How to Build a Compact Raised Bed for Patios

A paved corner beside a shed, a strip along a north-facing wall, the sunny patch where a bench used to sit — these are the spaces where a compact raised bed earns its keep. Built at roughly waist height and sized to fit between existing slabs, a simple timber frame turns hard landscaping into real growing space. It also solves the two problems patios create: shallow, compacted ground and nowhere to put good soil. This guide covers a bed around 1200mm long, 600mm wide and 300mm deep, which is easy to build in an afternoon with basic tools and modest materials.

Pick a Spot With Light and Level Ground

Before you buy timber, watch the corner you have in mind for a day. Most vegetables want six hours of direct sun; salad leaves, chard, parsley and mint will manage on four. A south- or west-facing wall reflects warmth and shelters plants from wind, which makes a real difference to tomatoes, chillies and courgettes in a British summer. Avoid spots under the drip line of a roof or a gutter downpipe, where compost gets waterlogged, and keep the bed within reach of a tap — a full 220-litre bed needs serious watering in July.

Check the paving is sound and reasonably level. A slight fall is fine and actually helps drainage, but if a corner dips and holds water after rain, pack it up with a thin bed of sharp sand or a couple of dabs of mortar before you build.

Size the Bed to Fit the Space and Your Reach

• Width: 600mm if you can only reach from one side, up to 900–1200mm if you can walk around it. Anything wider means standing on the soil, which wrecks the structure.
• Length: dictated by the patio. Two 1200mm beds with a 400mm path between them beat one long bed you have to climb over.
• Depth: 300mm is the practical minimum for salad, herbs, dwarf beans and most roots. Go to 400–450mm if you want maincrop carrots, parsnips or tomatoes with deep roots, and to save your back.

A 1200 x 600 x 300mm bed holds roughly 0.2 cubic metres of growing medium — about 200 litres, or four large bags of multipurpose compost. That is a genuinely useful amount of space: a row of salad, a courgette plant and a few herbs, or six tomato plants with a support frame.

Choose the Right Timber

Pressure-treated (tanalised) timber is the sensible default. Two courses of 150 x 47mm gravel boards stacked on edge give you a 300mm-deep wall that resists rot for a decade or more, and they are cheap and easy to cut with a handsaw. For corner posts, use 47 x 47mm or 75 x 75mm lengths cut about 50mm taller than the finished wall, so the tops sit proud and look deliberate.

Avoid old railway sleepers — the traditional creosote treatment is unpleasant to handle and not something you want around food. Untreated pallet wood will rot within a few seasons in contact with damp compost, though it is fine for a temporary bed. If you would rather avoid treated timber altogether, homegrown larch, oak or Douglas fir will last well, especially if you line the inside faces with a sheet of heavy-duty polythene punctured with a few drainage holes.

Build the Frame

• Cut four side panels to length. A circular saw makes quick work of it; a handsaw and a steady hand are fine.
• Cut four corner posts. Set them inside the corners so the side boards butt up against them.
• Drill pilot holes — treated timber splits easily — then screw the boards to the posts with 80mm exterior-grade screws, two per joint per course.
• Build the two long sides first, lay them flat, then join them with the short sides to keep everything square.
• Sit the finished frame on a sheet of permeable landscape fabric to stop compost washing into the paving joints, and check it for wobble.

Get the Growing Medium Right

Do not fill the bed with garden soil. On a hard surface it compacts, drains badly and turns into brick by August. Use a free-draining mix instead: roughly 60 per cent peat-free multipurpose compost, 30 per cent well-rotted manure or garden compost, and 10 per cent horticultural grit or sharp sand. Blend it in a barrow or on a tarpaulin, then add it in 100mm layers, watering each layer as you go to settle it without over-compacting.

Leave the mix about 30mm below the rim. That gap holds a mulch of compost or straw, stops water running straight off the surface, and gives you somewhere to top-dress each spring.

Plant Up and Keep It Cropping

Start with fast, forgiving crops: cut-and-come-again salad, radish, spring onions, chard, parsley, basil and dwarf French beans. Give the sunniest end to tomatoes, chillies or a single courgette, and use dwarf varieties throughout — a compact bed on an exposed patio is no place for tall cordon plants that catch every gust.

Water daily in high summer, ideally in the morning, and mulch to slow evaporation. Feed fruiting crops every fortnight with a potassium-rich liquid feed once flowers set. Sow little and often rather than all at once, and you will harvest from April through to October. Each spring, lift off the top third of the compost and replace it with fresh mix and a handful of slow-release fertiliser. The frame itself will quietly outlast several seasons of seed packets, and the paved corner will have become the most productive square metre in the garden.