Awkward concrete floor slab and dpm

[Griff84]

Hi guys

I'm renovating a property I've inherited and I'm now onto laying the floor slab, the issue I have is that the house was built in 1922 and it has stone foundations, a layer of lime/ ash / cement on top, then a layer of bitumin dpc then the house bricks,.

The stone foundations come out about 12 inches into the house and they are at the level I want the finished floor slab with the screed another 60m on top of this. I have the hardcore roughly 4 inches below these foundations ready to by compacted and bind with sand. The problem is that the dpm will have to come up the jagged stone foundations and across the stone to the wall which will leave 12 inches of bare membrane and the rest of the floor concrete.

If I disc cut the stone foundations at an angle and fill in rough areas with cement will that be ok and will having bare membrane around the perimeter edge be ok for the screed?

It makes me wonder if it's better to lay the slab, float this to a nice finish then lay a dpm on top and then lay screed on this.

I am a DIYer so be gentle if what I'm saying is nonsense

[Notch1]

I guess there is a risk the screed may crack where the new oversite meets the old spread footing, due to differential movement and shrinkage. You may need to incorporate something to strengthen the screed, like mesh. Im sure some builders on here will advise.

[davyp1]

I agree with Notch!
You would reinforce this screed with a light mesh or chicken wire!
Please be aware that screeding is quite a specialised job. If you do decide to employ a sub-contractor to do this work, you would need to ask him what he would recomend as reinforcement as it would be him who was carrying the risk.

davyp1

[Griff84]

Thanks both, would this be a liquid screed or the traditional dry mix type?

I'll definitely get someone in for the screed, the slab is quite easy but I've never done screening and I don't want to experiment now lol

Would it be better to lay the slab on top of the dpm then lay another dpm on top of the slab? Is this called a floating screed? Just wondering if having a ll the screed on the plastic would make it more uniform if you know what I mean?