If you plan everything in advance and stick to your plan, building a lean-to greenhouse can be a weekend task. To start, you’ll need to assess whether your available wall space gets enough daylight and whether the finished lean-to will obstruct your neighbour’s view. Always check the local building codes before you begin.
Tall Wall and Small Wall
Your lean-to will need a structure other than your house exterior to keep it stable. This is referred to as the tall wall, while the exterior wall is the small wall. To construct your tall wall, you will need to know the height of your small wall, the width of your lean-to and the angle of your roof. But don’t panic: no geometry is required. Simply get some help to hold the tape measure and then eye-ball the height of the roof. You’re aiming for a roof slope of 30 degrees. If you get this right, it’ll be easier to shape the top of the frame. Now construct your tall wall on the flat, using a carpentry square to ensure your right angles are precise. Use 2×4 blockings between the studs to strengthen the frame. Then lift the tall wall and attach to your house exterior.
The small wall is constructed in the same way with the top of the studs flat to ensure the roof sits tightly. However, you’re not going to build a roof and mount it. It is much easier to cut rafters. These will hold the small wall in place. The rafter edges will be cut to 60 degrees at he bottom where they meet the small wall and 30 degrees at the top where they meet the house exterior. Be careful not to make the mistake of cutting both ends the same.
The Front, Back and Door
The front and back of the structure are built in situ, but consult a plan to see how a door is constructed within the triangulated frame construction. Once all four walls are in place, reinforce the whole structure with metal braces using galvanized screws to protect against rusting.
Once more, the door will be constructed on the flat and lifted into place. You can choose to use a very simple design with just one horizontal for extra support, but think about a solid lower door to avoid kick damage. Using the same principles as the door, you can also include a ventilation window in the roof.
Polyethylene foil is the best option for a lean-to greenhouse. It’s easy to roll out, cut and fix in place. You can measure sections and cut it out on the flat, but always leave a little overhang for a margin of error. Secure to the frame with 1×2 wooden strips and screw down tightly. Once the screws are in and the greenhouse is weather-tight, it’s time for the finishing touches. Fill in all external screw heads with filler and, once dry, rub down with sandpaper. Once this is complete, your greenhouse is all finished.