Shed Base Estimator
Concrete slab, paving slabs or timber frame — complete shed base shopping list from your shed size. Sub-base, DPM and formwork all included. 2026 UK pricing.
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Build Guide
Which Base Type is Right for You?
Concrete slab — the permanent choice
Best for: workshops, larger sheds, metal sheds, anything that will stay in place for many years. Excavate 200–250mm deep, compact 100mm MOT Type 1, lay DPM, build formwork from 100×25mm treated timber, pour and tamp C20 concrete at 100mm depth. Allow 7 days minimum before loading — 28 days for full strength.
Concrete pours over 0.5m³ are generally more cost-effective with ready-mix. Under 0.5m³, mixing bagged concrete on-site is more practical. A standard 8×6 shed base (4.47m²) at 100mm = 0.45m³ — right on the boundary.
Paving slabs — the DIY-friendly option
Best for: smaller sheds on level ground, situations where you may want to move the shed later. Compact 75–100mm MOT, screed 25mm sharp sand, lay 450×450mm slabs in a grid. No mixing required. Less permanent and less flat than concrete, but much easier to achieve alone.
Gaps between slabs allow good drainage but also allow weeds — lay geotextile membrane under the sand layer to suppress them. Keep joints filled with kiln-dried sand to prevent movement.
Timber frame — best drainage
Best for: log cabins, eco sheds, sloping sites, situations where the shed manufacturer specifies a timber base. Compact 75mm gravel or MOT, lay geotextile membrane, fix 150×47mm or 100×47mm pressure-treated bearers to a perimeter frame at 400mm centres. The shed floor sits directly on the bearer frame, which lifts it clear of the ground for excellent airflow and drainage underneath.
FAQ
Common Questions
Yes — add 50–75mm on all sides. This gives a small overhang to shed water away from the edge and a margin for positioning the shed. For example, for a 2.44×1.83m shed, make the base 2.54×1.93m. Simply add your margin to the dimensions before calculating.
C20 (GEN 3) is standard for shed bases. Equivalent site-mix: 1 part cement, 2 parts sharp sand, 3 parts 10mm aggregate — or 1 part cement to 4–5 parts ballast (combined aggregate). For a ready-mix order, specify C20 GEN 3 or ST3. Do not use the very weak ST1/GEN 1 mix sometimes sold for garden use — it is too weak for a shed base.
Minimum 24 hours before light foot traffic. 7 days before placing a shed. 28 days for full compressive strength. In cold weather (below 5°C) protect with insulating sheeting and add 50% to these timescales. Never pour concrete when temperatures are below 2°C or forecast to fall below 0°C within 24 hours.
For a standard garden shed: no. The 100mm unreinforced C20 slab on a compacted sub-base is adequate for all normal shed loads. For a workshop with heavy machinery, a garage taking vehicle loads, or any base over 4m in any dimension: consider A142 mesh reinforcement laid mid-depth. Our estimator does not include reinforcement mesh — add it separately if needed.