What is the underpinning sequence?

Traditional mass concrete underpinning follows a hit-and-miss sequence: odd-numbered bays first (1, 3, 5...), then even-numbered bays (2, 4, 6...). This ensures you never excavate adjacent bays simultaneously, maintaining wall stability. Each bay must be fully concreted and cured (typically 7 days minimum) before the adjacent bay is excavated.

How wide should underpinning bays be?

Standard bay widths are 1.0-1.5m. The maximum bay width should not exceed 1/5 of the wall length or 1.5m, whichever is smaller. Narrower bays (1.0m) are safer for weaker walls. The bay width is measured along the wall face. Each pin should extend 150mm wider than the wall on each side.

How deep does underpinning need to go?

Underpinning must reach a depth where the soil has adequate bearing capacity and is unaffected by seasonal moisture changes. In the UK, this is typically 1.0-1.5m below ground level, but may be deeper near trees on clay soil (up to 3m+ in extreme cases). A structural engineer's assessment and trial holes are essential.

What concrete grade for underpinning?

Mass concrete underpinning typically uses C25/30 (GEN 4) concrete per BS 8500. Some engineers specify C30/37 (RC 30) for greater strength. The concrete is poured into the excavated bay and left with a 75mm gap to the underside of the existing foundation, which is later filled with a semi-dry mortar dry pack that is hammered tight to ensure full contact.

How much does underpinning cost in the UK?

Traditional mass concrete underpinning typically costs £1,000-1,500 per linear metre in the UK (2026 prices). A typical terraced house requiring 6-8m of underpinning costs £6,000-15,000. Costs vary significantly based on depth, access, soil conditions and whether internal floors need to be removed. Mini-piled underpinning is more expensive but faster.

TradeCalculator.co.uk

Underpinning Calculator — Bay Sequence, Volume & Concrete

Calculate underpinning bay count, sequence (1-3-5-2-4-6), pin volume and mass concrete for traditional underpinning.

Wall Length

Existing Foundation Depth

Depth of current foundations

New Foundation Depth

Required underpinning depth

Wall Width

Thickness of the wall above

Bay WidthMax 1.5m per bay

Concrete Price

How We Calculate This

This calculator determines bay count, excavation sequence, concrete volume and dry pack quantities for traditional mass concrete underpinning.

Bay sequence

Bays = ceil(Wall Length / Bay Width)
Sequence: odd bays first (1-3-5), then even bays (2-4-6). Never excavate adjacent bays simultaneously.

Concrete volume

Pin Volume = Bay Width x Pin Width x Pin Depth
Pin width = wall width + 150mm extension each side.
Pin depth = new foundation depth - existing foundation depth. Dry pack at 75mm depth fills the gap under the existing foundation.