Here’s how. I started with salvaged 2-by-8 redwood sills anchored into bedrock with expansion bolts. This part is key because a greenhouse has little mass for its size and can easily blow downtown in a stiff wind. Google “greenhouse anchors” or ask at the hardware store. Pressure-treated 2-by-6s are also fine for sills.
The frame is of 2-by-4s, toenailed together with deck screws. By this I mean a vertical stud is anchored to the sill by drilling through the stud and into the sill at an angle. Nails work, but mistakes are easier to correct with screws. Some people advise against normal 2-by-4s in a greenhouse because they can rot, but I’ve never had a problem with the space well-ventilated.
I framed up the two end walls first, one with room for a door, the other with space for a ventilation fan about halfway up. The door was about 30 inches wide and about 67 inches tall. The door we salvaged and cut to fit.
I framed out the front wall first, done the way you see it, with 45-degree angle braces in each corner for rigidity, then braced this in place while framing in the side walls, four-foot studs set 24 inches apart on center. I first fastened the stud bottoms to the sill, then set the horizontal top plate in place and secured it, again with deck screws. Both side walls went up, and then the rear end wall.
The roof slope is a 45-degree angle with a 2-by-4 ridge pole between the opposing rafters. I cut all the rafters at once for accuracy’s sake, laid them out, checked the top angle with a framing square, with a scrap piece of 2-by-4 between the rafters to be a stand-in for the ridge pole, and scecured each set in place with a cross piece just above door height. Use at least three screws where a cross-piece meets a rafter for rigidity. (These cross-pieces probably have names, but hell I don’t know what they are.)
So that’s the frame. Covering it will be four-year poly (they call it four-year but it actually lasts for five or more. The least expeneive I could find, cut to size, was from A.M. Leonard, mail order. Call and they’ll tell you what measurements you’ll need.
This I attached to the end walls first, fastening it at the top under lengths of wood lath, which is cheap and perfect for the use. I fastened the lath in place with panhead screws placed every 4-6 inches. Then do the bottom and sides, pulling it really tight and repeating the procedure until the poly was perfectly snug, with no wrinkles.
Then the same thing across the top, fastening it at the sills, throwing it over, and securing the other side. Be patient, this takes time. Once it’s secured to the sill and end walls, I secure it to the side studs and rafters.
And you’re done. If you have questions, leave a note.
I built the one you see in the Colorado Rockies, because tomatoes don’t grow outside at that altitude (9,000 feet), so I built two raised beds in the greenhouse with scrap lumber for the tomatoes, parsnips, and basil. I heated the thing with one of those oil-filled radiators, and it didn’t cost much because seeds need 70-degree F. temperatures to germinate, but can be cooler at night once they’re growing. So we germinated in the house (see “Seed-starting) and moved them out later.
For growing germinated seedlings for the garden, just set up sawhorses and plywood. You have a two-foot path in the middle, and three feet to work with on either side. That’s plenty.
And you know what? In January and February, when the sun is high and heating your greenhouse to 90 degrees, and it’s damp and smelling of rich earth and life and magic in there, and colder than a witch’s toe outside (did I spell that right?), take your sweetie to the greenhouse for glass of wine. Or just to sit. Now, just think what you’re saving over a trip to St. John.