Why and How to Build a Greenhouse Base
Posted: April 23, 2020
Categories: How To & Technical Advice
A greenhouse is a major investment in your garden and one you want to last. So, don’t just plonk your treasured structure on the soil and expect all to be well. Do the hard graft before your greenhouse arrives and you will still be enjoying it in decades to come.
Positioning is vital if you want your greenhouse (and its base) to last for decades. There are three watchwords to always remember when positioning your greenhouse:
SOLID, LEVEL and STABLE.
So for starters put away that thought of putting your greenhouse straight onto soil and expecting all to be OK. It may be for an hour or so, but the moment some of the soil gives way or is washed away (a storm will quickly reveal who has put up their greenhouse correctly and who hasn’t) your greenhouse will slip, sag and slump. Metal will buckle, glass could crack and your pride and joy will be consigned to the recycling tip. Sounds bad? Never going to happen to you? Ok, the best scenario if you put your greenhouse onto soil is that the door will become jammed resulting in a greenhouse you can’t actually use.
It’s also worth calculating the space you need around your greenhouse to ensure you can keep any cleaning and repairs up to date. Ideally you should be able to get a wheelbarrow around your greenhouse – this also allows air to circulate resulting in less moss and algal growth.
Before any structure is installed you need that base sorting.
A greenhouse ‘base’ is often a word used to describe two different structures.
Base = a plinth which your greenhouse sits on and is secured to. It raises the greenhouse a few centimetres above the ground level, creating a step in the process, but gives you a little more headroom and ensures the greenhouse itself is securely fixed.
Base = the actual flooring of the greenhouse. This can be virtually anything but solid concrete slabs are popular; a central path of slabs with soil borders either side is traditional.
Base referring to plinth: railway sleepers are often used to raise greenhouses off the floor. A simple course or two of bricks is often adequate and off the shelf plastic plinths are available. But it is vital that whatever you use as plinth has to be laid or installed onto ground that is: Solid, Level and Stable
There’s another good reason why greenhouses shouldn’t be put directly onto soil or turf. Moisture, damp and wet will all will rise up through your greenhouse frame and if it is wood then premature rotting could occur. Staining of your lovely wood is inevitable. And no-one wants that.
Using a Plastic Greenhouse Base
A sure-fire winning way to put a successful base beneath your greenhouse is using a plastic shed base kit. They are easy to install and will do all the hard work for you and your greenhouse. Choose the appropriate size and unpack everything – instructions are obviously included. Level out the ground where the base is to lie and ensure it is compacted. Level out any hollows with builders sand or pea gravel (and any spare pea gravel will come in handy later so get a few bags in) and keep compacting and checking levels.
Once you are happy that all is solid lay out the thick plastic membrane over the whole area to be covered by the base. Place the panels on the base, clipping together as you go to form a rigid framework. You can design this base to be the exact size of your greenhouse or slightly bigger – and that’s where the spare pea gravel comes in. Spread pea gravel into each of the cells in the base which will be visible once the greenhouse is installed. It creates a great looking finish and aids stability and drainage. Then get cracking on the greenhouse itself happy in the knowledge that your base is sorted.
Using Slabs as a Greenhouse Base
Slabs are used very much in the same way. Ensure the soil is compacted, level and screed off with sand or pea gravel. Then you can lay the slabs ‘dry’ i.e. not using any mortar, or ‘wet’ using blobs of mortar at the corners and one in the middle of each and every slab. Be warned though: slabs are and should be thick and heavy making them a challenge to use. But if you can manage it the resultant base, when laid correctly, will be solid as a rock. Always keep your spirit level moving and in use and don’t make the mistake of standing on slabs until any mortar has set.
It’s also easy to create a slabbed central pathway with soil either side. Tomatoes grow well in such borders in the warm and humid conditions your greenhouse will create.
Bricks can be used to create a solid base but reclaimed items, older bricks, will flake and crack in time.
Whichever you use, do it well, be solid, level and you will have decades of happy greenhouse gardening.