Foundations
Underpinning in Southport
Cracks in the walls are worrying. This page explains, in plain English, what underpinning is, who decides if you need it, and what affects the cost. When the engineer's design is ready, we dig and build it with our own machines.
What underpinning actually is
Every house sits on foundations: concrete or brick under the walls, below the ground. Underpinning means digging under those foundations and adding new concrete beneath them, so the house rests on firmer ground deeper down.
It is usually needed because of subsidence, which is when the ground under the house moves down and takes part of the house with it. It can also be needed when a new extension or basement next door digs deeper than your foundations.
It is a serious job, but it is a well known one. Done properly, in short sections, the house stays standing and you can usually stay living in it.
Found a crack? Ring your insurer first
Before you pay anyone, ring your buildings insurer. If it is subsidence, most policies cover it, and they will want their own engineer to see it before any work is done.
Take photos of the cracks now, with a coin or a ruler next to them, and the date. It shows whether they are getting worse.
Not every crack means subsidence
Most houses have some cracks. These are the ones to get looked at, and the ones that are usually nothing to worry about.
Get these looked at
- Cracks wider than about 3 mm, the thickness of a pound coin
- Diagonal cracks that are wider at the top, often running from the corners of doors and windows
- Cracks you can see on the outside and the inside of the same wall
- Doors and windows that start sticking for no reason
- An extension or bay window pulling away from the house
- Cracks that keep getting wider over weeks or months
Usually normal
- Fine hairline cracks in new plaster as it dries out
- Small cracks where a ceiling meets a wall
- A new house settling in its first few years
- Cracks that open a little in summer and close in winter, and never get worse
Fix the cause first
Underpinning a wall while the thing that moved it is still going on is money wasted. The usual causes are:
- Leaking or broken drains. Water escaping from an old drain can wash sandy soil away from under a foundation. Around Southport, a lot of the ground is sand.
- Trees and big shrubs. On clay ground, roots dry the soil out and it shrinks, worst in hot, dry summers.
- Soft ground. Peat on the mosses, made up ground, or foundations that were simply too shallow for the soil they sit on.
Very often, once the cause is fixed and the ground settles down, the house only needs its cracks repairing.
Cracks near a drain
We do CCTV drain surveys, where a camera goes down the drain to show cracks and breaks, and we repair or reline what we find. That is often the first thing an engineer asks for. See our drainage work.
Who does what
Underpinning involves a few different people. Knowing who does which part saves a lot of worry.
| Who | What they do |
|---|---|
| Your home insurer | If it is subsidence, most buildings insurance covers it. They usually send a loss adjuster and an engineer, and decide what work is needed. |
| A structural engineer | Works out why the house is moving, often watches the cracks for a few months first, and decides if underpinning is needed at all. If it is, they design it: which walls, how deep, which method. |
| Building control | Underpinning normally needs building regulations approval. The council, or a private inspector, checks each section of new foundation before concrete goes in. |
| A party wall surveyor | Only if the work is on a shared wall, or digs close to and deeper than a neighbour's foundations. Notices go to the neighbour before work starts. |
| Darker | Digs and builds the underpinning to the engineer's design, a section at a time, and fixes any broken drains we find on the way. We take the spoil away on our own grab lorries. |
Ways of underpinning a house
The engineer chooses the method from what is under your house. These are the main ones.
| Method | What it is | When it is used |
|---|---|---|
| Mass concrete | The traditional way. Short sections dug out under the wall one at a time and filled with concrete, down to firm ground. | The most common method for houses where firm ground is not too deep. |
| Beam and base | A reinforced concrete beam built under or through the wall, resting on concrete pads spaced along it. | Where firm ground is deeper, or the load needs spreading along the wall. |
| Mini piles | Narrow steel or concrete columns driven or drilled deep into the ground, joined at the top by a beam that carries the wall. | Soft ground, peat or deep fill, where digging down to firm ground is not practical. |
| Resin injection | A resin pumped into the ground under the foundation that expands and firms the soil up. | Some cases of soft ground. A specialist job, and only where the engineer says it suits. |
How traditional underpinning is done
Mass concrete underpinning is dug in a set order, a short section at a time, so the wall is always supported. This is called hit and miss.
- 1
Engineer's design
We work from the structural engineer's drawings. They show which walls, how deep, and the order the sections are dug in.
- 2
Notices and approval
Building control is told before work starts. If a neighbour's wall is involved, party wall notices are served first.
- 3
Mark out the bays
The wall is split into short sections called bays, usually about a metre long, each numbered in the order they get dug.
- 4
Dig one bay
One bay is dug out under the old foundation, with the sides supported. Never two bays side by side, so the wall above is always held up either side.
- 5
Inspection
Building control checks the bottom of the hole has reached firm ground before any concrete goes in.
- 6
Concrete and pack
The bay is filled with concrete to just below the old foundation. Once it has stiffened, the gap is rammed tight with a stiff sand and cement mix called dry-pack, which takes the weight of the wall.
- 7
Next bay
Once that bay has set, the next one in the sequence is dug. We work along the wall until every bay is done.
- 8
Backfill and tidy
The ground is filled back in and compacted, any drains we have repaired are reconnected, and the spoil goes away on our grab lorries.
The cost of underpinning
No two underpinning jobs are the same, so any price before the engineer has designed it is a guess. What makes the biggest difference is how much wall needs doing and how deep the firm ground is.
Send us the engineer's drawings, or ring and tell us where things are up to, and we will price the groundwork properly.
What the price depends on
- How many metres of wall need underpinning: one corner costs far less than a whole house
- How deep the firm ground is. Every extra bit of depth is more digging and more concrete
- The method the engineer chooses. Mass concrete is usually the cheapest, mini piles cost more
- Access. A clear side path is easier than working through the house or a tight back garden
- What is in the way: drains, gas and water pipes, tree roots
- Fees on top of the building work: the engineer, building control and any party wall surveyor
Questions people ask
What is underpinning?
Making a building's foundations deeper or wider, so they sit on firmer ground. It is done under the walls of a house that is already built, usually because the ground under the old foundations has moved.
How much does underpinning cost?
It depends mostly on how much wall needs doing and how deep the firm ground is. One corner of a house is a very different job to every wall. Once the engineer has designed it, send us the drawings and we will give you a price for the groundwork.
Will my insurance pay for it?
If the cause is subsidence, most buildings insurance policies cover it, usually with a bigger excess than normal claims. Ring your insurer first, before you agree any work, or they may refuse to pay. They will normally want their own engineer to look at it.
Can subsidence be fixed without underpinning?
Often, yes. If the cause is a leaking drain or a thirsty tree, fixing the drain or dealing with the tree can be enough once the ground settles down. Many subsidence claims end with crack repairs and no underpinning. The engineer decides.
Do I need a structural engineer?
Yes. Underpinning has to be designed by a structural engineer and approved by building control. We do not guess the depth or the method, we build what the engineer has designed.
Do I need planning permission?
Not normally. You do normally need building regulations approval, with the engineer's design, and building control inspect the digs before the concrete goes in.
Can we stay in the house while it is done?
Usually, yes, because most underpinning is dug from outside. If the engineer's design needs work under inside walls or floors, some rooms may be out of use for a while.
How long does it take?
It depends on the length of wall. Each bay has to set before the one next to it can be dug, so even a short run takes longer than you might expect. We give you a timescale with the price.
Will underpinning affect selling my house?
It has to be declared, and buyers and their surveyors will ask about it. What helps is paperwork: the engineer's design, building control's completion certificate, and the insurer's certificate of structural adequacy if it was a claim. Keep them all.
My extension is cracking away from the house. Is that subsidence?
It can be. Extensions often have shallower foundations than the house, or sit near trees or old drains. Get it looked at by an engineer. Sometimes only the extension needs underpinning.
What about my neighbour?
If the work is on a shared wall, or digs within 3 metres of their building and deeper than their foundations, the Party Wall Act applies. Notices go to the neighbour: one month before for digging near them, two months before for work on the shared wall itself.
Also worth a look
Send us the engineer's design
Phone lines open Monday to Friday 8am to 5pm, Saturday 8am to 1pm. WhatsApp photos of the job any time, we reply in office hours.