If you are planning a garden of some sort in an area that is topped with concrete, your first reaction may be to rip the concrete out. But this can be costly and it may not be necessary. Every situation is different, and what you decide to do with your old concrete surface will depend on multiple factors.
While it is possible to put soil on top of concrete, there are challenges you will need to consider. These include the location of the concrete, the weight of the soil, drainage factors, the possibility of chemicals leaching from the concrete, and, most importantly, the design of your new garden with a concrete base.
What to consider when putting soil on top of concrete
Before you rush out and order multiple tons of soil to top concrete, you need a plan. This will depend largely on the size of your concrete slab and its location.
Other factors to consider include the proximity of the concrete to your home and its height in relation to the surrounding area. Chances are you’re going to have to install or build some sort of edging to contain the soil.
Alternatively, you might consider an extensive raised bed or a series of small raised beds. You can increase the height of the bed to accommodate the root systems of whatever you are planting.
Are there planted areas next to or close to the existing concrete slab? Do you want to incorporate these into one single landscaping plan?
Perhaps you have an elevated concrete patio or even a flat concrete roof. Putting soil over concrete in this situation introduces its own set of challenges, particularly drainage and the weight of the soil. It would be prudent to consult with an engineer if this is the case.
What are you planning to plant? Do you want to plant over the entire concrete surface? If so, are you considering planting grass over concrete?
If the existing concrete abuts your house, don’t even consider topping the concrete next to the house with soil. This will invite serious drainage issues and could result in water seeping into the interior.
In any case, drainage is an important consideration. It is good practice to include a layer of fill at the base of raised beds, and the same applies when you put a soil top over concrete.
You need to channel excess water away from any buildings. This applies whether you build a series of raised beds or have an edging around the concrete to contain the soil.
You can do this with pipework, precast concrete, or even plastic channels. It’s like having drainage holes in a container, just not on the bottom, but on the sides.
Difficulties you might encounter when using soil on concrete
It stands to reason that if you are planning to top concrete with soil, you won’t pour concrete to create the surface. Rather, it will likely be an old concrete slab that you have inherited with your house.
While the possibility of chemicals leaching out of old concrete can be an issue, ironically, the effect of soil on concrete is usually more profound. But the chances are that the concrete you plan to grow over will not be functional. That is unless it’s an elevated terrace or roof garden.
Nevertheless, it’s a good idea to consider the possibility of leaching since this can alter the pH levels of the soil. The best course of action is to test the pH regularly and then make amendments or adjustments based on the results.
If the pH is affected, it’s likely that the soil will be more alkaline than you want it to be. You can decrease alkalinity cheaply and easily with elemental sulfur.
Drainage, mentioned above, is one of the greatest challenges and it can present you with the biggest difficulties. The problem is that if you don’t tackle the challenges of drainage, water levels are likely to build up quickly in wet weather and your soil may become waterlogged.
A caveat is that a raised bed built on concrete will usually drain faster than one built directly on the dirt. Also, because the concrete does tend to heat up and can raise the temperature of the soil and increase evaporation, you may need to water it more frequently.
How much dirt does it take to grow grass on concrete?
The question, can you grow grass over concrete is a common one. The answer is yes, though it’s not a great idea for large expanses, largely because of the maintenance factor.
The ratio of grass concrete heights will largely determine how much dirt you’re going to bring in. To be successful, you will need a suitable soil mix and not just any dirt.
Quantities can be a dilemma. But whatever you decide to grow on concrete, you need enough soil to accommodate the roots of the plants you plan to grow.
Generally, grass needs less soil than raised beds constructed for vegetables.
A good rule of thumb for veggies is at least 12 inches of soil. When established on hard surfaces like concrete, aim for 18-24 inches because the roots won’t be able to penetrate the base of the bed.
The type of grass you plan to grow will determine the depth of the dirt you need to accommodate it. If you don’t have enough for the roots to stay healthy, most types of grass will die. So, ideally, choose turf grass that has shallow roots.
Also, if there isn’t sufficient depth, you’ve got more chance of the ground getting waterlogged. If this happens, you run the risk of dreaded root rot.
You might get away with a 4-6 inch layer of dirt for grass, but 8 inches is a safer depth.
Take heart. If you have an unsightly concrete slab that you want to transform, you can put soil on top of it and grow grass and other plants.
It is, though, important to address the various challenges we have mentioned. With proper planning and continued maintenance, you shouldn’t have any unusual garden problems growing over existing concrete.