What is underpinning, and when is it needed?

What is underpinning?

Underpinning is the process of extending an existing foundation downwards, or transferring its load to something deeper, without taking the building down. In London, it most often means taking shallow Victorian or Edwardian footings down through the upper layer of London Clay, which shrinks and swells with the seasons, to ground that moves far less.

It is also used where a building needs to carry more load than its foundations were built for, or where new work next door will dig deeper than the existing footings. The method is chosen to suit the ground, the building and the reason for the work.

When underpinning is really needed

Most cracking in London houses does not need underpinning. When subsidence is caused by a thirsty tree or a leaking drain, removing the cause and then monitoring often stabilises the building, and the cracks can be repaired without touching the foundations. Insurers usually take this route first. Our guide on subsidence signs and what to do explains that process.

Underpinning becomes the right answer when movement continues after the cause has been dealt with, when the cause cannot be removed, or when the foundations are plainly inadequate for the building.

• Ongoing movement shown by monitoring, even after trees or drains have been dealt with
• A protected or neighbouring tree that cannot be removed
• A basement or lowered floor that takes excavation below the existing footings
• Extra loads from an added storey or a mansard
• A new extension or neighbouring building with deeper foundations alongside
• Footings that are damaged, very shallow or built on poor made ground

Traditional mass concrete underpinning

The most common method on London houses is mass concrete underpinning. The wall is dug out beneath in short sections, called bays, and each bay is filled with concrete down to the depth set by the engineer. A narrow gap is left at the top and rammed tight with a stiff dry-pack mortar so the wall sits firmly on the new base.

The bays are dug in a set order so that no two adjacent bays are open at once and the wall is never left unsupported over a long length. This hit-and-miss sequence is what makes the method safe, and the engineer's drawing shows it bay by bay. Depth depends on the clay, nearby trees and drains, and is covered in our guide to London Clay, trees and foundations.

Each excavation is normally seen by Building Control or the engineer before concrete goes in, so the formation can be checked and the depth adjusted if the ground is softer or wetter than expected. The dry-pack is placed only once the new concrete has gained enough strength, and the next bays follow in the order on the drawing. Done properly, this is slow, methodical work, and rushing it is where problems start.

Other underpinning methods

Mass concrete becomes slow and disruptive when the stable ground is deep, access is tight or the ground is wet. In those cases the engineer will look at alternatives.

The choice depends on the depth to stable ground, the space for equipment, how much disruption the occupants can live with, and the loads involved. Piled solutions need specialist rigs, but small rigs can work inside a house or down a narrow side passage, which makes them practical for many London terraces.

• Beam and base: a reinforced concrete beam is formed under or through the wall and carried on a smaller number of deeper concrete bases
• Mini-piled underpinning: small-diameter piles are installed either side of the wall and connected by needle beams or a capping beam that picks up the load
• Piled raft or ring beam: used where a whole building or extension needs new support, with piles carrying a continuous beam
• Resin injection: expanding resin is injected into the ground to stiffen it, which can suit some lighter structures and specific ground conditions

Full or partial underpinning

It can be tempting to underpin only the corner that has cracked. Sometimes that is the right call, for example under a bay window or a back addition that has moved on its own. But stopping the underpinning partway along a wall creates a junction between a deep, stable foundation and a shallow one that still moves with the clay. That junction can crack in future.

The engineer weighs this up using crack patterns, monitoring results and trial pits. Partial underpinning is often reasonable if the rest of the building is stable, but the design should deal with the junction deliberately rather than leaving it to chance. Our underpinning design and sequence service sets out the extent, depth, bay layout and dry-packing in drawings the builder and Building Control can follow.

Approvals and neighbours

Underpinning is building work, so it needs Building Control approval and inspections, particularly of each excavation before concrete is placed. If you are digging within 3 metres of a neighbour's building and deeper than their foundations, the Party Wall etc. Act 1996 applies and notice must be served. Underpinning a party wall itself also needs notice.

If a public sewer runs nearby, Thames Water may need to agree the work. Where the underpinning is part of an insurance claim, the insurer and their engineer will usually agree the scheme before it starts. Our basement conversion guide covers the extra steps when underpinning forms part of a new basement.

Living with an underpinned house

A properly designed and inspected underpinning scheme gives a building a sound, deep foundation, and many London houses have been underpinned without further trouble. Keep the Building Control completion certificate, the engineer's design and any insurer's paperwork together. Buyers' solicitors and insurers will ask about past underpinning, and a complete file makes those conversations short.

After the work, there may be minor cracking as the building settles onto its new foundations. Once that has stopped, the cracks are filled and redecorated. If cracks keep opening after that point, ask the engineer to inspect rather than assuming it is cosmetic.