Introduction: Making a Greenhouse / Hothouse Using Recycled Windows

By vivier.net
About: Artist, creator, musician, garden enthusiast

I've always wanted a four-season greenhouse that could keep plants alive during Mid-Atlantic winters. When I stumbled upon a set of windows that were going to be thrown out, I knew this was my chance.

That discovery set the wheels in motion. My challenge was that I had very little experience in building any kind of structure. This is how I broke down each step to get comfortable with how to approach it.

While the steps are all here, I think it's important to emphasize that the process is probably more important than the details. Given the likelyhood that your windows' dimensions won't match mine, your end product may not either. So this is more of a document on the process of "how to go about designing and building a greenhouse."

By the end of this, you should be able to understand:

  • What kinds of materials and tools are needed.
  • What the process is like.
  • How much money you may need to spend, depending on available materials and tools.
  • How much help you may want.

I hope you find it useful and inspiring. Please let me know your thoughts. Follow me here or on Instagram for more.

Make sure to see the "What I learned" section at the end. I'd probably do this differently if I had a second go-round.

Supplies

Materials

  • Recycled Windows
  • Glazing compound if repairs are needed
  • Lumber: (Your needs may vary)
  • 2"x6"x6' (QTY 4) for foundation.(Require treated)
  • 2"x4"x8' (QTY: ~25) for frame (Suggest treated
  • Stain/Paint (Untreated must be painted)
  • Plywood (or recycled clapboard for siding)
  • Nails: 8D 2 1/2" Galvanized - 5 lbs
  • Screws: 2 1/2" exterior 3 lbs
  • Spare 8' 2x4's for temporary bracing the frame.
  • Cinder block (6"x8"x16") QTY: ~32 (Your needs may vary)
  • 2-3 bags of 80 lb Masonry Mix - Type S
  • Trowel
  • Mixing tray (or spare plywood)
  • Masonry String with line level
  • Optional bag concrete mix
  • 8 - 3/8" x 6" galvanized carriage bolts with nuts and washers for attaching frames to foundation.
  • Clear polycarbonate roofing panel (3-4 12' panels, your needs may vary)
  • Roofing screws (1 lb)
  • Spray Foam
  • Sand or gravel for foundation under cinder blocks
  • Stones or gravel for interior floor (around 5' x 5' x 6"). Could do it with 4". (Your needs may vary)
  • Paint/Outdoor stain (optional, unless you use untreated wood and then this is required)
  • Hinges for top windows (2 per)
  • Hinges for door
  • Door latch
  • Anti-Sag kit (like for fence gates) if you make your own door.
  • Design phase: Cardboard, scissors, post-it notes, grid paper, scale ruler

Tools

Step 1: Get Windows and Measure Them

I was lucky enough to find 17 windows in an alley while walking my dog. A neighbor was having their old windows replaced and these were left near the trash. You can purchase old windows at a local "architectural salvage" shop or just keep an eye out for the local 'Window Replacement Business' trucks and go ask them what they're doing with their old windows.

Once you have the windows, you'll want to strip off all of the extra metal and ropes. Keep a bucket around to toss this stuff and wear gloves so you don't cut yourself on old pieces of metal.

You'll then want to take inventory and measure all of your windows, determining the width and height of each. Label them as you measure them. I grouped similar window sizes and labeled them "A,B,C" etc, so I could keep track of them, writing the labels on the glass with a sharpie.

Step 2: Design Phase

Once you have your windows, you'll need to determine placement and size of structure that you like. Be wary of any building code restrictions in your area that influence the size of a structure. My greenhouse had a 5'x5' footprint which put it in the category of "shed", not requiring any building permit.

Using a scaled ruler, I measured corresponding dimensions on post-it notes, and cut accordingly. I then organized them in patterns that I liked, remembering that I wanted opposing walls to be the same dimensions to keep the structure at least rectangular. I also wanted windows on all sides because I thought it looked cool to see all the way through it. I then cut out pieces of cardboard to correspond to the walls necessary, and started to think about the roof. You'll notice that I changed from a hip roof to a lean-to. I was hoping that a lean-to would have less angles to get correct. I feared the angles.

Don't forget the door! I had a couple of super thin windows that didn't fit anywhere else, but when turned on their sides, made a pretty cool door. This did make for a heavy door though.

Just for kicks, I cut out a little person that was also to scale so that I could get a rough idea of whether this was going to be enough space. Turns out 5'x5' doesn't feel bad if you have 7 - 8' ceilings.

At this point, I was able to make an estimate of how many 2x4's I would need. I made a trip to the store and purchased about 25 8' 2x4's. Your design may vary.

Step 3: Frame the Walls

I decided to start with building the walls instead of building the foundation first. Since I didn't have a lot of experience with framing, my confidence on being able to accurately predict the size of the walls was low. The foundation would have to be within an inch, so I decided to build the walls first.

I laid the windows down on a flat space, and started to cut the 2x4's to fit. I opted on a 7' height wall. Before I nailed (or screwed) anything together, I checked the fit one more time. There is a lot of "measure twice, cut once" going on. Maybe even measure thrice.

I quickly realized that I needed corner bracing. I added braces to the inside below the windows and on the outside at the top. The top braces would be removed once the walls were attached. Braces are cut at 45º degrees and ideally butt up against the horizontal window frame bottom.

Note: If you haven't already, this is a good time to verify the dimensions of the hinges that you want for your windows and door. Order them ahead of time so they're ready when you need them.

Step 4: Build the Foundation

Once I had the exact wall measurements, I knew how big the foundation should be. I advise making a diagram of how the corners would connect before determining the final dimensions for the footprint. Using graph paper, I drew out my options for cinderblock placement. This helped me figure out what the pattern was going to be, and how it may differ depending on what size blocks I picked.

I settled on the 6" blocks. While the cinderblocks are not actually 16" long, you can still use this as a measurement during planning since you allow space for mortar.

I decided to go with two courses (rows) of blocks. I dug my trench, and then dug it again just because I can't measure things right the first time. My ground was pretty firm so opted to use sand underneath the blocks to level them. It's probably more advisable to use gravel, to decrease settling over time.

I put fence posts in the ground at each corner and pulled a level string across each at the desired height of the blocks. I then placed the blocks, without mortar, to verify the spacing was going to work as expected. It also needs to be square. There are a few techniques such as for checking the squareness of blocks: using a large metal square, measuring the diagonals, and frankly, just looking at the blocks to see if they line up. On this small of a space, the blocks should help you keep things square though. Truth be told, I did a bad job here but didn't notice it until I needed to put the roof on. Maybe come up with a better way to square the bottom and let me know.

I did have to split a couple of my cinderblocks to get them to fit. I used a block chisel for this. Just to note, it is not easy to cleanly break blocks in half. You may need some extra blocks in case this goes wrong.

Once I had the blocks positioned correctly, I mixed up the mortar and applied it with a trowel. I had about 30 minutes of work time with the mortar, so I opted to only mix half of a bag at a time since I figured that I would be slow. When complete, I checked to make sure everything was still level and adjusted if anything that was off. You should also take one of your straighter 2x4s, turn it on it's side and check the level of the diagonals.

Let the first row dry for a few days. Then apply the second row of blocks in an alternating pattern so that gaps between blocks on the second row aren't on top of gaps on the first row. This is for stability, not just looks. After laying down the second row, check the level and adjust the amount of mortar if necessary. Let dry for a couple of days.

After all blocks were dry, I started placing the carriage bolts to attach the footer to the foundation. I filled the "cinder block holes" for where I wanted carriage bolts with gravel, checking the position on the corresponding wall, since I would need to drill a hole for the bolt to fit. Placing the bolt in the gravel head-side-down, adjust the depth so the tip of the bolt sticks up enough to go past the footer to allow for the nut and washer. Once you have all bolts and gravel positioned, mix a half-bag of the remaining mortar cement and fix the bolts in place in the center of the blocks. You may instead want to use a harder quickret for this, like a concrete mix and not the mortar mix. I got lazy and used the mortar mix, so I'll let you know how long that lasts. Make sure the bolts are straight up at a 90 degree angle, have enough space to go through the 2x6 and have room for a bolt. Fill the hole completely with concrete, pushing it down to get rid of any air bubbles. Scrape off any excess from the top. Let dry for a couple of days.

Using the 2x6" treated lumber, cut your footer to fit cleanly on the top of the cinderblocks, covering the holes completely. This is your footer and you will nail the walls to this board. Once cut, place the pieces on top of the hardened bolts and tap with a hammer, leaving an indentation of the bolts on the footer lumber. This will give you the locations of the holes that you'll drill. Drill 3/8" holes and slide the footer over the carriage anchor bolts. Repeat, until the entire footer is in place, apply washers and nuts to hold footers in place. Make the nuts snug, but don't over-tighten. You don't want to pull the bolts through the concrete.

Celebrate. This took a while.

Step 5: Raise Walls

Before raising the walls and putting them on the footer, you'll need to drill 1 1/2" holes in the bottom so they fit over the nut and bolt sticking out of the footer.

With the help of a friend, you can now put the frames in place over the bolts. Attach a temporary wall brace to the frame, helping to hold it in position. You can do this by partially nailing a single nail into the end of the brace and attaching it somewhere at the top of a wall, then pivoting the brace, putting it in the ground, until the wall is in position (and level).

Repeat until you have all four walls up. I also added temporary braces across the top before putting the roof on. These helped me to adjust and position things before committing to securing with nails or screws. Once in position, start screwing walls into the footer and to each other at the corners.

After firming up the walls, I recommend checking to make sure all windows are still going to fit where you expect them to go. You may need to make adjustments if they don't.

Step 6: Add Roof

The roof was tough, not gonna lie. The angle part was a bit tough to figure out, but I'm glad that I kept it simple.

I started with building a simple frame/box for the front of the roof with three evenly distributed supports. Once attached, I measured the height and span to the back wall to get the slope (slope = height / width). And from there, calculate the angle in degrees.

Place the uncut rafter on top to determine the location of the first notch to be cut. Check the length of your rafter as well, and make them shorter if necessary. I left mine at 8'. You want a little overhang at the front and the back. If you want to be double sure, you can always whip out the pythagorean theorem and determine how long you want the rafters to be with additional overhang.

Set your miter saw to the angle of the degree of your roof slope, and cut a right angle notch out of the rafter lumber. Use galvanized hurricane ties to secure them to the frames.

I highly encourage you to take a look at this video (and other "lean to shed" instructions) to determine the angles and structure of a roof and how to cut the right angled notch out of the rafter.

Once the rafters are in place, you need to add bracing across the tops of the rafters. This will help to stabilize the roof structure and provide a surface to attach the roofing material.

Now measure, cut and nail/screw the roofing on with roofing nails/screws. There are screws with rubber washers made specifically for this roofing material. It helps to grab the plastic to hold it and make the new holes water tight.

Step 7: Add a Door

This is a later view of the door, but you'll get the idea.

The door was a frame within the frame that I made during the framing process. To attach it, I used some "barn hinges" that ended up being kind of junky. I think I want to redo this to use a mortise-style hinge since they're so much sturdier.

This kind of door is very heavy and really needed a diagonal support, so I needed to install an anti-sag cable like you'd use for a gate.

Step 8: Add Windows

Now it's time to add the windows! For temporary placement, I held the windows in place by angling a screw into the frame at the base of the window, without driving it through the window. I put two at the bottom and top on the inside and outside which wedged the windows in place.

With the windows in position, you can now add the hinges to the top of each that you want to be able to swing open. To save money, I didn't order hinges for every window, just a few on the top. Take each window out, one at a time, and attach the bottom half of the hinge to the top of the window. Pre-drill holes so that you don't split the wood on the windows.

If you have a second person, now is a good time to get them to hold the windows in place while you pre-drill and screw the tops of the hinges into the frame. If you do not have a second person, I found a pry-bar to be adequate at raising and holding a window in place. You still need a leg, elbow or extra hand to hold the pry bar while you're pre-drilling, but it can be done. You'll want to place at least one strip of wood on the inside of the window frame so the hinged window can rest against that when its closed.

For windows that don't have hinges, you can come back and cut thin strips of wood to go around the inside of the frames to hold the windows in place. The size of the gap that you're filling is directly proportionate to your accuracy of measuring to this point. Good luck.

I used little strip of wood with a single screw to provide a "latch" to keep hinged windows closed.

Step 9: Interior Rocks

For the floor, I added a bunch of nice river rocks that I got from the local landscaping supply. The goal was to 1) have a nice surface to stand on 2) Well drained because there would be a lot of water inside and 3) help hold heat during the winter.

For a very reasonable price of $40 I was able to get a "half bucket" into the back of my truck. Getting rocks from a landscaping supply place is the absolutely cheapest, but may not be an option if you don't have a truck. They scoop them up with a front end loader and simply drop them into your truck. If you have a nice truck, you'll probably want to put some plywood in the bottom to keep it from denting up the bed when they drop the rocks. Or ... just break it in and take the dents. Or get an old truck. Delivery can be expensive and add another $200 to the cost.

Step 10: Weatherproof

Depending on when you build this, you'll want to prioritize one type of weatherproofing over another.

If you're close to winter, you want to seal the big holes starting with the big ones around the roof. I filled the side with leftover corrugated roofing, plexiglass and strips of plywood. Obviously the clearer the material, the more light you'll let in. I also sprayed foam in cracks and cut up pieces of foam stripping to fill some holes. Budget conscious people can get more creative here.

For the summer, you'll want window vents. A couple of these automatic vents are perfect to place on the hinged windows. These automatically expand when they get hot, no electricity or automation required.

Step 11: Monitor and Enjoy!

In the summer, I also added some basic shelving, which allowed me to hold 15 flats of seedlings! There was even enough space to have a chair where I could sit and soak in the bright sun on cold winter days.

I highly encourage you to pick up some of these cheap remote thermometers and monitor your greenhouse before putting any important plants in there. Summers can get hot in a greenhouse and winters can still drop below freezing. In the winter I realized the floor was 10 degrees cooler than the middle. I eventually placed a small ceramic heater in my greenhouse and was able to keep it above freezing after closing in more gaps.

Next up for me is to write an instructable on how to effectively monitor and automate your greenhouse.

Let me know what you think and if you found this helpful or inspiring.

Step 12: P.S. What I Would Do Differently

The main thing I would do differently is that I would build the foundation first, and add the frame on it, more like a traditional build. Since I didn't have all of the material in front of me to measure, I realize that I had doubts about actual sizes of things, which made me hesitant to rely on guessing ahead. The problem with this approach was that I had a less sturdy frame. I eventually secured it enough, but I feel like my walls (especially corners) would have been sturdier if I had set it up first. However, there is still a risk here and you may find that your windows simply don't fit.