Shed Base Calculator (Concrete & Paving)

Shed Base Guide

Why a Good Base Matters?

A solid, level base is the most important factor in shed longevity. Without a proper base: the floor rots as moisture wicks up from the ground, the structure goes out of square as the ground moves, doors and windows jam as the frame distorts, and vermin can access from below. The base should be 100–150mm larger than the shed footprint on all sides. A concrete base is the most durable; paving slabs are a popular DIY alternative; eco-grid on compacted gravel allows drainage.

What's the key thing to understand about Concrete Slab Base?

100mm concrete over a 75mm compacted hardcore sub-base is the standard. For a 2.4×1.8m shed (base 2.7×2.1m = 5.67m²): concrete volume = 5.67 × 0.1 = 0.567m³. Hardcore: 5.67 × 0.075 = 0.425m³ ≈ 0.75 tonnes. Concrete mix: 1 part cement, 2.5 sand, 3.5 aggregate (C20 general purpose mix). A 25kg bag of ready-mix covers approximately 0.01m³ of finished concrete — so roughly 60 bags per m³. Always add a damp-proof membrane (DPM) over the hardcore before concreting.

What's the key thing to understand about Paving Slab Base?

600×600mm paving slabs on a 25mm sand bed over 75mm compacted hardcore. For 2.7×2.1m base: 2.77m² ÷ 0.36m² per slab = 8 slabs minimum (always buy 10% extra for cuts). Dry-lay first to check the layout. Use a rubber mallet and spirit level to ensure the slabs are level (slight fall of 1:100 away from the shed for drainage). Ensure slabs are supported at all four corners — a central spot of sand is insufficient and causes cracking.

What's the key thing to understand about Planning Permission?

A shed base itself does not require planning permission in England and Wales. The shed does not require permission if: it is in the rear garden, has a ridge height under 2.5m or eaves under 2m, does not cover more than 50% of the garden area, and is not in a conservation area or AONB. Check the current rules at PlanningPortal.gov.uk before starting work, as rules vary and may have changed. Council tax is not affected by a standard garden shed.

What do I need to know about Shed Base Size vs Shed Footprint?

A shed base is almost always built slightly larger than the shed's own footprint, not sized exactly to match it, and getting this margin right matters for both the finished look and the shed's long-term condition. A common rule of thumb is to make the base around 50-100mm larger than the shed on each side, meaning the overall base is roughly 100-200mm wider and longer than the shed itself, which gives a small amount of tolerance for the shed frame to sit square on the base even if the base isn't dug and levelled with absolute precision, and avoids the shed's timber frame overhanging the edge of the base where it would be exposed to standing water and ground moisture. Oversizing a base by too much, however, wastes material and can leave an unsightly exposed concrete or paving margin visible around the shed once it's installed, which is why this calculator's oversize input is intended to apply a sensible margin rather than an arbitrary large one. For sheds that will have a felt roof with an overhang, it's also worth checking the oversize allowance still leaves enough base for the shed's actual footprint once any projecting roof eaves or gutters are accounted for, rather than basing the oversize purely on the floor dimensions.

What should I know about Site Levelling Before Laying a Base?

Whatever base type is chosen, concrete slab or paving slabs, the ground beneath needs to be properly levelled and compacted first, and this preparation stage matters as much to a shed's long-term stability as the base material itself. The area should be excavated down to firm, undisturbed ground, removing any topsoil, grass and organic material, since organic matter left under a base will eventually decompose and create a void, leading to settlement and an uneven base years after installation. A compacted hardcore sub-base layer, typically well-compacted crushed stone or similar granular material, is then laid and levelled before the concrete or paving slabs go down, giving a stable, well-draining foundation that resists frost heave and waterlogging better than building directly onto undisturbed clay or soft ground. Checking the base is level in both directions using a spirit level or string line across the full width and length, not just checked by eye, avoids a shed sitting slightly tilted, which over time can cause doors to bind, roof felt to pool water in a low corner, or timber cladding to take on water unevenly on the lower side, shortening the shed's working life even though the shed itself was well-made.

What do I need to know about Plastic Grid Shed Base Systems?

Plastic grid base systems (interlocking honeycomb-style panels, often filled with gravel or laid directly on a prepared, compacted sub-base) offer a lighter, faster alternative to a full concrete slab for smaller and medium sheds. They work by spreading the load over a wide area while allowing water to drain through and away rather than pooling, which suits sheds in gardens with naturally free-draining soil. Installation involves excavating and levelling the area, laying a weed membrane, adding and compacting a sub-base of hardcore or sharp sand, then clipping the grid panels together on top and filling the cells with gravel (or sometimes leaving them open under a timber floor). They're significantly quicker to install than concrete — often a weekend job rather than requiring days for curing — and avoid the cost and mess of ordering or mixing concrete. However, they're generally only rated for lighter structures and lower point-loads than a solid concrete slab, so they're best suited to standard timber or smaller metal/plastic sheds rather than heavy workshop-style buildings, large log cabins, or anywhere a vehicle might need to cross. Manufacturer load ratings should always be checked against the shed's actual weight and intended use before choosing a grid system over concrete.

Worked Example: Concrete Base for an 8x6 Shed

An 8ft × 6ft shed (approximately 2.44m × 1.83m) is one of the most common garden shed sizes, and its base should typically extend slightly beyond the shed footprint to support the full frame.

This base needs roughly 0.59 cubic metres of concrete, which, like the garden shed example elsewhere on this site, typically falls below most ready-mix suppliers' minimum load — worth checking before assuming a truck delivery is the cheapest option, since a job this size is often better served by bagged concrete mixed on site or a smaller specialist minimum-load delivery if the supplier offers one. The 100mm margin added around the shed footprint matters structurally, not just cosmetically: it ensures the shed's base timbers or bearers sit fully on solid concrete rather than overhanging the edge, which over time causes the frame to sag at the corners and the shed door to stop closing properly.

Drainage Considerations for Shed Bases

A shed base that looks solid can still cause serious problems years later if drainage wasn't considered at the planning stage. Any concrete or paving slab base should be laid with a slight fall — typically a gradient of around 1:60 to 1:80, barely noticeable to the eye — sloping away from the shed's doorway or away from the house, so rainwater runs off the surface rather than pooling against the shed's timber base and causing rot. A base laid perfectly flat, while easier to achieve with basic tools, is actually a common cause of long-term shed timber decay because water that lands on it has nowhere to go and sits against the wood until it evaporates or soaks in. Underneath the slab, a compacted sub-base layer of hardcore or crushed stone (typically 100-150mm) serves a drainage function as well as a structural one, allowing any water that does reach it to disperse into the ground rather than pooling beneath the slab and causing frost heave in winter. For sheds sited in naturally damp or low-lying parts of a garden, raising the base slightly above surrounding ground level, or adding a French drain or soakaway nearby, prevents the much bigger problem of water pooling around the base after heavy rain and slowly undermining it. Plastic grid shed base systems address this differently by allowing water to drain straight through the open grid structure into the ground below rather than needing a fall at all, which is one reason they've become popular on sites where getting a precise concrete fall right is difficult.