Digging out floor?

[trumansparks]

Hi,

I am about to dig out the lounge floor in prep for a new concrete slab, my questions relate to this

Am i safe to dig down level with the interior walls or do i risk undermining the wall. I am concerned that the 1850's stone cottage may not have too much depth to the walls.also i was just going to use the dpm pulled up as the edge of the slab or do i need to construct a frame?

should i take any safety precautions when digging out this floor?

also if someone could confirm the construction and add depths i would be gratefull? seems to bea few differing opinions on the web,

screed
concrete
dpm
sand
hardcore



thanks

truman

 

[RedHerring2]

First of all, I'm only a DIYer so can't offer definitive advice. But I did my floor on the house restoration in France last summer;
//www.diynot.com/forums/viewtopic.php?t=237971

My observation would be;
the walls are probably about 900mm thick and have been standing for 150 years, so the short time that they are without a little extra support isn't going to matter too much.
Are you on a level site or sloping site? A level site and you can assume that the perimeter is supported down to at least 400-500mm. A sloping site and the higher external areas may mean shallower 'foundations' to the wall. All of the above may depend on soil type, the harder it was to dig out in the first place the shallower the foundation will be.
Are you including insulation? You haven't mentioned it and it would seem foolish to ignore it at this stage. 100mm expanded polystyrene or 80mm of the more sturdier insulation. Either above or below dpm. If above dpm then a further layer of polythene to prevent concrete seeping down the joints or taping the joints.
You could save on the screed by ensuring a level and trowelled surface to the concrete, even a power float or even polished concrete.
25mm insulation upstand against the wall to prevent cold bridge. Then a 50mm X 25mm batten fixed above that to provide tamping levels and hold the dpm in place. Fix a datum line around the walls (With a laser level) and measure down from that to provide the finished floor level (ffl).

Depths are:
~40mm screed (You can reduce this by addition of SBR, but that's another story)
concrete 100mm with rebar mesh (in the centre of the concrete layer)
~100mm insulation
dpm
50mm sand blinding
~150mm hardcore, well whacked to refusal.

If you lay your insulation below your concrete and dpm you can possibly omit or reduce your sand blinding, depending on the level of your hardcore.
My walls were stone with a cob like mortar so I found the easiest way to fix the 25mmX50mm batten was 100mm or 150mm nails straight into the mortar when I couldn't get a plug and screw fixing into the stone.

Incidentally, another consideration:
If you've had breathable floors up till now, you're going to be sealing the damp into the floor with the dpm. The only way the damp can escape is into and up the walls. So you might just make any rising damp in the walls worse than before.

 

[noseall]

Material depth has been covered mostly by Red Herring but i would be very wary about removing floor spoil that exposes the soil beneath the wall structure.

Some buildings will fare better than others so each has to be judged on its own merits.

I have witnessed the damage that this sort of excavation can incur and it ain't pretty.

 

[trumansparks]

Many thanks red herring and noseall for your detailed replys. site is level by the way.

I had forgotten to include the insulation

I am now thinking

of;
screed
concrete
insulation
dpm
sand

and hardcore unless i can ditch this due to lack of depth?
(thanks for the tip about the edgings and levels redherring2)
I am starting to regret this job! it all started as surveyor said it needed doing as room was damp and was an old flagtone floor (2 missing replaced with concrete). It has lime walls and probably would of been ok had i replaced the missing flags but they are long gone now!

Now i am faced with the depth issues and worried about the walls falling down or pushing damp into them at the very best!

I now will have to approach the issue of a dpc in the stone walls or will I?

any final comment advise or even encouragement would be very welcome.


ps what happens when you can only go so far down with the depth what, gives?

cheers

truman

 

[RedHerring2]

Can you confirm the thickness of the walls? I was guessing at 900mm, if so there's no way you can insert a dpc, phyisically or chemically.

If you dig a test hole you'll soon know what depth you can safely dig to. Dig the test hole then come back for more advice if necessary. It may be easier to dig outside than internally. But then there's no real guarrantee that the both depths of foundations will be the same. The test hole will not be any danger to the walls. E.g. if you were going to underpin the walls you'd be digging down below the foundation for short lengths.
And there are alternatives to provide support to the walls, which I'm not fully clued up on, like casting something that looks like a concrete skirting board, but is perhaps 50 - 100mm thick, incorporated into the floor slab. You may need structural engineer's adive for that

Also don't forget, with judicial arrangements, you can maybe omit the sand blinding (by placing the insulation under the dpm) and the screed (by finishing the concrete carefully or appropriately), or use a thinner screed or floor leveller. Or even re-consider your chosen finished floor.

When I did my floor in France I had to use 2 X 40mm slabs of insulation. In hindsight and for the next bit of floor I'll place one slab below and one slab above the dpc. The insulation below dpc protects the dpc from the hardcore and the insulation above protects the dpc from the rebar mesh and chairs, (the method to raise the mesh up by 50mm, like bits of flagstone )

Finally, if you can't insert a dpc in the walls there's other ways to deal with the damp:
insulated plasterboard, whether d&d or on battens, metal or wood, with or without ventilation. You must ensure the plasterboard is not in contact with the floor.
Or, lime plaster the walls to allow the walls to breathe. (and use a breathable paint or lime wash)

 

[spc]

how do yo feel about putting a bitchamin floor covering,then install a floating floor filled between floor joists with celotex. studding out from the wall 25mm and using a 75mm stud incorperating 75mm of celotex between studs, vapour barier then plasterboard. this i know reduces the size of the room, but any moisture pushed out to the lime pointed walls will not directly affect the walls as you have the 25mm air gap which will not be bridging directly onto the walls. the vapour barrier will be for condestation cold spot only. not a waterproof barrier. All this of course is dependent on ceiling hieght.