Trench Fill Foundations
One pour. Done.
Replace three visits with one. Our flowable C25/30 or C28/35 mix fills the trench in minutes, no underground masonry, no cold joints, no delays.
The Mix You Need
C25/30-C28/35 Trench Fill
- Mix grade
- C25/30 or C28/35 (two-storey)
- Slump
- 100-125mm (flowable)
- Aggregate
- 20mm
- Typical width
- 450-600mm
- Fill depth
- Full trench less 150mm to ground level
- Sulphate-resisting option
- Available on request
Volume Guide
How much for a trench fill?
Figures based on ~850mm pour depth. Your actual depth depends on BCO requirements and soil, use our calculator or call for a tailored quote.
Small extension
4m × 4m footprint
16 lm × 450×850 trench
Medium extension
6m × 4m footprint
20 lm × 450×850 trench
Large two-storey extension
8m × 5m footprint
26 lm × 600×850 trench
New-build bungalow
10m × 8m footprint
36 lm × 450×950 trench
Why It Matters
Get this right and you won't redo it for 30 years
- Trench fill is the modern UK standard, no underground masonry, less labour, and foundations finished in one pour instead of three visits.
- On clay soils or anywhere NHBC heave precautions apply (near trees), trench fill with compressible liner is almost always the chosen method.
- A typical extension needs 5-15m³, a single ready-mix truck drops that in 15 minutes. Barrowing from a site mixer takes days.
- C25/30 meets domestic strip/trench fill spec. For two-storey or high-load walls, step up to C28/35, same price range, more margin.
Step by Step
How to prepare & pour
Design & soil investigation
Trench fill is only as good as the ground it sits on. Soil investigation (even a simple desk study + trial pit) tells you depth. On clay or near trees, NHBC foundation depth tables apply, often 1.5-2.5m deep.
Excavate deep & narrow
Dig the trench to full depth in one go with a mini-digger. 450mm wide is standard, 600mm for heavier loads. Keep sides vertical, trench fill relies on the concrete's self-support during the pour.
Heave precautions (clay soils)
On reactive clay, line the sides with compressible material (e.g. 50mm Clayboard) before pouring. This stops ground movement cracking your foundation in a dry summer.
Building Control inspection
Open-trench inspection is non-negotiable. Book the BCO for the morning of the pour so you can start as soon as they've signed it off.
Order flowable mix
Ask for a higher slump (100-125mm), this is trench fill, you need it to flow and self-level. We deliver anywhere from 1m³ to full wagon (8m³) loads, multi-truck deliveries arranged for bigger pours.
Pour & strike off
Discharge direct into the trench from the chute, no formwork, no tamping. Leave 150mm gap to ground level for the brickwork to start. Strike the top level with a spirit level and you're done.
FAQ
Common questions
Why choose trench fill over strip?
Speed. Trench fill replaces three visits (strip footings, then 4-5 courses of underground blockwork, then backfill) with one pour. On clay soils where you're going 1.5m+ deep, trench fill is almost always cheaper once you factor in labour and time on site.
How deep will my trench need to be?
Standard domestic minimum is 1.0m below ground. Clay soils: 1.0-1.5m. Near mature trees: 2.0-3.0m, governed by NHBC tables (tree species + distance determine depth). Your BCO or structural engineer will confirm.
Is trench fill more expensive than strip?
More concrete, less labour. For domestic work at ~1m deep, the cost per metre works out close. At 1.5m+ on clay, trench fill is usually cheaper total, because the labour saving on underground masonry exceeds the extra concrete.
What mix grade for trench fill?
C25/30 is the minimum spec for most domestic work. For two-storey extensions, heavy masonry, or sulphate-bearing soils, go to C28/35 or specify sulphate-resisting cement. Cost difference is small, margin is worth it.
How much wastage should I add?
Order 10% over your calculated volume. Trench walls are rarely perfectly vertical once the digger's been through, and a short-pour on trench fill means ordering a second truck, much more expensive than having a little over.
Can you supply large volumes for a new build?
Yes, full wagon loads up to 8m³, multi-truck for bigger pours. For plots needing 20m³+ we schedule back-to-back deliveries so the pour is continuous. Call 024 7636 1448 for new-build orders.
Ready to order?
Instant price in 60 seconds. Next-day delivery subject to availability. With volumetric, pay only for what's poured.