Also called a glasshouse, a greenhouse is an enclosed structure with a roof and walls made of glass or plastic. Both materials, glass and plastic, absorb the sun's heat and keep plants warm. You do not need to spend money to build a professional-looking greenhouse in your backyard. With some spare old windows and a few appropriate tools, you can be on your way to making a greenhouse for personal use in your desired size to extend the growing season for your favorite plants.
Select a spot that gets plenty of sunlight; decide the size of your greenhouse depending on the number of windows you have. Measure and demarcate the foundation for your greenhouse with powdered chalk.
Dig a 2-foot-wide and 1-foot-deep trench along the parameter of the demarcated lines. Check your local construction code for appropriate width and depth. Lower concrete footings into the trench, or make forms with ¾-inch thick plywood sheets. You can also rent prefab forms from your local cement supplier or contractor. Also, reinforce the footings with rebar or steel rods tied both vertically and horizontally inside the forms.
Leave ¼- to ½-inch D-anchor bolts protruding from the top of the footing while pouring concrete, so the walls can be attached securely to the foundation later. Also, leave 3-inch drainage holes--spaced 4-feet apart in the foundation. Allow the footings to cure before removing their forms.
Dig a 6-inch deep trench for the floor of the greenhouse. Level the base with the back of a spade before adding 4 inches of crushed gravel over it. Make a vapor barrier by placing construction plastic or tar paper over the layer of gravel before adding a 2- to 4-inch layer of fine sand over it. Tamp it well so it is level.
Place bricks for the floor; fitting them flush against each other. Pour fine sand over the bricks and sweep it into gaps and crevices between floor bricks. Your flooring is complete.
Attach vertical 4x4 wooden posts to the foundation (with the D-anchor bolts set in concrete). Cut the tops with a saw to allow for a slope that provides drainage for the roof.
Assemble walls using 2x4 or 2x6 lumber. Hold them against the vertical posts and nail them in place. Also, assemble the wooden roof frame and nail it in place. Cover the roof frame with roofing felt cut to size.
Attach glass windows to the sides of the greenhouse by inserting nails through their frames.