Affiliate disclosure: As an Amazon Associate I earn from qualifying purchases. This article contains affiliate links.
Raised beds on concrete are a practical way to grow vegetables, fruit and flowers where there is no open soil, but they behave differently from beds on the ground. Because water cannot drain downwards, you need to plan side drainage, choose a deeper fill, and design the bed dimensions around how you will reach and work in it.
Table of Contents
Drainage: letting water escape sideways
Concrete is impermeable, so excess water must leave through the sides of the bed. The RHS advises drilling plenty of holes in the sides of a timber bed. For stone or brick beds, leave gaps at 45cm intervals in the first two courses, cover the inside of each gap with fine wire mesh, and heap gravel against the mesh when filling to stop soil clogging the gaps.
Do not rely on a few small holes; water needs a clear route out. If you are using a ready-made timber kit, check whether it has pre-drilled side drainage holes. If not, follow the kit maker’s permitted drainage arrangement before filling. For recycled plastic beds, follow the manufacturer’s instructions, as some designs include drainage slots and others have closed sides that need holes added.
The growing medium also matters. On hard surfaces, the RHS recommends a mix of three parts organic matter, four parts sharp sand and seven parts topsoil. This is sandier than the standard raised-bed mix, which helps water move through and out. Be prepared to refresh or replace the soil regularly, as you would with a large container, to maintain good drainage.
If you are building on tarmac rather than concrete, the RHS school gardening guidance suggests covering the area with weed-suppressant material first. A weed membrane is not a chemical barrier or a substitute for drainage. Check the surface’s condition and the system’s suitability before growing food over it.
Depth: giving roots enough room
On open ground, plant roots can grow down into the soil below the bed. On concrete, the bed is the only root zone, so depth is more critical. The RHS general advice says 30cm is deep enough for dwarf bulbs, salad leaves and strawberries, but most plants, especially fruit bushes, shrubs and small trees, need 45cm or more. For beds on tarmac, the RHS school guidance states they need to be at least 50cm high.
For a productive vegetable bed on concrete, aim for at least 45–50cm of growing medium. This gives most annual vegetables and soft fruit enough root space. If you want to grow deeper-rooted crops or permanent plants, go deeper still. Remember that the frame height and the soil depth are not the same: leave about 5cm below the rim to allow for watering and mulch.
Fill the bed mainly with soil rather than compost, because compost breaks down and slumps over a few seasons. Bagged topsoil is convenient but expensive; you can also use soil from elsewhere in the garden, enriched with organic matter. Avoid filling the base with rubble or subsoil on concrete, because that reduces the usable root depth and can interfere with side drainage.
If you plan to grow ericaceous (acid-loving) plants in a masonry bed on concrete, line the sides appropriately to separate mortar from the root zone, keeping drainage outlets open; the growing medium and water supply must also suit acid-loving plants. This is the same precaution as for masonry beds on soil.
Access: designing for reach and mobility
Raised beds on concrete are often chosen to make gardening easier for people with restricted mobility, including wheelchair users. The hard surface can provide a level, firm path, but the bed dimensions must be planned around the user.
Width: the RHS advises keeping beds under 1.5m wide so you can reach the centre without standing on the soil. If the bed is against a wall or fence, make it narrow enough that you can reach all parts from the accessible side; this usually means reducing the width compared with a freestanding bed.
Height: there is no single correct height. The RHS school guidance says to check with wheelchair users which height suits them best. For standing gardeners, a higher bed reduces bending, but the RHS notes that higher beds need more soil and more watering during hot weather.
Pathways: the RHS gives 1m as a wheelchair path starting point and 45cm for a wheelbarrow, but those dimensions do not establish comfortable access. Thrive recommends paths at least 1.2m wide and substantial turning space; measure the actual chair, turning needs and user reach before building. On concrete, you may already have a hard path, but check for slopes, cracks or drainage channels that could make wheelchair use difficult.
Practical checks before you build
- Check the concrete is level and sound. Prepare continuous, stable support to suit the frame instructions; do not balance a filled bed on improvised shims. A damaged or uneven slab may need repair.
- Ensure the bed will not block existing surface-water drainage. Water from the bed will run onto the concrete, so plan a suitable drainage destination without causing ponding, slips or runoff onto neighbouring property. Do not rely on sweeping away repeated flooding or direct nutrient-rich runoff into an unidentified drain.
- Consider the weight. A large bed filled with damp soil is heavy. Check the support for the full wet weight. Raised balconies, roof terraces and suspended slabs need an assessment of the permitted loading before a bed is installed.
- Plan for watering. Raised beds dry out more quickly than open ground, and on a hard surface you may need to water more often. Install a water butt nearby and check soil moisture regularly.
Product types to consider
When buying a raised bed for concrete, look for these features:
- Timber raised bed kit – pressure-treated softwood is durable and easy to drill for side drainage holes. Check the height: many kits are only 30cm tall, which may suit shallow-rooted crops but gives less flexibility on concrete. Look for kits 45cm or taller, or use a kit explicitly designed and braced for stacking.
- Recycled plastic raised bed – rot-resistant and often low-maintenance; strength, bracing and available dimensions depend on the kit. Check whether the design includes drainage slots or whether you need to drill them. Some recycled plastic beds have closed sides, so side drainage must be added.
- Masonry raised bed blocks – height, wall thickness, reinforcement and foundations must be designed for the retained wet soil. Do not assume an existing patio slab is an adequate footing merely because it looks thick and level; substantial masonry beds need competent construction advice. Leave drainage gaps in the first two courses as described above.
Useful products for this job
The links below open Amazon UK product searches. Check the exact item, seller, current price and compatibility before buying.
- Timber raised bed kit: Pressure-treated timber is easy to drill for side drainage holes, which are essential on concrete. Check the internal height is at least 45cm and that you can drill side drainage holes without compromising the frame; confirm the timber is pressure-treated for outdoor use. Search Amazon for Timber raised bed kit.
- Recycled plastic raised bed: A rot-resistant option if the frame, bracing and drainage are designed for the intended depth and hard-surface installation. Check whether the sides are pre-drilled or slotted for drainage; if not, confirm the material can be drilled and that the bed height is at least 45cm. Search Amazon for Recycled plastic raised bed.
- Masonry raised bed blocks: Masonry can form a durable bed when walls, drainage and foundations are properly designed. Have wall and foundation requirements checked for the retained wet soil; confirm block suitability and planned drainage before relying on an existing slab. Search Amazon for Masonry raised bed blocks.
Another supplier for this job
Elixir Garden Supplies is an affiliate partner. We may earn commission from purchases through these links. Check the selected product, current price and delivery terms.
- Elixir graded and screened topsoil: A mineral-soil option to compare for raised-bed filling. Check the soil specification, quantity and delivery access; adapt the growing mix to your plants. View Elixir graded and screened topsoil at Elixir Garden Supplies.
Related garden guides
- How to Build a Raised Garden Bed: A 2026 UK Guide
- How to fill a raised bed without wasting good compost
- More planters & pots guides
Sources and further guidance
Guidance researched in September 2026. The feature image is an AI-generated illustration. Product guidance is based on published information, not hands-on testing.