Facts about greenhouse bases:
A greenhouse base's primary purpose is to support the frame of the greenhouse.
Sometimes the base forms the entire area under glass within the greenhouse, however the base is often just like the frame of a picture frame.
This base will have four edges but an empty, hollow inside.
This will give better access to the earth and ground below, allowing more room for planting whilst still giving support to the greenhouse glass.
The essential thing is to support the frame which carries the glass.
The provision of a secure and safe base in order to support the structural frame of the greenhouse:
Remember, a greenhouse has a lot of weight pressing down and a large glass surface that receives wind like a sail. In both ways - pressure downwards or sideways - it needs to be secure and safe!
Some greenhouses come with an integral base meaning that it is part of the frame when installed.
Others have optional bases at an additional cost.
The following greenhouses do not have a base as standard and are an optional extra
In the majority of cases a greenhouse is purchased for growing plants directly into the existing soil. Therefore the support for the base will correspond to the dimensions of the frame and will not cover the entire area under glass. (See below.)
Concrete blocks are laid lengthways (on their sides) on a mixture of sand and cement (concrete) filled trench. The standard builders block is 9" high when laid like this. This raises the height of the greenhouse by the same amount. That's too high especially at the entrance. However, the block can be lower than the ground level within the trench and held there by a sand and cement mixture.
Alternately, a brick can be used instead of regular blocks. They come in two types, concrete stock brick or redbrick. In this case the extra height is about 3 - 4" over ground level depending on which side it is laid and this is normally satisfactory. These can be laid so that they are finished to ground level. With care, a very acceptable appearance can be achieved.
Whether there is an integral base, an optional base or no base at all, Lenehans recommend that a sand and cement mixture together with bricks or blocks is employed. As previously noted it is essential to secure the greenhouse from movement of any sort whether subsidence or wind.
In some cases a greenhouse with a metal base (integral or otherwise) can be fixed directly to the ground.
Any solid, flat, level surface will suffice.
The base can be fixed to (say) a concrete area with expanding bolts anchored in pre-drilled holes.
The greenhouse frame is fixed with nuts and bolts to the base.
If the base is an integral one, it is fixed directly to the concrete area. This applies to such areas as a patio, a paved area, a parking area or any similar place.
(However, the consequence is that plants must be grown in pots or grow bags.)
If the primary reason for the greenhouse is recreational rather than horticultural this type of installation can suit many people.
A well sheltered location with winter sunshine provides a very attractive sitting area under glass while being 'outside' at the same time. Lean-to models are often used in this way.
Many people like to use their greenhouses for keeping their potted plants in or as a sunroom – they're really warm all year round in sunshine – or for 'pottering about'. In this case it is nice to have a concrete floor or even a tiled one. If this is the purpose of the greenhouse consider laying a damp-proof sheet ('visqueen') before pouring the concrete. It can more comfortable under foot.
The importance of precise measurements cannot be overemphasised! Lengths and widths are relatively straightforward but square corners (i.e. 90⁰) are vital as well. These are achieved by equalising the diagonals.
The base must be perfectly level.
Remember - A greenhouse is not a conservatory.