I needed a greenhouse to keep succulents alive through the winter and potentially grow more tropical plants, but I wanted to do it cheaply, but also make something that would last outdoors.

The pop-up greenhouses work, but they only lasted a year or so outside before rusting and ripping. With treated wood and glass, the greenhouse should be much more reliable.

This is a very easy project to make, and needed only a couple basic tools.

Saw - I used a circular saw, but a handsaw would work just fine.
Tape Measure - To make sure things fit.
Drill - To drive screws - I used self tapping, but you could also drill pilot holes if you prefer.
Screwdriver bit - The bit came with the screws.
#9 2 1/2" Deck screws (number needed will vary, I used ~30) - Bought at local Home Improvement store, but you could also get them online.
Old Windows (I used 5, but could scale up easily)- Neighbor was replacing windows so I asked if I could have a few of the old ones and got enough to make two greenhouses this size. People also offer them on Craigslist, Freecycle, and many others. The thing to look for is physically sound wood, ie. not rotted, and intact glass panes.
Scrap wood - I found an old pallet, which is made from pressure treated hardwood and typically has already spent a long time outside without deteriorating. I also used a scrap of 2x8, but this is optional depending on your final design.
Window Glazing (I bought 1\2 pint and had plenty left over)- My windows had some glazing that was cracked and broken, I'll explain the process in a later step. You can get at any hardware store, or buy online.
Putty knife - Needed to scrape off old paint and putty, and also to apply glazing if needed.
Outdoor Paint (I used 1 pint)- Any outdoor rated paint or even stain works perfectly.
Hinges - I made the roof hinged for easy access, but depending on final design, you could make a lift off roof or a removalble side, if that is better.

This was a very economical project. I already owned the tools, the windows and pallet were free, so I only had to purchase the screws, glazing, and paint.Total cost was $25 including tax.

Step 1: Step 1: Figure Out Size and Shape

The windows I got were several different sizes, so I spent some time with a tape measure and put the windows together in various configurations until I got what I wanted.

Luckily I had at least two windows that were the same size to make up the sides.

Since this greenhouse is sitting on my patio, it doesn't have any shelter from the rain, so I decided to make the roof sloped. Originally I was going to make a two sided sloped roof, but didn't have two other windows that were the same size, so I opted for a single slope.

While the size windows you get likely will vary from the ones I used, mine were:

Two 48" x 24" for sides
24" x 24" for the front
32" x 24" for the back
48" x 24" for the roof

I also cut the pallet base to two pieces 26" long and two to 48" long

Step 2: Step Two: Assemble Base

Assembly is very simple.

I used 3 screws to attach the pallet base to each 48" wide window, and 2 screws for the 24" wide windows.
Use 2 screws to attach a 24" wide piece of pallet base to the 48" wide pallet base, creating an "L" shape; repeat for the other side. The pallet is the wood that is in contact with the ground to protect the wood in the windows from water damage.
Note: Check you are attaching the correct sides of the windows - I put all of the original inside facing sides of the windows facing in.
Drive three screws through base of 24" window into the base of the 48" window to secure them together.
While I didn't use a square, (so the two "L" shapes of the base were initially not 90 degrees) the windows all are square, and there was enough give in the base that I could make it square when I screwed the two "L" shapes together.

Step 3: Step 3: Add Roof

I wanted at least most of the water to drain off the roof after it rained, so I made the roof angled, and hinged it to allow for ease of working in greenhouse.

There wasn't a specific angle I was going for, I just used the sizes of the windows I had to determine something that looked sufficient. In my case, the back wall was 8 inches higher.

The trickiest part of the build was making the two triangular pieces to fill in the gap created by the roof angle - so if you can transfer the measurements of a triangle to a piece of wood and cut a straight line, then you can do this project. :-)

For mine, the back was 8" higher, and the length of the gap was 48", so cutting a 48" long piece of 2x8 in half from one corner to the other would create a very close fit. (Not exact since the kerf of the saw blade is not calculated into that, I'll describe the final fit in a moment.)

Drive three screws through top of triangle into the 48" long sides to attach. I did need to drill a hole the size of the screw head part-way through to allow the third screw to actually reach the wood of the greenhouse side.
Attached two door hinges to back wall of greenhouse and then to roof to allow ~120 degrees of movement. It is enough to open the roof straight up without it falling back, though I still use a stick or a wedge when I'm working in the greenhouse to make sure the wind, etc. doesn't close it on me.

Step 4: Step 4: Finishing the Greenhouse

At this point, the greenhouse is basically complete. I needed to remove some of the old glazing and put in new, and scrape some old paint before repainting. You may not need to do this depending on your starting windows, but in case you need a basic primer, here is what I did.

Use putty knife to scrape and chip off any glazing (the grout holding the glass in the panes) that is loose or crumbly. Also scrape off any old paint that is peeling off.
Brush or dust off all the debris.
Roll a ball of the glazing between your hands to soften it and create a long "snake". Don't worry about size, using more than needed is fine.
Press the snake along the edge of the glass where the old glazing was with your fingers.
Use the putty knife to press the glazing into the wood and make good contact with the glass, and also remove any excess, which you can roll back into a snake for the next pane.
Allow the glazing to cure, which is typically a couple hours, but I waited over night.
Paint the greenhouse with an exterior grade paint.

Alternately, you could strip off any remaining paint and apply an exterior stain instead, but for my purposes, the old windows have enough charm on their own.

The greenhouse has been outside for about a year now, and successfully wintered over a bunch of succulents such as aloe vera, and they actually even grew slightly over the winter.

The only other things I did, which are optional is adding some leftover window weather stripping to make a better seal between the roof and the triangular pieces. Some water still is able to get in, but once the sun comes out it keeps the humidity level inside the greenhouse high enough for tropical plants. I also added a thermometer so I can monitor the temperature and crack the roof to allow excess heat/humidity out if needed.

Overall, it works amazingly well.

Enjoy!