a cold frame/propagation bench from the Western Red Cedar Association:   This was more like it, a mini-greenhouse that you worked in from the outside, small enough to fit in the space available and with good looks.  The “glass” part is the hinged lid that the user lifts up to access the plants.  And best of all, the plants are elevated to a more comfortable working height, at least they are off the ground.  I set out to design a structure that was large enough to overwinter some greens and also harden off seedlings in the spring. 

This is the general idea of the thing on which my plans are based.  I drew up plans for a larger structure.  The depth of the box was increased from 2 to 2 ½ feet and the length from 4 to 5 feet. The planting box was raised - their plans had the box only 18 inches above ground.  As I looked at the drawings for ideas  I began to think that the designers never actually built this.  There were parts missing in the design that were structural essentials. 

I drew schematics of critical sections of the structure, but left many details to work out as it is built.  It’s really hard to get a sense of the “rightness” of the design until I can see it “in the flesh.”  Since all the components are screwed together, it’s easy enough to back the screws out with a cordless drill when I want to make changes.

The challenge in designing a small garden structure is to make it light enough to move but strong enough to last.  I’ve found that most garden structures use more wood than necessary, erring on the side of using more lumber than is needed so no high stress area has inadequate wood.   It's a challenge to find a design that works without weighing too much. 
The first day I set out to build the basic framework.
Here’s the front section, 2 x 2 cedar legs about 3 feet tall and a 1 x 8 cedar cross piece which will support the bench floor.  The 2 x 4 at the bottom of the photo is a temporary spacer to hold the legs at the proper distance.  Once assembled the section was squared and braced.  The back section was built the same way, but with 2 x 4 cedar legs 8 feet tall.  At this point I did not how tall the finished back frame would be so I used the full  8 foot 2 x 4, intending to cut it later once the dimensions are worked out. 

I attached the side 1 x 8’s to the front section.  

The front and back sections were supported upright on the floor and the unattached end of the side 1 x 8’s (top of previous photo) attached to the back section.  Additional braces were added to square and brace the structure.  The 8 foot long 2 x 4’s made the back section cumbersome to work with.  If I build this again, I’ll know the exact length to cut them, but the first attempt is a learning process.

A diagonal from the top of the front to the back legs was made 2 x 2’s.  When I’m satisfied with the geometry they’ll be marked, removed and cut to fit, but for now they are just screwed onto the outside of the legs to get a sense of the final dimensions of the structure.  I’ll see how it strikes me the next day and may move the connection on the back up or down a little, but this is the general sense of its form.  I’ll make another post in a few days on its progress. 

(Next day).  Well I had a look at it this morning and realized that some things should be changed.  First of all it seems too deep to reach the back easily.  I may reduce the sides from 30 inches to 27 inches.  Changing the depth will also reduce the height, which is a little high at about six feet.  I’m also thinking about replacing the front 2 x 2 legs with 2 x 4.  I may put wheels at one end and the 2 x 2’s aren’t large enough to attach wheels to.   Finally I plan to lower the box two inches.  All these changes should make the mini-shed more right-sized.  Now if I knew just what to call it.

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