Underpinning

[davidm]

I have a lower room in an old 1880 stone cottage with a thin concrete floor directly on bare earth. This contributes to a damp issue and I have just stripped all the plaster off the walls, repointed with lime mortar and am now breaking up the concrete floor prior to putting a limecrete floor down. The room was actually built onto the side of the house in about 1920 and I've just found that the footings for the original outside wall of the house (which is now the inside wall of the lower room) are shallow, on bare earth and nowhere near as deep as I need to go to lay the limecrete floor.

If I break up the rest of the concrete floor and dig the soil out under the floor to lay the limecrete, can I be causing problems with the wall with the shallow footings? Do I need to underpin the wall? The house is built on a slope and I haven't found the footings for the other three walls yet so they hopefully will be deep enough.

 

[Gervase]

Underpinning is expensive and not witout risk, so it's not to be undertaken lightly.
Ideally you would need to seek the advice of a surveyor experienced in traditional buildings (not that common, I'm afraid).
But...
Such footings are very common, and the usual way of laying a limecrete floor in such situations is to ensure that the excavation is done is such a way that you leave a plinth on which the walls can sit by digging down at 45 degrees. The means, of course, that your limecrete floor becomes thinner as it gets to the walls, but most of that reduction is taken up by the LECA or glass underfloor aggregate rather than the slab itself, so the load-bearing qualities aren't compromised.
If you dig straight down to achieve what looks like a nice, square-side hole, then you probably will need underpinning.
Once the slab is cast, it will serve to hold the walls together, but much depends on the degree of slope you've got, the quality of the soil and the weather. Are there any signs of movement or cracking in the walls at the moment?

 

[Stephen]

Gervase is correct that underpinning can be tricky, etc. Further, if you follow his advice you must 'batter back' at 45 degree angle from the bottom outer corner of whatever footing presently exists.

However, there is another option - DIY.

If you carefully excavate under the present footings in no more than 1m lengths at a time down to a suitable depth you could build up a new brick footing (I assume you will use NHL5 mix). Do hit and miss sections around the walls - dig 1m out, leave a couple of metres then dig another 1 out, etc around the walls. Build your new footings in the excavated sections, leave to set for a while then do another series of hit and miss 1m lengths. Ensure you never excavate two 1m lengths adjacent to each other.

It will take a short while, but will give you the extra depth you need for your floor. You can also adjust the depth to suit your desired finished floor level.

You might want to get an engineer to have a quick look and do some basic calcs, but generally the principle of the DIY underpinning is sound if you take care. There is no mystery to it.

 

[davidm]

The room is some 4 x 3m with the wall in question being 4m. If following Gervase's idea, would it be better to do in it sections one at a time, digging them out and laying the limecrete in say 1m wide sections or is it likely to be safe enough to do the whole floor in one block?

 

[Tom]

Stephen's idea is good and this is what we had to do with a section of wall carrying the weight of a stone staircase and upper wall. Split the wall into 4 x 1m sections, underpinning sections 1 and 3 first. Be sure everything has gone off completely before moving to sections 2 and 4. Then you can dig down in a straight line to lay your LECA/Limecrete floor.

By the way, have you thought of installing underfloor heating, too? That's what we did with our LECA/Limecrete floor and it works a treat, freeing up wall space and doing away with unsightly radiators. It also means that the warmth is at the level you want it instead of being up on the ceiling!!

Jaqy

 

[Gervase]

No problems - you can do the whole floor at once.
As for the DIY underpin, it sounds a good plan if you've got dressed stone walls. I'd be wary of doing a DIY underpin on a rubblestone wall without grouting it first, however - any movement in the boulders at the bottom is liable to make the core (usually bound with mud and chicken crap rather than lime!) shift and could prompt movement. If a stone drops you've got an almighty job to get the beggar back again.

 

[chuckey]

I have to drop the floor level on my rock and rubble barn. I sought advice from a local builder who did a wonderful job on his own barn. What he did was to dig down to his level in 9 " strips going from the outside to the centre of the wall. I presume that is so the throughs take the weight of the under dug wall. He used cast-in re-bar to hold the slices together. He must have done about 100'linear feet of wall!

I was thinking of digging down to the lower stone level (inside and out),about 2' wide banging a bit of re-bar straight through under the bottom stone then strapping the ends of the re-bar unwards and fixing the straps into the wall at a height of about 6'. Then digging the remainder out from under the wall and laying foundations and concrete blocks. To get the bottom of the lowest stone to sit on the foundations I was thinking of imbedding a "jack". This will be a bit of 12mm stainless steel studding about 2" high with a nut on it. So tightening the nut under the bottom stone will push it upwards under the wall and down to the new foundations. The gap being filled with a rich sharp sand mortar.

Is this OOT?
Frank

 

[Gervase]

Blimey - that sounds like a heck of a lot of work! I'd only go down that route if a surveyor told me I was in danger of losing the wall if I didn't. I'm a great believer in the "if it ain't broke..." maxim. Taking away soil and replacing it with a limecrete slab is going to help stabilise the structure anyway.

 

[Essex]

We had a similar problem although the wall did go down, in many areas below the excavation depth. There were areas where there was no brickwork under the frame (it is a timber frame house) where we excavated to put in limecrete footings and a brick dwarf wall to support the frame (note that the house had been sitting, with no brick support, since the 15C). We excavated to the same depth as the existing brickwork. However we ended up having a monumental row with Building Control who wanted the footings to be excavated to building regulation depth. This would mean that we would have a couple of metres 900mm deep a couple of metres 500mm deep, then a couple of metres 900mm deep, a recipe for differential settlement??
We had to pay (grrh) a conservation surveyor to say exactly what we had been saying before building control would agree that it was OK.
Good luck

 

[charlie-ia]

underpin on a rubblestone wall without grouting it first,

i thought grouting was what you did to tiles? whats the difference between that and pointing?
can you tell us more about this please?

 

[Gervase]

Grouting is where you use a slurry of hydraulic lime to consolidate the rubble fill of a wall - usually gravity fed by tube at several points and allowed to flow down inside the wall, filling all voids, until it starts to trickle out of 'tell-tale' holes drilled at the bottom. Give it 28 days to cure and then - fingers crossed - lower elements of the wall can be removed or exposed without the fill or any other part dropping.
It's often used where the two 'skins' of a rubble wall are starting to lean apart, or where rats' nests and the like have opened big voids in the fill. It's quite a palaver to do, but sometimes is the only way to save an old wall. York Minster has hundreds of tonnes of grouting.