This is a 12’x24’ design with a 6’8” ceiling height. You will want to keep the doors open all summer, and closed most of the winter. In total it is just shy of 300 sqft. I have a starting table, some tools and a few chairs to take it easy with friends and enjoy the view, so it’s closer to 200 sqft of growing space. Still, plenty for me.
Tips For Picking Your Site:
- Firstly, you will always want to run this East to West so that the southern sun can come through the long side.
- If you are in the Northern states you will want to avoid shade until about 4 or 5pm during summer.
- If you are below 40 degrees latitude, the sun is pretty intense and any poorly designed greenhouse can cook your plants.
- Since this design does not include sides that roll up, Southern residents will want to incorporate a time or two where there will be shade from a tree or other structure. It’s hot in North Carolina in general, and in summer I have a Hickory shade it from 9am to Noon, and a birch shade it from 4pm until 6pm. This cools the inside significantly, and using trees is a great idea since the leaves fall and more heat gets in as winter comes on.
- Using an immovable structure for shade can create unwanted cold in the winter. Keep these on the North side to block wind and trap heat behind it.
- Pick a level spot, or one that can be leveled.
- Pick a spot that is up-slope of any drainage zones like hills or ditches, you do not want water coming under and getting your roots soggy.
- Notice on the left edge of the photo that the low point of the afternoon sun in winter will stilljust touch the edge of the greenhouse. Five feet left would have been a poorer choice. Honestly, it took me two years staring at light angles to figure out where this should go, so don’t rush it.
Tips For Prepping the Ground:
- You will need to spend some time prepping the ground if you use the native soil as a base layer. This will include tilling in anything currently growing.
- You can also roll out fabric and put soil on top, but I feel that limits growing deep rooted plants.
- You will then want to check the PH of the native soil and amend as needed. A soil sample will not be necessary because this layer will be beneath 4 to 6 inches of good topsoil to be brought in.
- Try not to mix the native soil with the stuff you bring in – this will keep the weeds down and help control any soil diseases.
- I built mine in early Spring and used leftover wood ashes to amend the soil and adjust the PH. I felt this was a great natural and sustainable option over using lime. I also made a bonfire in the middle with branches from trimming a nearby birch, an equally sustainable choice for soil amending. Went from 5.2 to 6.5.
Note: All wood used is pressure treated and should be left outside in the rain away from the greenhouse site for a few weeks. If you can afford cedar or locust then do so, but it will definitely be over $500. If you are trying to certify organic, or practice 100% organic, you may not use pressure treated wood, but this design will still work great for whatever you choose!
The Base Frame
- You will need 6 pieces 2x6x12.
- Scab two together with a 2×4 to make a single length of 24’ – repeat for the second side.
- Form a rectangle with the boards over your site.
- Check to make sure the frame is squared up – screw in the East side, stake it at the corners to hold it while you adjust the West side, then stake that side (place temporary wood stakes on the outside of frame )
- Using 3” screws connect the boards as shown – The 12” length needs to be inside the long sides so the hoops will be exactly 12’ apart, ensuring adequate headspace in the vault.
- Admire how perfect all your angles are, fix any that aren’t and take a break.
The Rebar Hoop Anchors
- You will need 26 pieces of 2 ft. reinforcing steel, or rebar. Use ½” (10mm) steel and buy 10ft. pieces if you can. If you have to buy 2ft. pieces that are pre-cut you will go over budget.
- Clamp the bars and cut them into 2ft. pieces with a reciprocating saw (use a metal cutting blade, $3)
- Pound rebar 1/8” away from the side of the bottom frame, at 2ft. intervals, starting and stopping at the corners. The gap is room for the hoop to slide on.
The Door Frames
We are going to plant the door into concrete post holes and then tie it to the bottom frame. This is a key part of the “windproof” factor. These door frames will also be buttressed against the ridge posts, and they don’t budge one bit in a stiff Carolina wind.
- You will need (5) 2x4x8’s and (2) 50lb bags of concrete.
- Build the door 3’ wide and 8’ tall using one top header and leaving the bottom end open.
- Find the center of your East and West bottom frames.
- Mark 1’6” in each direction, this will be the tie in point for the door frame studs.
- Dig a 1’6” hole flush with the endwall right where you marked for the studs.
- Plant the door into the post holes.
- Make sure it is level using the header board.
- Make sure the top and bottom studs are equidistant so the door goes on smoothly.
- Screw the studs to the bottom frame 2x6x12 board.
- Pour concrete into the post holes and let cure 1-2 days.
- Prop with 2×4 to keep plumb while drying.