Floor slab in before or after walls

[frank999]

Hi,
Ground Floor extension 6x6mt, render finish.

Not sure if walls will be block cavity or solid block at this stage.

Should the floor slab go in before the walls go up from DPC level, or after ?

Contractor will lay the footings and blocks, I will be laying the slab.

One method I observe is to run your visqueen (or other 100 gauge) DPM over your wackered and blinded MOT, then up and over the footing blocks, an additional harder wearing textured DPC on a roll - is either on top of, or under that DPM, and if a cavity is present - a tray is created, the concrete slab is poured - and using the raised blocks with DPC and DPM sitting ontop as a level, the slab tamped down. Block laying then continues up from there.

This creates a seamless DPC without any joins or overlaps, apart from where any cavity may be present.

I have a block layer suggesting that blocks go all the way up in one go, the slab then goes in after ... presume I would then have to 'glue' the latterly installed DPM to the stiff DPC plastic that I presume would be left jutting out 100mm or so at DPC level.

Putting the slab in before walls go up is easier for me, but involves block layer having to do the job in two parts. If I have to do it after, leveling will be bit more of a circus, and unsure how to join/glue the overlap between the DPM and stiff DPC.

Any thoughts appreciated.

 

[^woody^]

Tell the "block layer" to do one.

There is no reason to go to the inconveneince of putting a slab in afterwards and trying to place concrete and level it when surrounded by walls. And why wouldn't a proper bricklayer want to work off a nice level surface?

 

[frank999]

Thats what I thought ... you get a nice seemless DPM to DPC pouring slab first.

However, just spoke to Visqueen tech, they advise that the DPM should not be bent 90 deg. and run up the blockwork to meet the DPC - it should be kept flat - otherwise it could tear if there is movement - which makes sense, and a 'Lap tape' used to bridge the gap:

Something like this:
https://www.visqueen.com/sites/default/files/standard_detail/JD-02 ECO - DPM to DPM Jointing Detail.pdf

Taken from:
https://www.visqueen.com/products/damp-proof-membranes/ecomembrane

Not sure how you span the 300mm difference in level up the blockwork (50mm blinding, 100mm PIR, 150mm MOT)

... Visqueen have some films you can watch:
https://www.visqueen.com/installation-videos

Guess it depends on how much movement you are expecting to get over 10-30 years, if the DPM tears, only a tiny bit - it does defeat the purpose.

Running a slab inside - you would have to install tamping 'guide' rails along the walls, pour, let it harden enough to stand on, then rip out the rails and fill in ?? a lot of extra faff - and a discontinuous slab (at wall edges). Any better methods ?

I'll grab a cuppa.

 

[^woody^]

However, just spoke to Visqueen tech, they advise that the DPM should not be bent 90 deg. and run up the blockwork to meet the DPC - it should be kept flat - otherwise it could tear if there is movement - which makes sense, and a 'Lap tape' used to bridge the gap:

Nonsense. Then you are introducing a joint right on a 90° point - and risk failure unecessarily.

And how does this rep propose to "meet the DPC" - it requires not just to meet, but to lap the DPC - at 90° at that.

If there is movement enough to tear thick gauge polythene, then a tear is the least of your worries.

 

[frank999]

Umm, I guess if you cut a 500mm slither of the DPM to run up the sides of the insulated foundation bricks and to over lap 100mm top and bottom, and used double sided lapping tape - it might have more give - the Rep suggested the tape was like Blue tack - so moveable.

My BCO however seems happy for me to run it up the insulation and over as one peice, and suggested as you say that if there is that much movement that it tears - then I have other more serious stuff to worry about.

Its an 18 metre run (3 adj. walls of 6mt each) of 400mm (h) foundation blocks - not sure if its cavity or solid block yet.
Could I expect that to be laid in a day.