If you are wanting to build another edition of the Biosphere line, it cannot be done on a budget. However, if you want a compact greenhouse for starting plants and other garden projects, there are some inexpensive ways to produce a greenhouse. The main thing to remember if you want to build a greenhouse without taking out a second mortgage is to use items that you already have or that can be acquired cheaply.
Decide on what size greenhouse you need.
Many gardeners who just want a place to give their tomatoes a boost or start some other types of plants do not need a huge space. By using shelves, you can turn even a small greenhouse into a much large growing area. A greenhouse that measures about 3 feet by 4 feet and is 3 feet high can give enough space for many plants. This size greenhouse can be constructed in a couple of hours for under $35. If you have some of the materials already, it can drive the price down a lot.
Sketch out how you want the frame work of the greenhouse to be made.
Keep this part as simple as possible. If you make the greenhouse box shaped, you do not even have to worry about getting the peak of the roof to come out right. For box shaped units, you should make one of the long sides slightly taller than the other. This will give you a sloped roof that will shed water better when it rains. If the water pools up, it can tear through the plastic covering.
Use light weight lumber for the frame.
If you purchase 1×2 pine lumber, it will be extremely cheap. To build a greenhouse of the previously listed dimensions, you will need 9 boards that are 8 feet long. These cost about $2 each. Larger lumber can be used, but it will make your greenhouse bulky and heavy. Use drywall screws to assemble the pieces to make the project go faster.
Buy a clear plastic drop cloth for the fabric to cover the greenhouse.
Using a heavy weight plastic drop cloth like you would use for painting will give enough strength to the exterior of your green house. These cost about $5 to $10 at most discount stores. You will use tacks or staple to attach the plastic to the frame. Do not cover the floor. It will just stay open.
Cut the boards to the right sizes.
You will need 5 pieces of board that are 4 feet long, 7 pieces that are 3 feet long, 3 pieces that are 3 feet 6 inches long, and 3 pieces that are about 39 inches long. Do not cut these last 3 pieces until after the rest of the frame is assembled. You will need one last piece that is about 33 inches long. Cut it at the same time as the last 3 pieces.
Assemble the boards.
Make a rectangle out of two of the 4 foot boards and two of the 3 foot boards. Put the long boards on the outside of the shorter ones. Take 2 of the 3 foot boards and attach them to each end of one of the 4 foot sides inside the rectangle. Put them in the corners. Take the two 42 inch boards and attach them inside the opposite corners. You should now have a rectangle with 4 boards sticking upright.
Attach one 4 foot board to the top of the two three foot boards and another to the top of the 42 inch boards. Attach one 3 foot board to each of the ends stretching from the top of the upright 3 foot boards to the 42 inch boards. Make sure that these boards are level and attach at the 36 inch point on the 39 inch boards. Attach your final 4 foot board at the 36 inch level between the 42 inch boards. Your project should now look like two rectangles one over the other with a little extra sticking out of one side of the upper rectangle.
Fasten the a 3 foot board from the lower rectangle to the upper rectangle at the middle of the shorter side for a brace. Do the same with the last 42 inch board on the long side. Use a tape measure and find the distance from the top of the short side to the top of the long side. This will be the exact length of the boards listed as about 39 inches. Cut these boards. Measure the distance inside the uprights on the 3 foot side of the frame. It should be roughly 33 inches. Cut a board to about 1/4 inch shorter than this measurement.
Take the three boards that are the same length, and attach them to each end of the greenhouse frame from the short side to the long side. Fasten the 3 board in the middle of the frame matching the two end pieces that you just added. The final piece will be used later.
Begin to add the plastic covering.
Cut two plastic rectangles that will cover the 3 x 4 sides of the house. Do not cover the upper 6 inches of the long side yet. These pieces should be a little long so that they will wrap around the corners of the house. Fasten these to the greenhouse. Cut pieces that will cover the odd-shaped ends of the greenhouse. Make them long enough to wrap around the upper boards. Fasten one to one end completely, The second one will be used as a door. Attach it from the top down about 6 inches on the sides and let the rest hang loose.
Fasten the remaining board to the loose end of the plastic to act as a weight. This piece of plastic should hang to the ground with the wood attached. At the spot where it starts to be loose, screw in one long drywall screw on each side but leave most of the screw sticking out and pointing slight upward. This will act a a holder for the board on the door to hold it open.
Now cut a piece of plastic long enough to reach from the bottom of the shorter 4 foot long side over the top of the greenhouse and about 15 inches down the other side. Attach it to the frame up the short side over the roof and along the top of the long side. Now you should have a flap that can be tucked into the opening that is at the top of the long wall. This will give you a vent to keep the greenhouse from overheating. If you find it is too much of a vent, you can attach part of it to the frame later.
It is time to stake it down and start using it.
Your green house should now be complete. Attach strong cords at each of the 4 bottom corners and use tent stakes to anchor the greenhouse to the ground.