How to build a greenhouse

This page shows a unique and cheap method of how to build a greenhouse. The method used is author Mike Gilmore’s own design and came about because he needed a cheap DIY greenhouse build solution for his extensive alpine plant collection – an alpine house.
Introduction
The construction details for this greenhouse build can be used for any size of greenhouse. The greenhouse plans for this particular greenhouse were used to build the all-important glazed section that was built over the existing base which we wanted to make use of because of its fantastic rare floor. But any Self Builder simply needs to extend the sides down to the floor for his own use. It’s that simple. But first let me explain how we got there by way of introduction.
My family took over the restoration of a large overgrown and derelict walled garden in 1999, among the items we uncovered beneath the overgrown jungle was the brick base of a 60ft of what was once a very exotic heated cucumber/marrow house. The first photo gives you an idea of what we found after we had removed the overgrowth! The new greenhouse was to remain like this for 2 years before we got around to clearing it properly.






We discovered that the original greenhouse was smashed by the previous owner and all the glazing was left on the growing beds or swept under the floor out of sight. Walls were damaged and needed rebuilding, but eventually, after several bloodied fingers the job was done and we could consider building the greenhouse.

Greenhouse Build

The first job in constructing a new greenhouse is to build a level concrete base. Anyone debating with themselves about its cost should kick the idea into touch and get on with it. A concrete greenhouse base is THAT important.
Building the walls
It makes the build easier and more accurate. Once finished a concrete base is easier to walk on, easier to clean and helps maintain a  cleaner, healthier plant environment. The next job is the 2×4 base plate that runs around the floor perimeter. On my greenhouse its on the wall, but for you its on the floor, for both its a base plate. The important thing here and now is to mark it out and get it ready.
If your greenhouse is as long as mine you will need to make the necessary scarf joints using the single halving technique. Basically, at each end of timber that you’re joining together, cut a half-deep section of timber out that’s about 8″ long. Match both halves of each timber together. Easy. But DO NOT fix the floor/wall plate to the concrete base.
Now mark and cut your vertical ‘wall’ timbers. It’s easier to do this on the ground. Align them at regular intervals and fix them to your base plate by knocking 6″ nails through the base plate and into the verticals.
Do the same for an identical wall plate that will run around the top of your greenhouse, which you will attach the roof to.
You should have now completed a simple wall framework on the floor.
Now, turn it upright. You will need an extra pair of hands to align it to the base plate where you want it, while you now secure it to the floor.
Repeat for each wall. Your greenhouse should now look like this.
In the picture three walls are up and so is part of the roof ridge in the centre. The diagonal lightweight timbers are roofing lathes which were used to brace the structure and ensure everything was square and upright. Use the 3-4-5 triangle technique from your school days to check everything is SQUARE and a level that everything is UPRIGHT.
The picture is very self-explanatory and clearly shows the construction of the central roof ridge.
Please note the ridge is a DOUBLE RIDGE. Slightly more difficult to build, but it makes for easier maintenance.
Plastic corrugated roof panels were used in this greenhouse build example because the greenhouse is located beneath Larch trees which drop branches as a matter of habit through the winter.
Although none have needed to be replaced yet, this is only due to the improved resilience of the plastic. Glass here would be a different story.
This next photo shows my economical screen material – ordinary windbreak. The trick to making this more permanent is to thread galvanised wire through the windbreak material along the edge you need to secure. It takes time. Once stretched over the framework hammer U-nails across the threaded wire. The result is most unlikely to rip.
Windbreak fabric makes a cheap greenhouse. It makes an excellent shade house for anyone living in sunnier climes. In this case I was building an alpine house, these diminutive plants are not keen on wet UK winters but they relish plenty of airflow all year-round. This design provides them with the best of both worlds. Plus, there’s no risk of me leaving the windows closed accidentally on a sunnier day and losing my stock!
The completed greenhouse build viewed from inside.
This greenhouse has extensive roof overhangs to prevent wind-blown rain entering. The roof sections are ten feet long and the extra overhang means I don’t need to cut them.
So why the double ridge construction you may well ask? The roof span is pretty extensive because of those uncut roof panels described above. Each springtime the roof can be easily cleaned with a power washer when I walk along the ridge and spray downwards on to the roof. Much easier and more effective than if I tried cleaning the roof from ground level.

Here’s the shading effect.
Again the shading is simple and effective. Windbreak material is threaded through with wire once more and the shades are pulled open or closed at will.