How Much Concrete for Footings?
Strip footings are the standard foundation for garden walls, boundary walls, and low retaining walls in the UK. Building regulations require footings to be at least 225mm deep and extend below the frost line. For 3m of footings (450mm wide x 225mm deep), you may need approximately 0.304m³ of concrete, which is around 38 x 20kg bags of ready-mix. This guide takes you through trench excavation, levelling, and pouring footings that comply with UK building standards.
Planning and Preparation
Dig the trench to the required depth using a mini excavator for long runs or a spade for shorter sections. The trench bottom must be on firm, undisturbed ground — never pour footings onto loose backfill. Standard strip footings for a garden wall should be at least 450mm wide (three times the wall width for a single-skin wall) and 225mm deep minimum. Check the trench is level along its length using a spirit level and straight edge. If the ground slopes, you can step the footings in increments equal to the brick or block course height (typically 225mm). Remove any loose soil from the trench bottom before pouring.
Mixing Concrete
For this project our estimate suggests around 0.304m³ of concrete. You have two options: use approximately 38 x 20kg bags of ready-mix concrete (such as Blue Circle, Postcrete, or Hanson Multicem), or mix your own using a C20 mix ratio of 1:2:3 by volume. To mix your own, you'll likely need around 83.5kg of Portland cement (4 x 25kg bags), 178.2kg of sharp sand, and 300.7kg of 20mm gravel aggregate — a total of roughly 562kg of materials. For small batches, mix by hand on a clean board or in a wheelbarrow: combine the dry materials first until the colour is uniform, then gradually add water until the mix holds its shape when squeezed but is still workable. For larger volumes, hire a cement mixer — a half-bag (electric) mixer handles most DIY jobs and costs around £25-35 per day from tool hire shops. Avoid adding too much water, as this weakens the finished concrete significantly.
Pouring Technique
Pour the concrete directly into the trench in one continuous operation if possible. The concrete should fill the trench to the marked level on the profile boards. Use a shovel or a piece of timber to work the concrete into the corners and against the trench walls. Tamp the surface level with a straight edge or a length of timber. The top of the footing does not need to be perfectly smooth, but it must be level so that the first course of bricks or blocks sits evenly. If the trench is deep, pour the concrete in layers of about 150mm, compacting each layer before adding the next. For stepped footings on a sloping site, pour the lowest step first, then work uphill, allowing each section to support the next.
Weather Considerations
Footings are typically poured into a trench, which offers some natural protection from wind and direct sun. However, the same temperature rules apply: avoid pouring below 3°C and take precautions in very hot weather to prevent rapid drying. If the trench fills with rainwater overnight, bail it out and let the bottom dry before pouring. Pouring concrete into standing water will wash out the cement and produce a weak mix at the base of the footing. Cover the top of the footings with polythene sheeting after pouring and leave for at least 24 hours before starting brickwork. In cold weather, extend this to 48 hours and use insulating material over the polythene to retain the heat of hydration.
Curing Times
Footings should be left to cure for at least 24 hours before any brickwork begins. For garden walls, 48 hours is a safer minimum, especially in cooler weather. Structural engineers typically specify 7 days before loading footings with significant weight. The concrete reaches approximately 50% of its final strength after 3 days and 65% after 7 days. For a standard garden wall, starting brickwork after 48-72 hours is common practice among builders, provided the weather has been mild. Keep the surface of the footings damp during curing by covering with polythene. In summer, this prevents the surface from drying out and forming a weak, dusty layer that reduces the bond between the footing and the mortar bed for the first course of bricks.
Estimated Cost
Prices for concrete materials in the UK vary by region and supplier. Using ready-mix bags, approximately 38 x 20kg bags may cost around £152-£228 at typical retail prices of £4-6 per bag. If you mix your own, the individual materials may cost roughly £95-£147 (cement £20-£32 for 4 x 25kg bags, sharp sand £35-£55 for 1 bulk bag, aggregate £40-£60 for 1 bulk bag). For a job this size, mixing your own concrete from bulk materials may be significantly cheaper than buying individual ready-mix bags. Consider ordering a bulk bag (dumpy bag) of sand and aggregate from a local builders’ merchant for the best price. If you need to hire a cement mixer, expect to pay £25-35 per day from most tool hire companies. A wheelbarrow, shovel, and spirit level are also essential tools you may need to buy or borrow.