Greenhouse Base Preparation Guide

Why do I need a foundation for my greenhouse?

Every greenhouse, no matter how big or small, should be fastened down to a solid foundation of some sort to prevent weather damage, warping and uneven stress. If you do not fix your greenhouse down, it will not last. Without a level base, erecting your greenhouse will be difficult, and in the longer term could cause glass to break or polycarbonate sheets to pop out. Doors and windows could become stiff or stuck if the frame distorts.
Your warranty will not cover a greenhouse suffering from poor base preparation.

A Hall's greenhouse base with the metal spikes cemented into the ground at each corner.

Most of our greenhouses either come with a rigid metal frame called a 'greenhouse base', or have this as an optional extra that you can choose to buy for an additional cost. We always recommend the use of a base to provide extra stability, anchoring your greenhouse to the ground during bad weather. It will also ensure that the frame sits flat and level, allowing all the parts to fit together as they should, making the assembly process a lot easier. Most importantly, it will help your greenhouse to last for years longer by making it more secure and stable. The general idea is that the greenhouse base (an inflexible frame) is fixed to the ground first (best cemented in) and then the greenhouse erected on top of this rigid, sturdy frame.

What kind of foundations can I choose from?

1. Compacted soil/earth

It is possible to fix a greenhouse onto adequately compacted soil. Most greenhouses have the option of a metal plinth. These plinths come with anchor 'spikes' on each corner which, when splayed out and cemented into a hole, prevent the base from lifting up. The ground must be flat and levelled off using a spirit level. It is also advisable to compact the ground using a roller to help prevent subsidence. Bases of gravel or hardcore are not suitable because they are not stable enough.

Advantages

  • Good drainage
  • Cost effective
  • Use of existing ground for growing
Disadvantages
  • Not suitable for anything over the size of an 8ft x 10ft because of the frame weight.
  • Potential risk of subsidence that could cause glass to break, polycarbonate sheets to pop out or warping of the greenhouse frame
  • Must have a metal plinth to site on to the soil
  • Difficult to get level and accurate
  • Difficult to suppress weed growth, pests and water logging
  • Soil and grit will regularly clog up door runners so regular maintenance will be required

2. Perimeter bases

Another good money-saving option is using either edging kerb-style slabs, breeze blocks, bricks or concrete to create a solid foundation only directly under the greenhouse frame, leaving the central floor portion free. You can either build up a foundation from ground level, or cut a suitable trench and fill this in with your chosen medium.

Advantages

  • Offers just as strong a foundation as a fully slabbed base
  • Cost effective because less materials are used
  • Lots of options inside the greenhouse, either growing from ground, gravelling or a central pathway
  • Simple to prepare
  • Can be more decorative than other options
Disadvantages
  • Measurements are crucial to get right because there is less room for error.

3. Slabs or Paving

Our preferred method if you are happy to grow from growbags, a slabbed base will last as a solid and suitable foundation for years to come.

Advantages

  • A clean and simple environment for your greenhouse
  • Can fasten your greenhouse using screws and rawl plugs for better strength in windy conditions
  • Better natural drainage than concrete because of the cracks in the slab joins straightforward to get level and flat
  • Provided they are bedded well, they will not move and warp over time meaning your greenhouse won't suffer under any strain.
Disadvantages
  • More expensive than simpler options
  • Restricted to growing from grow bags and pots

4. Concrete Pad

Another potential hard-standing option, a solid concrete pad can be a good option for larger buildings.

Advantages

  • A generous concrete pad will last for a long time
  • Can use expansion bolts for larger buildings to increase strength
Disadvantages
  • Can be quite expensive
  • Standing water can be an issue unless you think about putting drains or drainage holes in