This is a brief guide on how I took some old windows from houses they where tearing down in my neighborhood and turned them into a small greenhouse in my back yard. I collected the windows over the course of a year and a half and the build took about 3 months, spending one day a week on it. I spent about $300 for the lumber for the frame and screws, caulk, latches, etc. That's almost 10% of what a greenhouse kit would cost. The size I built was 7 ft high x 10 ft deep x 6 ft wide. But the size of your greenhouse will depend on your windows and the time you want to put into project.
Step 1: Collect Windows and Plan Two Pair of Equal Sides.
Look for old windows and save every one you get. After you have many, lay them out and play a game trying to make two pairs of "walls" both the same height. Two to three inches won't matter as you can cover the difference with wood. Smaller holes will need to have glass cut for them or filled with something else. Keep in mind that one end will need a door and the other a hole for a fan.
Step 2: Create a Frame
Using the windows you chose as a guide, construct a frame for each wall. Use good lumber for this, as it is the structure that holds all the weight. I used all 2 x 4s for the studs and 4 x 4s for the corner posts. Choose a length that allows at least 14" of the stud to be placed in the ground for support.
Step 3: Brace Walls
Start placing the walls up, bracing well so they don't fall over. Be sure to check that they are level.
Step 4: Make the Foundation Secure
To avoid certain problems with pesky city building permits, I built the structure shed height and did not pour a concrete foundation. Instead I buried cinder blocks to stabilize the 4 x 4 corner posts. They keep it from moving an inch.
Step 5: Screw on Windows
I used some nice coated deck screws to affix the windows to the frame. This will allow for easy removal and replacement if any break. This side facing the camera has the empty window for a fan.
Step 6: Get a Floor
I was able to find someone who needed rocks removed from their yard. Using rocks or stones is good for two reasons: good drainage and heat storage.
Step 7: Build the Roof
This was tricky. I ended up getting siding from an old shed someone had torn down. Any material you use, look for lightweight and waterproof material. Be sure that you have some that will open for ventilation, at least 20-30% of your floor space. You can get by with less if you use a fan for ventilation. Also build the slant roof with at least a 4 degree pitch, otherwise rain may not sheet off well.
Step 8: Add the Shelves and Fans.
I found an old picnic bench table and this fan and shelf in the garbage. I figured I can use them in my greenhouse and save them from a landfill.
Step 9: Caulk and Paint
Use a good outdoor caulk and seal all the cracks and holes between windows. Paint the wood to protect it from the weather.
Step 10: Update: Spring 2009
The winter this year was especially bad near me. We had several feet of snow weeks on end. Luckily, I had already emptied the greenhouse and removed the roof panels in late November. I live in a zone 5 area. During the last month I brought out an electric heater to keep the temperature more consistent overnight.
This year I was able to obtain a large picture window and decided to install a windowed roof this spring. It will allow much more light in and therefore heat. I used the same deck screws to affix the windows to the roof frame I already had built. For the roof vents, I took two windows and screwed them together. I found old door hinges and used a piece of PVC as a brace. I added a screw holding it to the frame as a cotter pin. Lastly, In case a huge gust of wind came along and tried to yank open the windows, I nailed a small chain to the frame and window to prevent the window slamming backwards onto the rest of the roof.
I also modified the south facing bench. It connects to the frame on one end and still uses cinder blocks on the other. This will hopefully allow me to utilize the space inside better. It's filling quickly!
Now that the roof will allow so much light through, cooling will be a greater issue this summer. I may place some of the old panels back up in July/August to reflect some of that light. I also have obtained some reflecting fabric.
Lastly, I think in the future, I will completely rebuild the roof, using the windows for a gable type structure. It will force me to use some sort of poly material to cover up the gable ends. The current pitch of the roof is not enough to slope water off the windows completely.
Step 11: Edit: Fan Window
I was unhappy with having to remove the fan/vent window and having to prop it against something while cooling the greenhouse during the day. The frame was already designed to fit the window into it. I decided to have it slide up and be held in place. I started by salvaging some hinges from an old entertainment center. They are the kind that sit completely outside the door. Plus these had a unique shape that fit around a right angle. This allowed the wooden "stops" to swing in place and hold the window up while I was venting or when the fan is in place. Across the frame I nailed some boards to hold the fan window against the frame. Lastly, I found an old pulley and fastened it to the window so I can pull it up easily.
Step 12: Spring 2010 Roof Vent Upgrades
Had a major score! A local community greenhouse was torn down and replaced. I was able to get some great parts. Here is a picture of the new window system. It originally opened the windows on the side of the greenhouse. The wheel is turned and rotates the gear attached to the pipe opening the windows. Makes opening and shutting easy. While every window now must be open at the same time, I can control the angle at which they are open.
Also pictured is a gutter claimed from the trash. The hinge side of the roof windows always leaked profusely. The gutter catches the water and stores it in a bucket for easy watering.
Step 13: Spring 2010 Shading
Bought secondhand some rolling shades which are working great. They easily roll up and down the south facing wall while not taking up too much room.
Step 14: Winter 2012
Here is the greenhouse in a mild winter. I overwinter many potted perennials inside. To insulate the roof, I stretch a sheet of poly across the top to keep out the drafts. Last October, 2011, I repainted both the inside and out. All the wood is doing well. I hope that, with care, the greenhouse will last over 10 years. It has changed the way I garden, making my back yard much more productive.