Building Raised Beds on an Old Concrete Slab: The Right Way to Do It

The Scenario

A homeowner recently asked:

I have an uneven 3.5m x 4.5m concrete slab in my garden and want to install four large raised beds on top. I'm worried about leveling the surface, potential chemical leaching, and how to prep the slab for the wet UK winter.

The GardenOwl Diagnosis

The Assessment

You have a 3.5m x 4.5m concrete slab in your backyard. It’s uneven, it’s ugly, and it significantly detracts from your home's curb appeal. It’s currently collecting dead leaves and potentially creating The Capillary Wick Trap. You want to turn this wasted space into a productive vegetable garden by putting raised beds on top. You are worried about the concrete leaching chemicals into your food, and you are trying to figure out how to level the beds on a bumpy surface before the UK winter sets in.

The Trap: The "Soil Shim" and the Claustrophobic Layout

Most homeowners try to solve the uneven concrete problem the intuitive way: they shovel dirt under the raised bed frame to fill the gaps. Do not do this.

Soil placed directly on concrete under a wooden frame acts like a sponge. In a wet climate (especially a UK winter), that soil will stay saturated, wicking moisture up into your timber and rotting your expensive beds from the bottom up. Furthermore, the first heavy rain will wash that loose soil out, leaving your beds wobbling anyway.

The second trap is the math. You mentioned fitting four 120cm x 180cm beds on a 3.5m x 4.5m slab. On paper, the square footage fits. In reality, you are building an obstacle course. Once you account for the physical width of the timber, you are likely leaving yourself with paths narrower than 50cm. You can’t kneel to weed, and you certainly can’t fit a wheelbarrow. A garden you can’t work in is just a wood box full of weeds.

The Solution: Drainage, Air Gaps, and Better Math

Here is how to build this system so it lasts for years.

1. Level with Hard Materials, Not Soft

Ignore the bumps in the concrete. Build your timber boxes first. Set them in place, and then use rot-resistant shims (like slate tiles, plastic packers, or pressure-treated wedges) to level the frame. You want the wood lifted slightly off the wet concrete in the low spots. Gaps are good—they let water escape.

2. The Liner is Your Safety Net

You are worried about chemicals leaching from the concrete. While cured concrete is generally stable, the real issue is keeping your soil in and the heat regulated.

Line the entire bottom and sides of your beds with a high-quality, non-woven landscape fabric or geotextile.

• Why? It acts as a filter. Water drains out onto the slab and runs off, but your potting mix stays inside. It also creates a barrier between your roots and the concrete.

3. The "Thermal Break" Layer

Concrete is a thermal battery. It holds cold in the winter and bakes in the summer. This can stress plant roots near the bottom of the bed.

Since your beds are deep (45cm), use the bottom 10cm for a drainage/air layer. Lay down coarse gravel or, even better, a layer of thick branches and twigs (a technique similar to Hugelkultur). This creates an air gap that insulates the soil from the concrete’s temperature swings and ensures the bottom of the soil doesn't turn into anaerobic sludge.

4. Fix the Layout

Don't force four beds. If you crowd this slab, you will regret it every time you try to turn a hose around a corner.

• The Rule of Thumb: You need a minimum of 60cm (24 inches) for a walking path, but 75-90cm is better if you use a wheelbarrow.
• The Fix: Reduce to three beds, or change the dimensions to narrower, longer troughs. This improves airflow around plants, which is critical for preventing mildew in damp climates.

Visualizing the Result

Before you buy a single piece of timber, you need to see the spacing. Drawing it on graph paper is fine, but it doesn't show you how tight the corners will feel.

This is where GardenDream saves you money. You can upload that photo of your slab and overlay the beds to scale. You might realize that an L-shape or U-shape configuration gives you more growing space and better movement than four parallel blocks. It’s a blueprint tool that lets you make mistakes digitally, rather than with expensive lumber.

If you want to test this on your own yard, upload a photo and see what this design would look like using our Exterior Design App.