Underpinning Walls in Old Houses

Now if you’re reading this because you have a settlement problem then stop, take a deep breath and try to relax. I know cracks, settlement and underpinning walls can induce bowel clenching fear into the strongest heart. But fear not; it’s often quite a simple repair. Note, I said simple, not easy! It is almost certainly going to be hard work and it’s fiddly to do in confined spaces. But it is doable if you have the time and a little patience. It’s not even terribly expensive material wise, concrete and mortar are cheap enough.

Coincidentally; I recently found myself deep underneath an old house after a worried call from an old client. When I arrived and saw the state of the wall, I realised this was one rather tricky underpinning job. But I’m glad they called me, I quite like a challenge! To be honest, underpinning walls is not something I do every year, but once in a while I get the chance to happily crawl around underneath a house and prop up a problem wall or two.

collapsed wall
Collapsed wall showing the loose fill construction…

The image above is in the cellar underneath the house and this is where I was working. This is pretty unusual, normally you’d be digging down around the outside of the wall to get underneath it. But on this house I had a head start as the cellar floor was already about a meter below the outside ground level (plus it was dry, about 13 degrees C and wet outside…).

800mm Thick Stone Walls

This job wasn’t a regular brick wall though, and it wasn’t a simple case of digging underneath and filling the resulting holes with concrete. No, this was a random, loose filled stone wall built around 120 years ago, and here’s the thing; it was 800mm or 31.5″ thick, almost like a castle!

thick stone walls
The concrete was to be almost 800mm wide and 600mm or so deep at this point, fully underpinning the wall.

Undisturbed, walls like this can stand strong for centuries, but because of their somewhat ‘dry built’ nature, if something moves, or if you pull a stone out, whole sections can tumble down like a house of cards. Tricky work indeed.

This particular wall wasn’t sinking though, just in a precarious position due to some digging in the past to create some extra space in the cellar. Plus, some recent blasting nearby resulted in a largish section of stonework collapsing into an indoor rockery with no hope of a redeeming water feature…

Not Much in the Way of Foundations

Doubly tricky because there were no real foundations as such in those days and certainly no concrete footings. Back then, you’d just build in a very shallow trench on a layer of fine blinding (small crushed stone usually). The ground in this area is a mix of hard stone with occasional soft, crumbly areas, which is fine in most cases to build straight on.

wall hanging on
The largely dry stacked wall was precariously sitting on a little loose fill blinding, on top of sloping bedrock…

I couldn’t just go in and dig out large 1m wide bays to underpin as normal, as I’d risk the wall above collapsing totally. I needed a softly, softly approach. With one exception (where the stone wall itself was missing), I went for half meter wide bays as this gave me the best chance of not losing any more walling during the work.

excavate bays under wall
Here is a half meter wide bay dug underneath the stone wall down onto good ground…

Instead, I excavated each bay gently, rather like an archaeological dig. The aim was to remove any soft topsoil and loose material until back onto good ground; rock preferably, or as in some parts, a stable looking, ballast type subsoil. I wondered if there had been water running through this gap in the bedrock at some point in the past (hence the sand).

underpinning walls with concrete
You always need a gap above the formwork or shuttering to get the concrete in…

Working like this meant I could use the very minimum of new concrete under the wall to support it. Often forgotten, but minimising the weight should be a factor in underpinning. This makes sense if you think about it, because more weight means more downward pressure on the ground; ergo the better the ground needs to be to support the load. And we already know this wall is in trouble!

Commonly, walls which need underpinning are sinking on some poor ground maybe, or on top of leaking drains etc. Throwing in tons and tons of extra weight via concrete could, in theory, make things worse.

bay of underpinning
Transferring the weight of the wall onto the bedrock below…
steel reinforcing underpinning walls
Adjacent bays are linked together using thick steel reinforcing bars…
Don’t forget to liberally splash a thin cement slurry everywhere before pouring the concrete; this reduces the suction between the new and ‘old’ concrete. Mix water and cement into a thin, easily splashed slurry and flick on with a large brush.

Underpinning Walls Traditionally

Often when underpinning walls, folks dug small pits under the brickwork and flooded them with concrete. I had a friend who used to do this full time. All his digging tools had really short handles (to make it easy to dig in a small square pit underneath a wall) and he drove a tiny little van too. He used to joke that he bought them as a job lot from a Gnome…

A textbook underpinning bay layout, but each one tends to be pretty unique. This shows digging the pits from the outside, but if there is a cellar you could be digging from the inside.
Standard underpinning, (image taken from the book “How to be Handy“, or you can read more about underpinning here…)

But you need to be extra careful under a loose fill, random stone wall (or poor brickwork), because the wall would collapse into the pits before you can get the concrete in. So I opted for a gentler way to provide the support the wall needed without going crazy with the concrete (and even then, close to 3 tons of material went into the job).

underpinning walls
Finding solid strata is key, rather than just digging away blindly to hit a certain depth…

I decided to use a kind of hybrid repair to underpin these walls, because they were not sinking, just unsupported. Again, there was a real risk of the wall collapsing if I went crazy, digging out big bays etc. Plus I wanted to take advantage of some good bedrock not far away, which was perfect for supporting the walls.

I filled all the soft areas with mass filled concrete and supported the good areas with concrete pinned to the bedrock. After all, there is no point in removing good supporting ground just to replace it with concrete.

Once I had stabilised the ground underneath the wall with concrete, the wall was once again fully supported. Next was to rebuild the fallen area of stonework. Random doesn’t even begin to describe it! All sorts of sizes and even types of stone went back into this rough wall. And because old habits ‘die hard’, I finished off the joints with a section of truck fan belt and a light brush to leave it looking tidy (even though hardly anyone will ever see it and the rest of the walls were very, err, rustic lets say!).

concrete bay under underpinned wall
Concrete in place, time to put together the giant stone jigsaw!

I don’t know why the spirit level is in the above image, I certainly couldn’t use it to build this wall. Building by eye was the only way, but I did have a string line right at the top so I could at least end up fairly straight!

finished underpinning walls
All finished…

Hard and steady work for sure, but rewarding in it’s own way I think, even if hardly anyone will ever see it. I reckon it should be good for another hundred and twenty years at least 🙂

Stay well,


Comments 11

  1. Thanks Ian for your detailed reply. I started digging a trial hole, 2m down and haven’t reached the foundation yet. I’ll be digging again once some other jobs are finished with on site. The soil so far is all backfill so I haven’t reached any of the original ground.

    1. Post

      Wow, 2m? I have not seen foundations that deep in all my years!
      Well, best of luck and feel free to send or post pics if you think I can help.
      Stay well,

  2. Why does this house have a castle style wall filling? Was the owner worried about sieges?

    1. Post
  3. Hi Ian, thanks for your article. What would you have done if you couldn’t find solid enough ground for to build on? Could you have looked at digging some trenches across the cellar floor from one wall to the opposite wall, filling with concrete and using them as a type of raft foundation?
    I’m asking as I’m in Ukraine and the topsoil is notoriously deep.

    1. Post

      Hey Noel,
      Hmm, if the ground is poor you’re going to need to increase either the area or the depth. By area I mean you need to try and spread the load over a wider area, like a raft as you say. Typically full rafts are used on poor ground so the load is spread out evenly over the whole house.

      I think digging across the whole floor of the cellar could be problematical as the ‘beam’ if you like, would need to be very strong to resist the downward pressure on its ends (if I’m picturing it correctly). More likely it would break the concrete some way inside of the walls. As you might know, concrete is incredibly strong under compression, but crazy weak under tension.

      Otherwise, what piling options are available? There are folks here who ‘drill’ holes into the ground, usually close to the wall and then concrete on top, putting the house on ‘stilts’.

      What you’ve got to watch is weight. Intelligent solutions are always better than just throwing concrete at it (folks often think this is the cure). Picture your soil, it’s already struggling to support the weight (hence the problem) so adding more weight has the potential to actually make things worse.

      I’d be inclined to at least dig a trial pit half a meter wide say and see exactly what you’ve got under there. Make sure also that the problem isn’t a localised one because of a separate problem (like leaking drains, tree roots etc. etc.). If you find a reasonable bearing sub-soil then go wide and cast a strip concrete foundation with steel and then build a wall off it and up to the bottom of your old walls. Maybe, difficult to say without being there!

      Let me know your thoughts…
      Cheers for now

  4. Many thanks for this detailed information Ian! very timely for me as I am about to complete underpinning on a Random rubble wall here in Normandy France (with large stones at the base)

    The adjoining wall needs to be supported I believe by a Buttress, would you be willing to cover this operation in a future article please?

    Bien Cordialement


  5. Very very interesting. I would have fear of working in a basement which could collapse at any moment so I guess there may be an element of divine trust. The finished job looks so professional and must give you a lot of satisfaction. The construction process was well explained and the diagrams and photographs were so helpful that I would be tempted to have a go myself, although age is not in my favour. Obviously insurance cover could be a big factor when undertaking this sort of project.

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      Thanks John.
      To be honest, I assessed that the risk was close to negligible. Because it was a wooden house sitting on top of a large wooden ground beam which would be able to span quite a distance, should part of the wall have collapsed underneath it.
      Still, softly, softly approach is sensible in most cases.
      Thanks again for your kind words John.

  6. Hi Ian,

    Fascinating account, it’s this sort of descriptive sequence and the reasons for the methods employed that I appreciate as an Engineer. I’m unlikely ever to do anything related to building works but I still like to understand what is required – “knowledge is power” as they say. I’m off now to read your articles on lath and plaster as I’ve an 1930’s house with some walls and ceilings with the same which may need attention.

    Best regards, Victor.

    1. Post

      Thanks for the kind words Victor!
      Ah the beauty of the internet, we get to read all sorts.
      Last night I read about the couple who were arrested for stealing from Bernie Ecclestone’s daughter, which lead me to read about Bernie, which led me to the yacht he sold to Eric Clapton and that lead me to read a long article about what was done to the boat as part of the 5 million pound re-build!
      Good luck with the lath and plaster, let me know if you have any questions…

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