Shallow footings and suspended floor problems

[oldbutnotdead]

Could do with some advice here.
The problem- timber floor a bit tatty, weak spots in the bays by the chimney breast, usual stuff.
Lifted some floorboards to find zero void beneath the joists which were supported on random bits of brick. Joists in surprisingly good nick (3" x 2" old fine-grained, pretty straight, some of the tails rotted out).
Exploratory dig found the bottom (soldier course) of the front wall (no concrete underneath) at about 450 below ffl (that to the top of the soldiers).
So plan was dig out the rubble and some clay to get down to about 450mm below ffl, this leaving room for 50mm sub base, 200mm void, 45 x 170 joists with 200mm rockwool between them (held up with pvc netting).
Annoyingly, the other walls have shallower footings, the worst one being the spine wall with top of soldiers at about 250 below ffl.
Annoyingly 2 the clay is very hard so tends to come out in chunks so its been almost impossible to just leave a 45° slope of undisturbed clay near the wall- so for now there's a 300mm wide strip of undisturbed clay along most of that wall which tops out at about 190 below ffl.
Given all that, the next bit of the dig (going down to 600 below ground for footings for dwarf walls) seems unwise. So contemplating alternatives;
1 Floating floor- concrete to 140 below ffl, dpm, 100mm pir, t & g loft boards glued then t & g floorboards nailed on top.
2 Half breed floor- concrete to 250 below ffl then dpm then concrete to 200 below ffl then 45 x 150 joists on dwarf walls (to give room for insulation & reduce thermal bridging) then t & g floorboards on top. No ventilation to timbers- have read that this is OK but i can't remember where. Possibly on here somewhere..
With either of the above, will i be better off leaving the clay next to the spine wall or digging it out to expose the bricks?
3 Carry on with suspended floor, set dwarf walls on top of the subfloor, work round the high ground level bit.
4 Raise FFL by 100mm - would rather not have a step from hallway into library but worse things happen at sea.
5 As it happens I could raise FFL (or more accurately not floor above that 300mm wide clay strip on the spine wall)- plan for that wall is all bookshelves so it starting 150 above ffl would not be a problem.....
Sorry for long and wordy post, there are some pics to follow.
My thinking behind both above sketches is pouring a decent chunk of concrete directly against the brick walls will give a bit of friction reinforcement to the walls and a bit of resistance to ground heave. I really don't want to start underpinning the (full brick) spine wall- i can't afford to get it done professionally and i'm not confident i'd be able to DIY it successfully.
Any suggestions gratefully received. Except the ones that say knock it down and start again or bodge it and flog it.

Comedy constructional hearth. Shame cos the tiles on top were in reasonable nick.

Chimney breast (on left) soldiers at about 450 below. Wall on right starts at about the same depth

But then comes up a course so soldier top now 350 ish below

Spine wall- right hand end of mattock level with bottom of soldier at 350 ish below, top of soldier 280 ish.

Don't know what is under that strip of clay but i'm sure it won't be pretty. Leaving it well alone, possibly for ever, possibly til an hour before the concrete pour for floating or halfbreed floor.

 

[Mr Chibs]

When I refurb'd mine (1900 house), I reinstalled suspended floors. The old floors were rotten and wormed, so had to be replaced and there were a couple of old concrete ones laid onto ash (no DPM's) and cold as the arctic.

My kitchen is now a solid floor with UFH. As it was my first dealing with UFH wasn't sure what to expect. Now I wish I'd not recreated suspended floors and made everything solid.

Woodwork is easier in my mind than borrowing hardcore and concrete it, but my feeling is, that concrete route is solid floors are fit and forget.
I used 100mm PIR, but you could adjust the thickness as and where needed to bring the levels the same.
My suspended floors have 175mm rockwool, but it comparison they are colder than the UFH one, even though the room is the same temp.

Floating floor on concrete would be my preference, if you can maintain ventilation on any other suspended floors.

 

[oldbutnotdead]

When I refurb'd mine (1900 house), I reinstalled suspended floors. The old floors were rotten and wormed, so had to be replaced and there were a couple of old concrete ones laid onto ash (no DPM's) and cold as the arctic.

My kitchen is now a solid floor with UFH. As it was my first dealing with UFH wasn't sure what to expect. Now I wish I'd not recreated suspended floors and made everything solid.

Woodwork is easier in my mind than borrowing hardcore and concrete it, but my feeling is, that concrete route is solid floors are fit and forget.
I used 100mm PIR, but you could adjust the thickness as and where needed to bring the levels the same.
My suspended floors have 175mm rockwool, but it comparison they are colder than the UFH one, even though the room is the same temp.

Floating floor on concrete would be my preference, if you can maintain ventilation on any other suspended floors.

If i do go the floating floor route i'll take a window out and chuck Readymix at it- getting too old to faff with 2 cubes of hand batch .
And good shout on the venting other rooms- yes i'd need to duct through to the kitchen.

 

[Mr Chibs]

I've been using volumetric concrete wagons, handy as you don't have to work out the quantity to precisely, as it's mixed there and then.

Good luck with it.

 

[oldbutnotdead]

Ta. It's the foundation bit that worries me more at the moment than the thermal performance. No-one likes doing a job twice, if i did have to revisit them i'd rather have timber and a subfloor to get through instead of a foot of concrete....but will the foot of concrete reduce the odds of having to revisit. Where's @^woody^ when you need him

 

[^woody^]

As if by magic.

What's wrong with leaving the ground as it is, and just replacing the floor and it being good for the next 100 years or however long the existing one lasted? Are there any problems now, apart from the tatty floor? You could deal with damp joist bearings at the wall

The problem with digging stuff out in situations like this with shadow foundations, is that the equilibrium is upset, and then things can change or even move. But, oh I see you've already started.

You could just lay a DPM, put the joists back, full- full insulation, and then lay the boards. It would be just like garage conversions are done, same principle.

 

[mrrusty]

7 years ago we had this dilemma. Back of house was flags on ash and cinders direct on the ground, front was rotten and wormed suspended floor with poor ventilation- shallow corbelled foundations on to clay. Damp in the corner when it rained. The problem was airbricks only on front wall. We did have a reasonable void - perhaps 400mm but the base of the corbelled foundation wasn't that much below. Initially we thought re-build the suspended, but in the end we discussed with builder and LA building inspector (who was primarily overseeing our extension, not the floor job) and builder hardcored, dpm'd insulation and screed.

Back of house and new extension likewise was insulated concrete floor. 7 years on and there is absolutely nothing to suggest this wasn't the right decision. And the flags are now our patio.

 

[oldbutnotdead]

Cheers both. @^woody^ yes there had been some ground heave at some stage (a while ago by the look of it) and same as @mrrusty poor underfloor ventilation, musty smell in the room.
Think it must have been garage conversions i had in mind for my plan 2 above- does full fill with rockwool still work with no ventilation in that sort of setup or does it have to be PIR? Does it need vcl or similar on top of the joists or am i overthinking it?