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.

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.

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.

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.