Greenhouse Design: A Cedar-framed Lean-to Greenhouse

Has your dream always been to design and build your own greenhouse? My greenhouse design choices was for a cedar-framed lean-to greenhouse.

I built my greenhouse several years ago after being frustrated with the existing greenhouse the previous owners of our house had built. They did it cheaply with discarded windows and scrap pieces of lumber. It looked awful, overheated in summer, cooled down too much in winter and bothered me for over 5 years.

It was one of my first more involved projects I took on to improve my garden to make it more tranquil.

So I started off on what I knew was going to be a major project with a design for the new greenhouse.

Here are my tips that I hope will help you with your own greenhouse design process. It is a case study and while your greenhouse may follow a completely different design, I’m sure you will get some value out of it. Even if you don’t build the greenhouse yourself and have someone else build it for you.

Overall Size

Since my greenhouse was going to be dependent on the size of the existing windows from the old greenhouse, that was the basis I started from. I measured the existing windows accurately. I had a few different sizes plus some were single-pane and some double-pane. This was all noted down.

As I wanted to use one of the 3 or 4 large double-pane windows sideways for the south side (right side when standing in front of the greenhouse), I knew I would have to increase the distance the greenhouse extended from the house.

However I had to make sure the slightly larger greenhouse would not impede traffic passing the right corner. Using a brick where the wall was going to end up, I practiced with my wheelbarrow to make sure it just fit through the narrow squeeze point. I do have alternative ways to bypass the greenhouse so this was not that critical, just a nice-to-have.

You’ll find as you design anything, you are going to have absolute-must-haves and nice-to-haves. It all comes down to time, space and money.

Then I had my deck on the north side and a peach tree (which died a few years after building the greenhouse) on the south side which dictated the maximum width. Now I essentially had my floor plan.

If you are building a completely free-standing greenhouse, you won’t necessarily run some of these greenhouse design constraints, but still you will need to keep the size of it in proportion with the rest of the garden. And your budget!

One consideration: try to design it with standard lumber lengths in mind. You will waste less wood, saving money and require less cutting, saving time. I didn’t have that luxury, so it cost a bit more and took more time. Don’t worry, the wood scraps were carefully saved for other projects or for burning in my wood stove.

Framing Style Dictates Size

I knew early on that how I built the greenhouse would also dictate the overall size, not just the space I had available. I had decided already on cedar and wanted to have 4×4 posts in the corners and for the top and bottom “sillplates”. The choice of what lumber dimensions to use was for strength but also for looks.

Combine that with the size of the existing windows, this determined the overall height and width of the walls.

Obviously I needed a door. I wanted to make mine wide enough so that I could easily carry things in and out. The old greenhouse has a very narrow door that made it impossible in some cases to move things in and out.

I also knew I wanted to have knee walls to help with insulating the greenhouse. That determined the overall height of the walls, since again I wanted to use standard lumber lengths to avoid wasting a more expensive wood like cedar.

So with these criteria in mind I started to design the framing.

General wall design

I wanted to design the greenhouse in 3D so I started to plan it out in Sketchup, which is a free 3D modelling software. It does however take some practice and time and effort to learn Sketchup. It will come in handy for other gardening projects as well, so consider learning it.

One advantage Sketchup gave me was the ability to model how my choice of construction would affect light levels during different times of the day. And any edits are very easy to make if your design needs to change.

However greenhouse design work can be also done by hand on graph paper with a scale ruler. You can even make mockups out of thick card, fold them up and tape or glue them together.

As with any project, it is best to start off with the easiest part. In my case this was the south wall, but in some ways it was also the most crucial since I was working with the size constraint of the windows I wanted to reuse and the framing sizes I planned on.

South wall:

Here is what I did in Sketchup:

1. Created the window of the right size.
2. On eithe