How to Build a Lean-to Greenhouse (Step by Step)

Lean-to greenhouses offer a budget-friendly alternative to stand-alone greenhouses. Built next to an existing structure, you get space to cultivate plants while saving on building costs.

For year-round growing also see How To Winter Sow Seeds.

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Build A Lean-To Greenhouse

One of the top benefits to building a lean-to greenhouse is the savings on materials because you are building against an existing wall.

One downside can be accessibility if your only option is to add a doorway at one end. The space needed for a walkway ends up taking half the interior space.

A solution to this is to add a door in the middle—if possible—or, do what I did.

I built my lean-to greenhouse next to a covered patio. This allowed me to make it accessible from the patio—there are no other entry points.

From the patio, I installed 3 windows on hinges at waist height. This means the entire 3-foot by 12-foot interior of the greenhouse is easy to reach and dedicated to plants.

When using trays for seed starting, I add some temporary wire shelves.

There is also a lot of extra space boxed into the base of the greenhouse. I decided to close it off for now but perhaps turn it into storage space in the future for things like plants pots, trays, and bags of potting mix.

I’ll share what I learned when choosing plans, which I ended up radically changing, how I kept my costs down, along with a video showing how the project progressed, and some step-by-step stuff to give you a good idea of what you’ll need to think about before building yours.

Contents

• Getting Started
• Adapting Plans to Fit Your Situation
• Watch How I Built My Greenhouse
• How to Build a Lean-to Greenhouse

Getting Started

When looking for lean-to greenhouse plans, it is important to be sure they not only fit the space you have available but also use materials you can obtain locally. Specifically, be sure the roofing material is available and meets your budget.

I used Suntuf polycarbonate panels as you’ll see in the step-by-step details below. They are available in different sun-blocking strengths (listed in percentages). The least expensive ones block out the most sun.

A lot of greenhouses use aluminum frames but I specifically framed mine with wood because it’s readily available, we do not have any issues with termites, I can cut it to size, and it makes it easy to attach the roof screws.

From there, it’s all about the interior space of the greenhouse and how you will access it. Most have a door at one end: mine has interior windows which open to our covered patio. It’s a shame to waste space for accessing the plants if you don’t need to—especially in a smaller space.

The whole advantage to this style of greenhouse is the cost savings gained by building next to an existing wall of your house, garage, or shed. These savings may allow room in the budget for some very nice-to-have extras like automatic window openers or some other wall or roof vents to allow for good airflow.

There is a big misconception that greenhouses stay hot 24/7. Both are incorrect. Greenhouses heat up in the sun and cool right down at night. This explains more about how greenhouses work. If you are in cold climate like I am, you have to add a supplemental heating system to retain warmth at night. Without this or some other protection, temperatures will be very close to outdoor temperatures on those cold fall and winter nights—something tender plants cannot withstand.

For design options, there are a lot of free plans online but I could not find anything in the style or size I wanted so instead I adapted existing plans.

Alternately, you could also buy a lean-to greenhouse kit.

Adapting Plans to Fit Your Situation

When I decided to build this greenhouse, I started looking online for ways to create a lean-to greenhouse next to a building. My situation is a bit unusual because, instead of accessing the greenhouse from one of the ends, it opens to the inside of the covered patio. And, instead of building a walk-in unit, I opted to create it at table-height for easy access.

The design is inspired by this barn greenhouse by Ana White which is a freestanding 8×12-foot greenhouse. Mine is 3×12-feet. So, I had to rework the plan to fit my situation. I swear it took longer for me to think through the plan than it did to build the thing!

To make it work, consider the height of the plants you want to grow. My greenhouse allows for 4-foot tall plants. If I wanted more vertical space, I could have built the base lower or just added a floor instead.

I started out intending to use a bunch of old wood-framed house windows leftover from building my mini-greenhouse. After some thought, I realized that it would be very hard to adequately frame the antique windows—which are all different sizes and in various conditions—and get the fairly airtight (and rainproof) space I was hoping for. Instead, I used clear 12-foot Suntuf polycarbonate panels for the exterior, and three of the old wood windows for the interior window-doors as you’ll see below. I’m very happy with all these choices.

The main goal for this greenhouse is to grow winter veggies like salad greens (mesclun mix), spinach, broccoli, Brussels sprouts, and kale in the cold months, and propagate cuttings and start seeds in spring and fall.

In summer, I’ll add a shade cloth to help keep the temperature down to a suitable growing range.

While I realize it’s very unlikely you will want to build a structure exactly as I have done here, I’ll walk you through the process in case you need ideas for your own unique building situation.

And yes, I built it all by myself without any help. I have no particular skills, but I just love making stuff and learning as I go.

There were quite a few challenges including the patio itself, which is not level or square, and some of the wood I purchased which was second-rate, and quite twisted. But, like any ordinary superhero, I managed to make it work.

How to Build a Lean-to Greenhouse

Here’s how the patio wall looked before I got started. I had covered it in screen and wood scraps a few years ago knowing I would be adding on a greenhouse at some point. This area, unfortunately, directly faces a neighbor’s patio, so it’s not great for privacy without a wall like this.

It would have been easier to make the greenhouse 4-feet deep instead of 3 (using Ana White’s plan), but I wanted to make it easy-to-reach in from the patio and 3-feet is a comfortable amount. Plus, a four-foot depth would have taken up too much space on the narrow pathway outside the structure.

Tools and Materials

• Wood: 2x4s, barn board siding (10-inches wide, 1-inch thick), 3/4-inch plywood for table top and gussets, misc. 1x2s for battens and trimming windows.
• Deck screws
• Roof screws with rubber washers
  I bought mine at Home Hardware.
• Suntuf polycarbonate panels (3) 12-foot panels – clear – allows 92% of light in, block 99% of harmful UV rays
  Notes: I bought mine at Rona.
  Watch out for different colors: light grey looks good but blocks too much sun.
• Miter saw
• Jig saw
• Kreg jig (to hide screw holes in some 2x4s)
• Kreg circular saw guide (I love this!)
• Electric drill
• Level, square, and angle measuring tool
• Old wood windows (3)
• Heated paint remover for removing old paint
• Primer, paint, stain, paintable caulk
• Measuring tape, pencil

Total Costs

I built this project in 2018 and the materials cost approximately $800 Canadian dollars ($635 US at the time I built it).

Steps

1Frame and Install Inner Windows

This was the inside of my covered patio as I was starting the build.

After removing the exterior wood on the patio wall, I stripped the paint off the old windows, caulked, primed, and painted them, and then framed them in place on the wall.

The windows were all different sizes and shapes, so I opted to line up the bottom edges and adapt with the rest.

I framed in the windows so they would close fairly snug, hoping this will give me some control over the temperature in the greenhouse in the winter. The covered patio is open to the backyard (there’s just screen on the other open side), so there won’t be much insulation to keep it warm on winter nights.

2Build Base

After the windows were set, I built the base of the greenhouse. I opted to box it in for now and perhaps turn it into storage space later. I can always build doors from the inside and add roll-out bins for things like potting mix and flowerpots. I could also add insulation. But right now it’s just an empty box with table top.

The heroic part of this build was figuring out how to make the base level and square. The patio wall actually drops several inches from one end to the other, so I just did my best to stay level-ish while blending in with the existing walls.

I admit I got rather excited when my plywood sheets fit perfectly in the box and the whole thing measured according to plan.

One blooper that came up later was, the barn board used for the siding came in two different thicknesses. Once I realized this, I had to move some pieces around so the thicker stuff was on the base and the stuff I planned for was up top.

That’s why, note to self, it’s always good to check the actual measures of each piece of wood before you build.

3Frame Roof

This is the part that is adapted from Ana White’s Greenhouse plan. In her plan, there are two 24-inch clear panels for the roof, and two for the sides. I had to change the angle of the trusses / supports so they would have just 3-feet of panels total (two on the slope, and one vertical).

I’m proud to say I went against my desire to just keep building and stopped and stained the wood as I went along. If I hadn’t, it would have been difficult (and in some spot, impossible) to do so once the panels were installed.

A few people have written to ask how I calculated the angles needed for the trusses.

I started to do the math and then realized that it was far easier to simply do a mock up. I played around with some 2x4s until they looked good (and would accommodate the sun panels properly, marked the wood, and cut it. The first one acted as my pattern for the rest.

It’s not so easy to see in the photo but each truss is two 2x4s plus a connector piece, also made from 2×4 wood. You may want yours to be more elaborate but I really do find they are not noticeable once the panels are installed.

4Install Roof

The sun panels are lightweight and fairly easy to install. I thought I might need help with this part, but I managed everything on my own.

I opted to use foam ridgeways (made to match the shape of the panels) along the support beams. There are also plastic ones available, but they cost a lot more.

The instructions for the panels advise using roofing screws with rubber washers on alternate ridges, but I found I needed them on every ridge to form a seal.

On the vertical parts, the roofing screws are drilled into the valleys.

In this next photo, you’ll see a clear panel on the end of the greenhouse with wood up top. After finishing the project, this side section leaked so I ended up filling it in with wood instead. You can see a picture at the end of this article. Thankfully, that’s the only leak I’ve ever had and it was easily remedied.

5Set Up Greenhouse

I’m pretty sure I will add some wire shelves inside eventually to increase the plant space. As I mentioned, my main aim is to grow winter veggies. The big bins in the photo (below) are for seed sowing. I drill drainage holes in the bottom and use the lids underneath as drip catchers.

I also added some of my favorite garden riff-raff including the gnome and toad.

And the first project was to take softwood cuttings to propagate for new plants. The free instructions are here.

I added a thermometer with humidity readings so I can start to learn how things are in the greenhouse compared to the conditions outside. You could also go extra fancy and get a wifi thermometer to monitor things from afar.

It’s not shown here but at one end of the greenhouse I added a window I can open and close for air circulation. Eventually I may control it with automatic air vents (hardware that allows windows to automatically open at a certain temperature), and consider getting a misting system plus a fan for more air circulation.

Here’s a look again at the outside. There’s one knot-hole in the lower wood that I must block off before the chipmunk finds his way in!

Here’s the view from the other side where I filled in the window with wood to stop a leak:

I was really hesitant to build this because I thought I might not make much use of it. Wrong! As I’m updating this several years later it’s still one of my best garden projects and I use it all year-long. And it’s really fun to come out to patio in winter and not have to walk through snow to visit my winter plants.

You can read more about my greenhouse here and how I use it today.

I hope you’ll try building your own lean-to greenhouse.

~Melissa the Empress of Dirt ♛