Shed Base Guide: How to Choose and Prepare a Base

A good base is the most important thing you can do for a new garden building. A firm, level base keeps the building square, so doors and windows open properly and the roof doesn't twist. It also lifts the timber away from damp ground, which helps your building last for years.

Here's how to choose the right base, get the size right and prepare it properly.

The main types of base

Concrete base

A concrete slab is the strongest and longest-lasting option. It's usually the most expensive, too.

• Best for: larger sheds and workshops, log cabins, garden rooms and home offices, and concrete garages, which need a concrete base.
• Good to know: concrete needs to be laid level and left to cure fully before a building goes on it. For bigger buildings, it's well worth having it laid professionally.

Paving slab base

Paving slabs laid on a compacted, level sub-base make a solid, practical base for most small and medium buildings.

• Best for: most garden sheds, summerhouses and greenhouses.
• Good to know: the slabs must be laid flat and level with each other. Uneven slabs will pass their bumps straight on to your building.

Timber frame base

A pressure-treated timber frame raises the building off the ground and lets air circulate underneath. It's quicker to put together yourself than concrete or slabs, and can be levelled on ground that isn't perfectly flat.

• Best for: timber sheds and summerhouses, especially on gently uneven ground.
• In our store: the Power Sheds Shed Base Kit, the Shedlands Timber Shed Base and the Suresheds Tanalised Airflow Base, each designed to suit that brand's buildings.

Plastic grid and plastic support bases

Plastic grid bases are interlocking panels that are usually filled with gravel. They're lightweight, let rainwater drain through and are easy to lay yourself. Plastic supports that sit under the floor bearers do a similar job, keeping timber clear of the ground.

• Best for: smaller and lighter buildings on firm, level ground.
• In our store: The Shed Boot plastic shed base kit, which lifts your shed's floor bearers clear of the ground to keep them dry and ventilated.

Which base is best for the UK climate?

Wet winters are the main enemy of a garden building, so the best base is one that keeps timber off damp ground and doesn't let water sit around it.

Whichever you choose, avoid low spots where water collects. There's more on this under drainage and position below. For the timber itself, see our shed timber guide.

How to prepare a shed base, step by step

Always follow your building's instructions first, as some manufacturers give an exact base size or method. As a general guide:

A paving slab or concrete base

1. Mark it out. Use pegs and string to mark a rectangle the same size as the building's floor, or very slightly smaller. Measure both diagonals: when they match, it's square.
2. Dig out. Remove the turf and soft topsoil down to firm ground, deep enough for the sub-base plus the slabs or concrete (see the depths further down).
3. Lay and compact the sub-base. Spread hardcore, such as MOT Type 1, and compact it thoroughly in layers with a plate compactor or tamper.
4. Lay the slabs or pour the concrete. Bed slabs on mortar and tap each one level with its neighbours. For concrete, set timber formwork to the finished height, pour, tamp it level and leave it to cure. Don't put a building on it for several days.
5. Check again. Check level along the length, across the width and corner to corner, and re-measure the diagonals before the building goes on.

A timber frame or plastic base

1. Clear the area. Remove the turf and level the ground as closely as you can. A weed membrane helps stop growth underneath.
2. Position the base. Lay the frame or grid out and check it's square by measuring the diagonals.
3. Level it. Adjust or pack it level as the manufacturer instructs. Plastic grids are usually filled with gravel.
4. Check before you build. A final check with a long spirit level saves sticking doors later. Note that plastic supports such as The Shed Boot are made for a hard, firm surface, not bare soil or grass.

Bases for garden rooms, log cabins and home offices

A garden room is a different job from a shed. It's heavier, it usually has glazing and doors that must stay perfectly square, and you'll be spending real time inside it, so damp and cold matter far more. That's why nearly every garden room and log cabin ends up on concrete.

What a garden room base needs to do

• Carry the weight without settling. A 44mm log cabin or an insulated garden room is far heavier than a shed, and a base that sinks even slightly at one corner will jam the doors.
• Stay dead level and square. Glazed doors and windows are unforgiving. A base that's out by a few millimetres across its diagonals shows up as a door that won't close.
• Keep damp out. A damp-proof membrane under the slab stops moisture rising into the floor — important in a room you'll be sitting and working in.
• Take services if you need them. If you're running power out to the building, it's much easier to lay the duct before the concrete goes down than to dig it up afterwards.

How thick and how deep?

As general guidance for a typical UK garden, on firm, well-drained ground:

So a garden room base is typically dug out to somewhere around 200–300mm overall, depending on how much hardcore the ground needs. Soft, clay or made-up ground, a sloping garden, or nearby trees can all mean digging deeper, and a larger building may need thickened edges. These are typical figures, not a specification — the right depth for your garden depends on the ground you've actually got, so have it assessed before you dig.

Concrete also needs time. It's usually walkable within a day or two but shouldn't carry a building for several days, and reaches full strength over several weeks. Don't book your installation for the morning after the pour.

• Vista Collection garden rooms are subject to a site survey before installation, which confirms your base and electrical connection.
• Power Sheds log cabins arrive as a self-assembly kit on a pallet, so your base must be finished and cured before the delivery date.
• Pent Haus garden rooms are assembled on site over around three days, so the base needs to be ready and signed off first.

Log cabin foundation options

• Concrete slab: the usual choice. It's strong, it stays level and it can include a damp-proof membrane.
• Paving slabs on a compacted sub-base: can suit smaller, lighter cabins, as long as they're laid perfectly level and the manufacturer allows it.
• Raised frame or ground screw systems: sometimes used on sloping ground or where digging is difficult. They must be designed to carry the cabin's weight and be accepted by the cabin's manufacturer.
• Power Sheds base kit: the Power Sheds Shed Base Kit is made to match the footprint of your Power Sheds building, including cabins. Check with us that it suits your cabin and your ground.

On uneven or sloping ground, don't try to level the cabin itself by packing under it. Get the base level first, which usually means digging out the high side or building up the low side. This is where a professionally laid base is worth it: send us a photo and we'll ask Midland Base Works to take a look.

Shed bearers explained

Most timber sheds and summerhouses sit on bearers — the pressure-treated timber rails that run underneath the floor. They're easy to overlook, but they do two important jobs: they spread the building's weight across the base, and they hold the floor a little clear of it so air can move underneath and the timber stays dry.

• Bearers usually run across the width of the building and are spaced evenly along its length, with the spacing set by the manufacturer. Check the instructions for your building rather than guessing.
• They must sit on a level base. A bearer bridging a dip will flex when you walk on the floor.
• Keep them out of standing water. Plastic supports such as The Shed Boot lift bearers clear of a slab or paved base and let air circulate.
• If you're replacing bearers on an older shed, use pressure-treated timber of the same section, not ordinary softwood.

Why bearers must be pressure-treated

Bearers sit lower than any other part of the building and are the first timber to meet damp, so they need to be pressure-treated (tanalised), where the preservative is forced deep into the wood. Surface-treated timber isn't enough here. Timber that will sit directly on soil or gravel should be rated for ground contact: the Power Sheds Shed Base Kit, for example, uses UC4 pressure-treated timber, which is rated for direct ground contact.

How far apart should bearers be?

Bearer spacing is set by the manufacturer to suit the floor boards, so it varies from building to building. On most sheds the bearers come fixed to the floor panels, so the spacing is already right. If you're making up bearers or supports yourself, follow your building's instructions. For plastic supports, The Shed Boot recommends fitting boots roughly 30 to 50cm apart along each bearer. As a rough guide, an 8x6ft shed needs around 48 boots.

Which bases do bearers sit on?

• Concrete or paving slabs: the bearers sit on the base and hold the floor clear of it. Lifting them slightly on plastic supports stops rainwater sitting underneath and lets air through.
• Timber frame base: the base frame lifts the bearers and floor clear of the ground. Use ground-contact rated timber for anything touching soil.
• Plastic grid base: the bearers sit on the gravel-filled grid, which drains freely.
• Plastic supports on their own: The Shed Boot clips onto bearers up to 2 inches wide and is made for a hard, firm surface, not bare soil or grass.
• Not straight onto grass or soil: bearers laid directly on the ground stay damp and rot much sooner.

What does a base cost?

There's no single price, because the cost is driven by the work involved rather than the building you're putting on it. The things that move the price most:

• Size. Cost rises with floor area, and a garden room base is usually thicker than a shed base as well as larger.
• Ground conditions. Firm, level ground is straightforward. Soft or clay ground, made-up ground or a slope means more digging, more hardcore and sometimes a deeper footing.
• Spoil removal. Digging out for a base produces a surprising amount of soil, and getting rid of it is a real cost.
• Access. If a mixer lorry can reach the spot, the job is quicker. If everything has to be barrowed through a side gate, it isn't.
• Extras. A damp-proof membrane, reinforcing mesh, thickened edges or a duct for electrics all add to the job.
• Doing it yourself. A slab or timber base you lay yourself costs materials and a weekend. A professionally laid concrete base costs more but arrives level, square and guaranteed.

The only reliable way to know is to have someone look at the spot. Get in touch with the rough size of your building and a photo of where it's going, and we'll ask Midland Base Works to quote.

Bases for specific buildings

• Greenhouses: most Elite greenhouses have an integral aluminium base, and Elite base materials are available to anchor and level them on soil, grass, paving or brickwork. Check the product page for your model.
• Lifelong steel sheds and garages: Lifelong's installers don't need slabs or concrete. As long as the ground is fairly level, they pack out the footings as part of the installation.
• Concrete garages and stores: a concrete base is required.
• Garden rooms: Vista Collection garden rooms are subject to a site survey to confirm your base before installation.

Do metal sheds need a base?

Most metal sheds need a firm, level base like any other building, and should be anchored to it as the manufacturer instructs. Lifelong steel sheds and garages are the exception in our range: Lifelong's own team installs them straight onto fairly level ground, packing out the footings as part of the installation, so no concrete or slab base is needed. If your ground slopes, ask us before you order.

How big should the base be?

Aim for a base that is the same size as the building's floor, or very slightly smaller. Don't make it bigger.

It might seem sensible to add a border, but a base that sticks out beyond the walls creates a ledge. Rainwater collects there and splashes back onto the bottom of the cladding, which encourages rot. If you'd like a path or hard standing around your building, leave a small gap or use gravel, which absorbs splashes instead of bouncing them back.

• Measure the floor or footprint given on the product page, not the roof. Roofs usually overhang the walls. Our size guide explains nominal and actual sizes.
• Always check the manufacturer's instructions for your building, as some give an exact base size.
• Leave working space around the base so you (or your installer) can assemble the building and treat or maintain it in the future. Lifelong, for example, asks for at least 2ft of working area around their buildings.

Getting it level

• Use a long spirit level and check in every direction: along the length, across the width and corner to corner.
• Check the base is square by measuring both diagonals. They should be the same.
• Small errors matter. A base that's out of level can pull a building out of square, so doors stick and the roof can twist.

Drainage and position

• Watch where the water goes. After heavy rain, look at where water collects or runs in your garden. Avoid putting your building in a low spot or in the path of run-off.
• Don't build straight onto grass or soil. Ground holds moisture, which can creep into the floor and cause damp inside.
• Allow airflow. Raised timber bases and floor bearers let air move underneath, which helps keep the floor dry.
• Think about access. Leave space to get round the building, and think about how you'll get parts to the spot on delivery day.
• Check planning rules. A raised base adds to the overall height of your building. See our planning permission guide.

When to call in Midland Base Works

We work with Midland Base Works, who lay concrete bases that are precision laid, level and load-bearing, with a fast turnaround. We'd recommend a professionally laid base if:

• you're buying a large building, log cabin, garden room or home office
• your garden slopes or the ground is soft or uneven
• your building needs a concrete base
• you'd simply rather not dig, mix and level it yourself

Contact us or call 01543 398778 to discuss your base. We'll check whether Midland Base Works cover your area.

For what happens after you order, see delivery and installation. Once your building is up, our timber care guide explains how to keep it in good condition.

Common questions

What is the best base for a shed?

A concrete slab is the strongest and longest-lasting, but paving slabs on a compacted sub-base suit most sheds. Timber frame and plastic bases are quicker to lay yourself and work well on firm ground.

What base does a garden room need?

Nearly always concrete. A garden room is heavy, has glazed doors that must stay square, and is a space you'll actually use, so it needs a base that won't settle and a damp-proof membrane to keep moisture out of the floor.

How deep should a garden room foundation be?

As a general guide, a garden room base is dug out to somewhere around 200 to 300mm overall: roughly 100 to 150mm of compacted hardcore with around 100 to 150mm of concrete over it, often with steel mesh. Soft or sloping ground can mean going deeper, so have the spot assessed before you dig.

How thick should a concrete shed base be?

Around 75 to 100mm of concrete over 75 to 100mm of compacted hardcore is typical for a garden shed or summerhouse. Larger workshops, log cabins and garden rooms need more.

How much does a concrete base cost?

It depends on the size, the ground conditions, how much soil has to be taken away and whether a mixer can reach the spot. There's no standard price, so send us the rough size and a photo of where it's going and we'll ask Midland Base Works to quote.

What are shed bearers?

Bearers are the pressure-treated timber rails that run underneath a shed floor. They spread the building's weight across the base and hold the floor slightly clear of it so air can circulate and the timber stays dry.

How far apart should shed bearers be?

It depends on the building, so follow the manufacturer's instructions. On most sheds the bearers come fixed to the floor panels at the right spacing. If you use plastic supports such as The Shed Boot, fit them roughly 30 to 50cm apart along each bearer.

Should a shed base be bigger than the shed?

No. Make it the same size as the floor or very slightly smaller. A base that sticks out lets rainwater collect and splash back onto the cladding, which encourages rot.

Does a log cabin need a concrete base?

A concrete base is the usual choice for a log cabin, garden room or home office, because these buildings are heavy and must stay perfectly level and square. Always check the manufacturer's instructions for your model.

Can I put a shed straight on grass?

It's best not to. Ground holds moisture, which can creep into the floor and cause damp and rot. Use a proper base that lifts the building clear of the ground.

How do you prepare the ground for a shed base?

Mark out an area the same size as the shed floor and check it's square by measuring the diagonals. Remove the turf and soft topsoil down to firm ground, then lay and compact a hardcore sub-base. Lay paving slabs level on mortar, or pour concrete into level formwork and let it cure. Check it's level in every direction before the shed goes on.

What is the best base for a log cabin on uneven ground?

Make the base level first rather than packing under the cabin. That usually means a concrete slab with the high side dug out or the low side built up. Raised frame or ground screw systems are sometimes used on slopes, but they must be accepted by the cabin's manufacturer. A professionally laid base is the safest choice on uneven ground.

Does a metal shed need a base?

Most metal sheds need a firm, level base and should be anchored to it. Lifelong steel sheds are the exception: Lifelong's own team installs them straight onto fairly level ground, packing out the footings as part of the installation, so no concrete or slab base is needed.

What is the best shed base for wet UK weather?

Concrete is the most solid, and paving slabs on a well-compacted sub-base work well for most sheds. Whatever the base, keep the timber off the ground: lift the bearers on plastic supports or a pressure-treated, ground-contact rated frame so air can move and water can't sit underneath. Avoid low spots where water collects.