Is this the right way to prepare this subfloor?

[snapperpb]

We have a garden shed I'm converting to an office. The slab has sunk in one corner (I've fixed the problem with the foundation).

 

You can see from the pics the back right corner is sitting about 13cm below the bottom plate of wall. (Whereas the closer left corner is almost about even with the base plate)

 

What is the best way of prepping this floor for tile?

 

My current plan is:

DPM paint + DPM runner along the side

Foam in edge cracks

Sand & Cement Screed

Tile backerboard with insulation (is this needed?)

Tiles

 

Does this sound about right?

 

I'm in the UK, not much frost heave or very low temps, but of course there's damp. 

 

I'm going back and forth on electric underfloor heating here as well, if anyone has any takes!

[TommoUK]

..put some insulation & dpm down before you screed. you probably then want to insulate the walls and ceiling

[snapperpb]

@TommoUKThanks for this, I was wondering about insulation under the sand & cement, but doesn't that mean I'd need 35mm minimum depth everywhere? (The high point of this slab is almost level with the base plate of the frame)

[Redbeard]

What's the roof made of, and how well is the base-rail fixed to the base? If the whole structure is not crazy heavy/wobbly a few jacks in sequence might get things level, allow a course of blocks and give the edge restraint for an insulated concrete (or not concrete) floor. Not as simple as 'leaving it be', but potentially not as complicated as one might think.

[TommoUK]

10 hours ago, snapperpb said:

@TommoUKThanks for this, I was wondering about insulation under the sand & cement, but doesn't that mean I'd need 35mm minimum depth everywhere? (The high point of this slab is almost level with the base plate of the frame)

from pic it looks like 800-100mm to your soleplate? besides there's more than one way to do it.  another would be to lay your dpm, then run smallish 'joists' (say 50mm x 50mm) and fill the gap between the joists with insulation before laying ply or t&g flooring and then your final floor finish

[snapperpb]

Yeah it's 1300mm to the soleplate in that far corner BUT it's almost level with soleplate in the bottom left (not pictured). 

 

Is it a problem if my final floor height is higher than the soleplate?

 

(Redbeard the roof is clay, frame probably isn't professionally built I think it was the old owner)

[TommoUK]

I see...so either break out that corner and make it level

or fill the rest up to make it level

or put in a false floor

 

 

..

 

 

[snapperpb]

Got it thank you! 

 

Does it matter if my floor rises above the the baseplate of the wall frames when its finished?

[Dave Jones]

dig it out.

 

working back from top.

 

65mm screed

150mm insulation

150mm hardcore

 

nice and toasty.

[TommoUK]

should be ok but bring your dpm up the inside of the wall at least 150mm (above the level of the outside ground level) before you fit your internal wall board