A greenhouse may be the way you are considering as a means to extend your growing season, but you may be concerned about what you should use as a solid base for your greenhouse. Some people use a brick base, while others simply place the greenhouse on the bare ground, but is it safe to use sleepers to support your greenhouse?
There are safety risks with putting a greenhouse on sleepers because of the preserving chemicals that are used in sleepers that can leech into the soil. Sleepers will also decay over time despite the preservation measures. There are better and cheaper options available to anchor your greenhouse.
Sleepers are sometimes considered as a base for greenhouses because the characteristics of these heavy pieces of wood solve several problems for greenhouse owners. However, there can be some concerns around using sleepers for this purpose, depending on the type of growing you do in your greenhouse and the type of sleepers you use.
Is It Safe To Put A Greenhouse On Sleepers?
The reasons for considering sleepers as a foundation for a greenhouse are because of the main characteristics of sleepers that present some solutions for some issues for greenhouse owners. The benefits that sleepers offer are considered to include the following.
- Sleepers are heavy. Sleepers are made from dense hardwood, which gives them substantial weight. This weight is an advantage for greenhouse owners to secure their greenhouse to these heavy timbers to prevent the greenhouse from shifting or getting blown away and damaged in strong winds.
- Additional height. Sleepers are normally quite large chunks of wood, which give the greenhouse a little extra height when they are used as the foundation for the structure.
- Durability. Sleepers are durable because they have been treated with chemicals to prevent rot and prevent the wood from being eaten by termites and other wood-eating critters.
These are the benefits that using sleepers offers for greenhouse owners, but there are some aspects of using sleepers as a foundation for your greenhouse that you should also weigh up and consider if the benefits for you outweigh the negatives.
What Are The Negatives For Putting A Greenhouse On Sleepers?
There are some negatives to using sleepers in the role of acting as a foundation for your greenhouse, but some will depend on how you are making use of your greenhouse and growing your plants.
- Shifting of the sleepers. Because the sleepers do not run the full length or width of the greenhouse, you may experience shifting of the sleepers where the lengths join, which can cause problems for your greenhouse. The sleepers may not be heavy enough in strong winds to prevent shifting unless they are secured to the ground. Shifting and movement can cause the greenhouse to twist and flex, which could crack the glass or other panels.
- Uneven settling of the sleepers. Once again, because the sleepers don’t span an entire length of the greenhouse, the sleepers may settle unevenly if they are placed directly on the ground. This can cause the same problems as with the sleepers shifting.
- Rotting of the sleepers. Even though sleepers are treated to make them durable and last a long time, they will eventually break down and rot, which will require them to be replaced before they cause a problem for your greenhouse.
- Soil contamination. The preservation chemicals that are used to treat the sleepers can be contaminants for the soil. Old railway sleepers, in particular, are treated with creosote, which is a known carcinogenic contaminant. The heat and moisture in a greenhouse can cause these chemicals to leech out of the sleepers and into the surrounding soil.
The contaminants that could potentially leech out of the timber and into the soil are probably only a contamination issue for you if you are placing your greenhouse on bare soil and also growing your plants directly in the soil inside your greenhouse.
If you are not going to be planting directly in the soil in your greenhouse, the contaminants leeching into the soil may still be a problem for plants outside of your greenhouse and also pose a contamination threat to underground water supplies.
- Old railway sleepers are probably the worst risk from a contaminant point of view since these timbers were generally treated without any particular concern for the environment.
- Many garden centers and nurseries sell timber sleepers for greenhouses that have been treated with more environmentally safe chemicals to preserve them.
- These sleepers are generally made from a less dense wood, which means they will not be as heavy as railway sleepers and will need to be securely anchored against winds.
The advantage of sleepers from a garden center is that they come in longer lengths that can be cut to the size of your greenhouse. This will reduce the problem of the sleepers settling in an uneven fashion or shifting and posing a problem.
Here’s a great video showing how a greenhouse can be built on sleepers:
What Alternatives Can You use To Secure Your Greenhouse?
So, if you decide that the negatives regarding using sleepers to secure your greenhouse are things you want to avoid, what alternatives are open to you for securing your greenhouse or increasing its height?
There are alternative methods that you can use, which may be better options for you than wooden sleepers, which will need to be replaced from time to time. Some options are more permanent choices, while others are temporary.
Lay a concrete foundation
You can lay a concrete foundation that extends around the greenhouse. This will provide a secure fastening point for your greenhouse, especially if you embed bolts or rebar in the concrete that stick up vertically as securing points for the greenhouse structure. A concrete foundation has the advantage that you can lay bricks on top of the concrete to raise the height of your greenhouse.
Use pegs or ground anchors
Long pegs or, even better, ground anchors can be driven into the ground to securely peg your greenhouse to the ground. Pegs or ground anchors are great options if you want to have a solution that you can easily remove to relocate your greenhouse and won’t leave an eyesore in the garden. It is also a good alternative if you intend to plant directly in the soil in the greenhouse.
Anchor plates are plates of steel that are buried in the soil beneath the greenhouse at intervals. The soil on top of the anchor plates is what holds them in place. They have a rod that extends above the soil surface to act as anchor points for the greenhouse. Anchor plates are good options if you don’t want to lay a concrete foundation and it is also easily moved.
Use concrete blocks with concrete adhesive
Concrete blocks or bricks can be used to build a base for your greenhouse without laying a foundation. There are concrete adhesives available from most hardware or home improvement stores that will bond concrete blocks together without the need for mortar.
You will, of course, have to level the ground for the blocks and make sure that each row of the blocks is level to keep your greenhouse straight and level. Once the concrete adhesive is set, you can drill into the concrete blocks to insert concrete screws or rawl bolts as anchor points for the greenhouse structure.
The disadvantage of the ground pegs or ground anchor option is that the greenhouse is fixed directly to the ground. Using this method will be difficult if you want to add additional height to the greenhouse.
- Anchor plates can be used in conjunction with bricks to add height to the greenhouse.
- Simply make sure that the rods that stick out of the ground from the anchor plates are long enough to extend beyond a couple of layers of bricks.
Then stack the bricks to the height that you need, place the greenhouse on the bricks and fasten it down to the anchor points on the anchor plates. The bricks do not need to be cemented in since the weight of the greenhouse fastened to the secure anchor points will hold the bricks in place. This is another option to use if you what a solid, secure structure but not one that is permanent.
The biggest enemy for a greenhouse is wind, irrespective of whether your greenhouse has glass or plastic panels. Even if your greenhouse is in an area of your garden that is fairly sheltered from the wind, you can get rogue gusts of wind coming through that can shift your greenhouse or blow it over.
This is particularly true if you live in an area that has many thunderstorms that can produce strong winds.
It should be a high priority to secure your greenhouse in some way to protect it from this kind of disaster. Sleepers are not always the best option for this, and I would recommend that you consider one of the alternatives we have proposed that will give you a longer-term solution without the problems associated with using sleepers.