Getting foundations right is not glamorous, but it is the bit that keeps everything else standing. Whether you are pouring a garden room slab, a single-skin wall footing or a stepped trench for an extension, the smartest money you spend is on planning the mix, volume and pour sequence. You will see how to measure and calculate volumes accurately and how to use an online calculator to sanity-check your figures. You will also learn why volumetric on-site mixing is a real advantage for staged pours, step footings and tight access.

First principles: soil, loads and footing sizes

Concrete only performs as well as the ground beneath it. Before you size a footing, confirm:

• Soil type and bearing: Dense gravel and chalk have high bearing capacity. Firm clays are common but can shrink and swell with moisture and tree influence. If in doubt, dig trial pits and consult local guidance or a structural professional.
• Frost and vegetation: Keep the underside of strip footings below the frost line and at a safe distance from trees and hedging to limit heave risk.
• Load path: A cavity wall or heavy masonry carries more load than a timber frame. Concentrated loads from point foundations under columns may call for pads or thickening.

Typical domestic rules of thumb (always confirm with drawings, Building Regulations and your engineer where required):

• Strip footings for low-rise cavity walls: commonly 600 mm to 1,000 mm wide, thickness 150 mm to 300 mm depending on soil and load.
• Trench-fill foundations: concrete to within 150 mm to 300 mm of final ground level, typically saving brickwork and speeding builds on poor ground.
• Pads for posts or garden rooms: often 600 mm square by 300 mm deep, spaced per design.
• Slabs for sheds and garden rooms: 100 mm to 150 mm thick on a compacted sub-base, with thickened edges where loads concentrate.

Can you pour concrete over dirt?

Not directly. Remove topsoil and organic matter, then compact a stable sub-base. Pouring onto soft or loose ground leads to settlement, cracking and loss of level. For trench-fill, excavate to firm strata with clean, straight sides and no loose spoil. For slabs, lay a compacted sub-base and, where specified, a membrane.

Volumetric on-site mixing vs drum deliveries

Volumetric delivery brings raw materials to site and batches concrete at the point of pour. That gives you several practical advantages:

• Exact amount: You only pay for what you use, metered on the truck. No panic about a shortfall or disposal of leftovers.
• Exact mix: Agree the mix and slump, then tweak if weather or finishing needs change.
• No ticking clock: Because materials are mixed on site, you are not racing a drum’s workability window. Staged pours, step footings and tight access become manageable.
• Cleaner operation: Controlled discharge, tidy wash-out, and a pace set to match your team.

Estimating volume without guesswork

Calculate volume in cubic metres (m³). For simple rectangles: length × width × depth. Convert all dimensions to metres first.

For stepped or variable-depth trenches, split into sections and sum the volumes. For circular or pad foundations, use area × depth. Add a sensible contingency for trench irregularities and waste from spillage or over-excavation, typically 5 to 10 percent for strip footings with good formwork. Avoid padding numbers “just in case” when you have volumetric on-site mixing, because you can meter exactly what you use.

Use our online calculator on our website to cross-check your maths and generate a quick estimate, then request a quote and book your preferred time slot. We will confirm your mix and volume and keep the site moving.