5. Buyers Guide - Preparing your base
Your choice of foundation should be given great thought, as it will affect both the building of your greenhouse, as well as its future use.
Whilst it’s the choice of many gardeners to build their greenhouse directly on top of soil, especially on allotments, it is by no means the ideal choice. Unless the soil is heavily compacted, it will not give a flat, level surface, making it difficult to build the frame square, and will prove problematic when you come to glaze. You would also be limited to the size of greenhouse, as anything larger than an 8’x10’ could be liable to subside over time, due to the weight, which could twist the frame and pop the glass. Other downsides are the problems of soil getting in and clogging the door runner, the need for weed control, and also the possibility of burrowing rodents.
Some manufacturers will imply that their metal base plinth solves everything and you don’t need to bother with a base. Our experience of seeing greenhouses erected on soil, a few years later, only reinforces our belief that your greenhouse will last longer with a good slab or other suitable base under it.
Simplicity greenhouses offer an optional metal base which comes with anchoring posts for each corner, and extra ones for longer models. These allow you to build on practically any surface, including soil if you so choose, and provides a strong, flat support for the greenhouse frame. However, the corner anchoring posts would need concreting in, if the base is being placed directly on to soil, to firmly anchor the greenhouse to the ground. This method still won’t overcome the problems of weed control and burrowing rodents, but it will raise your greenhouse by 4 inches, and unwanted soil won’t impede the action of the sliding door.
Alternatively, if you still want to build over soil, you could create a perimeter base from flag stones, concrete or brickwork, to bolt the base down to. This is a far more secure method, and still allows for natural, free-drainage, and won’t restrict you to only growing in pots and growbags. When constructing brickwork, you need to be in possession of the official base plan from us, as it needs to be built to the mm, as the cills often overlap the bricks.
Some gardeners prefer the solid permanence of a full concrete slab. Concrete won’t move or sink, it’s perfectly flat and even, and is ideal for the use of expanding anchor bolts, to firmly secure your greenhouse to the slab. The only real downside is that it has a tendency to collect pooling water, unless you create some form of drainage. No greenhouse is 100% waterproof, and water is usually free to seep under the cill or base. You will also add water when watering and the plants will then give it off through their leaves, creating condensation. Therefore, your base should be free-draining and always above the surrounding ground.
The preferred choice, and the one which we recommend, is a base of flagstones. Not only is it easy to secure your greenhouse to, if laid correctly, it will provide a solid level base for your greenhouse. Equally as important, is the fact that it requires little maintenance, apart from the occasional sweep; you won’t get any pooling of water, as it allows free-drainage through the chinks between the slabs, and it will also keep out any potential burrowing vermin from the garden.
If you decide to follow our recommendation to use paving slabs for your foundation, the accuracy of measurements is not critical, but please remember that the actual size of your greenhouse is larger than the nominal size often stated, and the size of the base you create needs to reflect this.
Please be aware that a 6'by 8' greenhouse is called that for ease. The actual dimensions are usually a few inches bigger e.g. 6'3" by x 8'5". To obtain the actual width and length of any greenhouse in this guide, please use the metric measurements (expressed in millimetres) found in the tables presented for each greenhouse. We will also send you a base plan with your order acknowledgement.
Anchoring your greenhouse base to flagstones or concrete is far more straight forward than anchoring to soil. As you buy your greenhouse, remember to order an anchoring set, consisting of 10 brackets, screws and rawl plugs.