Keeping and building on existing slab?

[Stoph43]

Hi team, looking for some advice:

We'll be conducting a new build using the previous slab from a barn. We're taking the barn down and keeping the existing concrete slab (approx ~200m thick and pretty level for a barn) The slab currently has its own DPC. We'll be digging down around the slab to instal new foundations, but we'll be building up above the slab breaking into it in some places to add foundations for supporting walls. The thought is to build up from the current slab with new insulation, under floor heating and screed.

 

My question is, can we simply put a new DPC on top of the existing slab, with insulation and screed on top? Will this still work and will it require any form of sand binding or similar underneath etc?

 

Rough picture below to help explain.

 

 

Thanks for your help

 

[nod]

Nothing to stop you as long as it’s not load baring 

Don’t forget to put a 300 mil high load dpc on the internal skin of block to attach you floor dpc to 

[Mr Punter]

Can you allow for 225mm above the slab to get to your new level?  That would give  150mm insulation, 55mm flow screed and 20mm floor finish.

[Russell griffiths]

Depends how poor the finish is on the original concrete. 
you can use 25mm of polystyrene insulation on top of the concrete and below the damp proof membrane to take out any lump n bumps or sharp areas. 
 

if it’s really bad a few bags of cheap self leveling compound to take out the worst bits. 

[Iceverge]

Agree with above.

 

From the top down

 

Screed with UFH pipes 

Slip sheet

Insulation layer

DPM

Layer of EPS insulation 

Concrete slab. 

 

 

 

[saveasteading]

I'm all for retaining slabs. Too many are broken out needlessly.

200mm is heavy industrial thickness for forklifts and racking.

You can tell how flat it is simply by looking for puddles. A few mm is of no consequence.

Why do you need a new dpm on top?

[Stoph43]

Thanks All, the concrete is in pretty good condition, i'll have a better idea on levels once its fully cleared out and @saveasteading i'll follow your method. 

@Mr Punter- levels we should be able to have room for 225 as you suggested, thanks for this.

@nod- Yeah agree on the 300 mil DPC

 

@saveasteading I automatically 'assumed' I would need a second DPM as to allow it to run above the slab and into the new internal skin block and join to the DPC. The current DPM would not have any excess to allow for the raised blockwork around the perimeter of the slab to meet the new floor level, i thought this would be a concern, hence a new DPM would need to be laid? 

[saveasteading]

54 minutes ago, Stoph43 said:

thought this would be a concern, hence a new DPM 

The cost is relativaly small and it removes some concerns so ok.

 

My quick and dry method of checking floors for level is a golf ball. Roll in several directions (across and diagonals) and see if it deviates.

 

The official check for smoothness is a 3m staightedge. Lay it down and also rock it, to see how big any gaps are. Do it all over. 3mm is the limit on commercial floors and sensible on domestic too. But the pir and screed will overcome any bigger gaps.