cedar lean-to greenhouse

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.

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South wall:

greenhouse design south wall

Here is what I did in Sketchup:

  1. Created the window of the right size.
  2. On either side of the window I placed a 4×4 post. It is crucial to leave a gap of about 1/4” on either side for expansion and the inevitable slightly-out-of-square framing. You can’t cut tempered, double-paned glass but you can cut lumber slightly longer to allow for this gap.
  3. Next up for the height I added a 4×4 header on top and 4×4 sill on bottom. This is mitred on the front side to make room for the second header that runs perpendicular on the front wall.
  4. Next came the 2×4 knee wall header. This is what the window will sit on and I knew it would bear a lot of weight so I also added…
  5. …a 2×4 knee wall brace in the middle of the wall. Most of the weight of the window sits on this one 2×4. In hindsight maybe it would have been better to add a brace on each post as well for the horizontal brace to sit on, but so far I have not had any issues.
  6. The knee wall panels came next. These were sized to avoid wasting wood, as the tongue and groove cedar is quite expensive. I didn’t want to waste a few inches from each board.
  7. Now I went back and sized the two posts on both sides of the window so that they reached the top and bottom framing members. In Sketchup this is easy to do with the push-pull tool.

That was the general framing design that I felt happy about. So I took extra care in building the south wall since I knew I would be using the same framing design on the longer west wall.

West wall:

greenhouse design west wall

I repeated the same process for the most part for the west wall. It shares the corner post with the south wall, so one less component needed. Same window size so all the components were the same size as for the south wall.

In fact in Sketchup I simply copied and pasted and rotated the south wall, removing the one corner post that was shared.

I also changed the top 4×4 header. I made this as long as I could and luckily it spanned the doorway. It was mitered on the right side to mate up with the south wall’s header.

  1. I had a window that was just the right width, so that was put in to the left of the door, again with 4×4 posts in between.
  2. The doorway width was established as the space left over and it worked out to an appropriate 36” width. I worried later about actually designing the door since that was very flexible. If you are using a pre-made door, obviously the gap will be more constrained.
  3. The only other odd part is a corner wall that had to be put in to go around a sink that I wanted to keep (the sink has since been moved). That was a fairly minor problem that could be designed later to fit in the space that was left. And Iended up having a narrow window that just fit!

North wall

greenhouse design north wall

So far it was going well, no major hassles. But now the north wall (left side when standing in front of the greenhouse) was causing me grief as I have the odd situation where it abuts my second floor deck.

And I had the entrance door to my workshop under the deck that I wanted to keep (it has since been converted to my garden shed).

D. Basically the design I came up with is that the last roof rafter on the north side is the top of that wall and the rest is simply filled in as best as I could. The bottom of the wall is another beam attached to the side skirt of the deck to leave the rest underneath open for access to the shed door.

So it required some special framing which I actually changed in the summer of 2017 to make it work better and more leakproof. But the general design stayed the same.

Roof Design

greenhouse design roof

This was what kept me awake at night after designing that pesky north wall. It had to be functional in that I didn’t want it to leak and make the greenhouse humid and damp, especially in winter.

Well, I failed on my first try! I designed the pitch to be steep to maximize light going into the greenhouse, especially in winter when the sun is lower. So that was okay. The design is based on how to attach a deck to a house, which is with ledgers.

The main design challenge here is that my ensuite bathroom juts out by two feet. So I had to design the roof around this and make sure that the two ledgers were at the right heights relative to each other so that the roof would be even all the way across.

E. I did calculate the rough position of the high ledger that is to the left of the bump out but decided to determine the final location once I had the lower ledger on the right attached. This was a good idea as the location of it was a bit different than on the design.

F. Next up were the rafters. These had to be cut at the same angle as the roof pitch and attached to the ledgers on one end. The other end had to sit on top of the front 4×4 wall header. Lengths were a bit hard to calculate so I cut only one end of each rafter, held it up on the ledger and marked where i thought it should be cut shorter. Had to do a bit of trial and error cutting to get it to the right length. And naturally all the rafters have to be in line otherwise you end up with a wavy roof.

The other problem to solve was how to support the polycarbonate panels that I planned to use on the roof. Initially I thought it would work just like with the windows, using strips of 1×1. However the first rainstorm I realized it didn’t work the way I wanted to – the top 1×1’s didn’t cover the edges of the panels enough so the rainwater could sneak by and leak inside.

And because of the 8’ length of the panels, I had the panels ending just shy of the edge of the top header. So rainwater pooled on top of the header and leaked into the greenhouse. I retrofitted these later the first winter by moving the panels so they overhung the edge of the header and added some metal flashing on the top edge but that didn’t work so well either and didn’t look that great.

G. So in the summer of 2017 I rebuilt the roof and use larger 1×6 cedar to cap each 2×4 rafter and make up the gap at the top of each panel so they could overlap the header on the bottom edge. Now the rainwater hits the 2×6 and rolls off of it onto the polycarbonate and then down to the gutter I installed (and bonus: fills up my rain barrel in the greenhouse!) Works much better!

H. Final problem – the roof vents. How to design them to open but also make them leakproof alluded me. After the summer 2017 roof redo they are better, but initially they had the same problems as the roof. Here is the construction diagram of how they now are constructed. There are still some joints between the wood framing that I had to caulk with brown caulk to fix some leaks. And one framing member on one roof vent needs to be moved as it is slightly too low on the roof and there are still some leaks.

So there is the greenhouse design. I hope that gives you some ideas for what you need to consider for your own greenhouse design. Keep in mind that you should always consult a structural engineer and other professionals, especially if you have high snow loads or are building a very large greenhouse.

And for some more reading on the topic of greenhouses, you should also check out 7 Useful Features You Need in a Passive Solar Greenhouse

Are you planning on designing a greenhouse? It would be great if you could share some details and we get a conversation started about your greenhouse design and hopefully others can chime in with their thoughts and experiences.

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  1. Hi Marc:
    Glad to have found your website. I am currently doing up plans for a City of Victoria permit for a greenhouse that we want to attach to the back of our house. It is a bit larger than yours 23′ x 15′ and will be a two part affair. We purchased a 2nd hand 11 x 15′ kit greenhouse a couple of years ago and want to add a second 12′ wide room between it and the house. This will be a fairly simple shed roof that carries the roof line of the existing addition on the house out the 12′ to the greenhouse. The whole affair will sit on a paved brick patio which will act as a nice heat sink. I would like to use cedar roof rafters and framing for the most part as well as coreplast roofing material to let in light. Most of the windows I will be using have openers so venting should not be an issue. Do you have any suggestions for size of material to use for the rafters that will satisfy the City Engineers who seem to think that I am building a house and not a greenhouse to grow some food in these Covid times? Any advice would be appreciated. Thanks.

    1. Hi Ken, Great to hear about your greenhouse! I always regretted not making mine bigger.

      As for the rafters, if you are not using glass, the weight won’t be much. And usually we don’t have to worry too much about snow load here in Victoria. I’m using 2×4’s – just make sure to get the straightest ones you can find without huge knots as I had to replace one that was bowed simply from a knot that eventually I guess dried out enough from the sun.

      But if attached to the house, you are correct: the City may see it as an addition and require 2×6 rafters properly braced and all. Could you make the roof free-standing at least on the side where the house is? Then you might get away with it simply being a structure rather than an addition.

  2. hI Marc,
    thankyou for your informative blog.
    I am planning on building a GH with triple polycarbonate roof.
    I would love to see your plan how to install a roof vent so as to minimize leaking?

    1. Hi Thao, that’s great that you’re building a greenhouse. Triple carbonate is even better for insulation but just keep in mind you’ll get less light penetration.

      My vents are a simple design, just a cedar frame with the poly panels set into a groove in the frame. The frame overlaps the framing of the roof by about an inch/2.5 cm. I do get a bit of water dripping through if it’s a heavy rain. And at the hinge side at the top I did install some plastic sheeting to seal off where I was getting some rain coming in.

      I don’t mind the occasional drip of water inside the greenhouse but find the place where it comes in most is where the horizontal frame of the roof meets the vertical frame pieces. I need to redo those, so when I do I’ll find a better way to join those pieces together.

      Good luck with your greenhouse!

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