Underpinning & damp!

JDLDIY · 10 Oct 2016

Hi there. We want to extend onto our 1920s house, both alongside and going up. Preliminary digs have revealed next to no foundation, the walls appear to be cavity brickwork with render. Not sure whether there is a damp proof course, but if there is it's not working, we get write a bit of damp.

So, want I wanted to ask was what is the process for underpinning? I've heard of people doing it themselves and I can't see why I couldn't, room by room. As I go I would fix the damp problems also, what round be the method/options v there also?

Many thanks for any advice.

Himaginn · 10 Oct 2016

Underpinning and damp-proofing are two different processes, not necessarily related.
Underpinning is what goes on under the base of the walls, damp-proofing is what goes on above ground/floor level.

Here's an explanation of underpinning:
https://www.thebalance.com/what-is-underpinning-how-to-repair-foundations-845100

Notice especially this comment:

Underpinning in foundation should be addressed and supervised by an engineer.

JDLDIY · 10 Oct 2016

Himaginn said:
Underpinning and damp-proofing are two different processes, not necessarily related.
Underpinning is what goes on under the base of the walls, damp-proofing is what goes on above ground/floor level.

Here's an explanation of underpinning:
https://www.thebalance.com/what-is-underpinning-how-to-repair-foundations-845100

Notice especially this comment:

Underpinning in foundation should be addressed and supervised by an engineer.

Thanks, that's a useful link. Yes, I totally understand that the underpinning and damp proofing are different processes, but whilst I'm wrecking an area I might as well attend to both!
I will be doing this myself and so I won't be sinking piles. Unless of course the SE insists, in which case I'll have to stand back and let others get on with it. So, can I just check my understanding of the poured concrete method?
1. Floorboards up inside.
2. Excavate to specified depth, starting at corner, to specified width and length.
3. Check condition of ground which concrete will sit on, compact somehow if necessary?
4. Shutter sides with ply, but not bottom, presumably to a height just above the bottom of the first course of bricks?
5. Mix and pour concrete. Use a stick/mixer to get rid of air holes?
6. Wait for concrete to set, then start again...

That about it roughly?

Himaginn · 10 Oct 2016

I'd say number 1 is to consult a SE, discuss your proposals and take advice.

If you're considering damp proofing by injection, it can be done afterwards, and will probably require removal of the lower one meter of plaster and replacing with a sand cement render, because of any residual damp left in the wall. There are some who would argue that damp proofing walls is a waste of time, or not required.
But consider the two operations, underpinning and damp proofing, as completely separate and distinct processes, not running concurrently.

JDLDIY · 10 Oct 2016

Himaginn said:
I'd say number 1 is to consult a SE, discuss your proposals and take advice.

Absolutely, no argument there. I just didn't put it in my list as I just want to nail down the physical process, should poured concrete be an acceptable solution.

Himaginn said:
If you're considering damp proofing by injection, it can be done afterwards, and will probably require removal of the lower one meter of plaster and replacing with a sand cement render, because of any residual damp left in the wall. There are some who would argue that damp proofing walls is a waste of time, or not required.
But consider the two operations, underpinning and damp proofing, as completely separate and distinct processes, not running concurrently.

The first thing to do with anything that's damp as far as I've always thought is to identify, isolate/remove the cause of the damp. Now, in this case it's not coming from above at all, so it must be coming from down below, or through the wall. Pouring what are effectively new foundations surely gives me the opportunity to insulate the wall from the ground somehow? What about using waterproof concrete?
I get what you are saying though. From my point of view I'll be working room by room as we'll still be living in the house, so they'll both need doing within the same time frame - part of the same job.

Himaginn · 10 Oct 2016

You don't insulate the wall from the ground, you insulate the floor. The walls are insulated from the exterior lower temperature, therefore the insulation goes on the face (interior or exterior) of the walls, or in the cavity.
It's possible to isolate the damp by insulation on the inner face of the wall.

It may not be feasible to underpin room by room. Talk to a SE. He/she might even say it's not needed or offer alternatives. But adding an extra storey would suggest that some inspection and possible up-rating is required, or an additional storey may not be possible.
It may even not be possible to build alongside without some work to the existing foundations.
Until you talk to a SE and some inspection work is undertaken, it's all very hypothetical.
Underpinning and insulation of walls require building control notification.

An old post from FMT, which demonstrates the paperwork involved in underpinning, and the potential repercussions for the future:

freddiemercurystwin said:
Ask the owners via the agent and they should have all of the documentation that went with it, so there will be a structural engineers assessment, a structural engineers design, a receipt/warranty from the contractor who undertook the work, confirmation that the works were undertaken in accordance with the design from Building Control. You also need to ask to see proof of current house insurance and proof that the current homeowners have disclosed the subsidence. .........including some bore hole tests, this can be from the original engineers. .....
If, upon request the homeowners are unwilling to get the big fat file out for you to look through when you ask then walk away. I would suspect they will be willing and should have all this stuff, that seems to be my experience when there is known subsidence from the outset.

..........and bear in mind that future buyers who buy the property off you in the future will also factor all this in too and some buyers will not even entertain the prospect, some people will just walk away when they even just hear the word subsidence but there is more to it than that .......t.

JDLDIY · 10 Oct 2016

Ah ok, apologies, I've used the wrong word - I meant isolate the wall from the ground, not insulate.
I appreciate I will need to go about the whole thing in accordance with advice from an SE. I have already had some advice from him, underpinning in some form will definintely be required, although perhaps not all around the property. I wanted to establish what was involved, physically what the process is and any tips for doing it efficiently.

vinn · 10 Oct 2016

OP,
Perhaps, what you should do is to open a new thread (with photos) ref. your damp issues?

JDLDIY · 10 Oct 2016

vinn said:
OP,
Perhaps, what you should do is to open a new thread (with photos) ref. your damp issues?

Thanks, but the primary concern is to get the structure right in terms of strength. I know I can sort the damp issues and taking photos will involve moving furniture etc which I can't do right now.

noseall · 10 Oct 2016

Underpinning is one of least favourite jobs in construction. It (typically) involves digging down below the bottom of the wall and then underneath the wall, in predetermined intermittent sections, then forcing concrete into each hole. Once the concrete has hardened, the remaining gaps are excavated and subsequently filled.
The amount that you can remove at any one time will be determined by the integrity of the wall and the loading and stability of the structure.

JDLDIY · 10 Oct 2016

noseall said:
Underpinning is one of least favourite jobs in construction. It (typically) involves digging down below the bottom of the wall and then underneath the wall, in predetermined intermittent sections, then forcing concrete into each hole. Once the concrete has hardened, the remaining gaps are excavated and subsequently filled.
The amount that you can remove at any one time will be determined by the integrity of the wall and the loading and stability of the structure.

Thanks noseall. Yeah, I'm thinking that as it's a job nobody wants to do, it'd be expensive...whereas if I can find a way to go about it myself at my own pace it just might save me a fortune. Next stop a chat with the SE....

vinn · 10 Oct 2016

Underpinning of any kind is not a job for a DIY'er or a non-specialist building contractor.
Specialists are skilled, experienced and insured for this kind of work - or they dont stay in business long.

Without rehearsing them here there's a whole list of variable & very expensive complications that you could run into - such as not being able to insure or sell your house.

Before consulting anyone else have a word with a conveyancing solicitor & see what she has to say about your proposal.

JDLDIY · 10 Oct 2016

vinn said:
Underpinning of any kind is not a job for a DIY'er or a non-specialist building contractor.
Specialists are skilled, experienced and insured for this kind of work - or they dont stay in business long.

Without rehearsing them here there's a whole list of variable & very expensive complications that you could run into - such as not being able to insure or sell your house.

Before consulting anyone else have a word with a conveyancing solicitor & see what she has to say about your proposal.

Hi Vinn, many thanks for your advice. I do think there is a big difference though between a house which has been underpinned as it's subsiding and underpinning the house which actually had no foundations to speak of so you can extend onto it. Obviously I would do it to an engineering design spec and comply with building regs etc.

I've tackled loads of DIY jobs in the past that folk have said are beyond DIY'ers, but as I said before, as and when it gets too tricky I'm happy to leave it to the professionals. I will have a word with a solicitor, good advice, thank you.

moreteavicar · 1 Mar 2017

Go for it JDLDIY - there appears to be some misconceptions that underpinning is a dark art performed by highly skilled craftsman / artisans. It is nothing more than digging a hole and filling it with (correctly mixed) concrete according to instructions. It is not for the builder/contractors to have their work insured, but the SE upon who's calculations and advice all work is carried out. My neighbours' property was recently underpinned and excavated by such "highly skilled" craftsmen working for a professional building company. They managed to get that bit right as it is hard to mess up (luckily concrete was bought in). Everything else they touched was a complete disaster, including the extension - laying the most uneven and twisting foundations known to man, and the to add to the woes the building company went bust. Eventually it was all rectified at time and cost, but in all, the SE and BC documents are what matters. The builder's insurance won't even deal with the more superficial damage they caused to both our properties during their activities. I'm in the process of doing something similar, and on the advice of numerous people I know in the building trade, I'm going to tackle it myself in pretty much the same way you're going about it, which appears to be sensible and methodical

moreteavicar · 1 Mar 2017

And regarding damp coursing, here are a couple of possibilities. I had been considering laying in membrane sheet in the hole for underpinning (SE doesn't see a problem with it, other than fiddly, but putting pressure on concrete will make the membrane take shape of the hole). Alternatively, as with my neighbours, no additional DPC was added in the wall or foundation, but instead a wall membrane was added, complete with drainage channel at wall/floor intersection. Water that gets trapped at the membrane eventually drains off down the channels, which gets piped off and runs out into the garden.

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Underpinning & damp!

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