Building internal block walls directly on slab?

[flanagaj]

Our current build has the internal blocks walls sitting on 450mm wide footings.  From a build perspective, this is actually a right pain and complicates the whole process up to finished slab level.   I know raft style slab would mean we could avoid this, but are there any options to create a giant reinforced slab that would enable the internal walls to be built directly on top.

The main motivation for this, is it will make it much simpler to power float a large 17 * 7m slab than it will if said slab is divided up into 4 smaller slabs. 

[ProDave]

Build internal walls of timber and plasterboard.  Then only supporting walls will need proper foundations.

[Conor]

That's what we did. Ground bearing raft. Where there were internal walls, the concrete was thicker with an extra layer of mesh. Walls were built off the slab. We also and compacted the Type 3, then the digger driver scraped out the areas for the walls, compacted, then blinding layer. Essentially extending the ringbeam internally.

 

 

[Conor]

Before it was finished, but you get the idea. Our reason for doing this was because it's a basement, we'd already dug out 400tonnes of earth and didn't want more coming from trenches.

 

Edited Friday at 11:22 by Conor

[saveasteading]

1 hour ago, flanagaj said:

right pain and complicates the whole process up to finished slab level

In what way? It's standard construction and means all your structural walls are sitting on the same strength of ground.

 

The only issue I can see is if you wanted a power floated slab. But it's a house not a warehouse. 

 

What am I missing,?

[-rick-]

1 hour ago, Conor said:

That's what we did. Ground bearing raft. Where there were internal walls, the concrete was thicker with an extra layer of mesh. Walls were built off the slab. We also and compacted the Type 3, then the digger driver scraped out the areas for the walls, compacted, then blinding layer. Essentially extending the ringbeam internally.

 

How did you handle floor insulation?

[Conor]

2 minutes ago, -rick- said:

 

How did you handle floor insulation?

200mm insulation then liquid screed. So fairly typical except for ring beams instead of trenches, as we were already down into firm ground. External walls were ICF with tanking.

 

We originally had it designed as an insulated raft, but SE couldn't get it to work.

Edited Friday at 11:39 by Conor

[-rick-]

13 minutes ago, Conor said:

200mm insulation then liquid screed. So fairly typical except for ring beams instead of trenches, as we were already down into firm ground. External walls were ICF with tanking.

 

That's what I guessed. Means it wouldn't work for @flanagaj if he wanted to powerfloat the slab, not put insulation and another concrete layer on top.

[-rick-]

1 hour ago, flanagaj said:

The main motivation for this, is it will make it much simpler to power float a large 17 * 7m slab than it will if said slab is divided up into 4 smaller slabs. 

 

As you know from previous threads I see the appeal of a floated floor for certain properties, at least as a phase 1 cost saving measure. But as soon as you add the complexities of not having a single level slab with no corners to negotiate I think you can throw any idea of cost savings out of the window and instead you are looking at a concrete floor as a premium option not a cost saver. If so, then there are other ways to go. If you are thinking of it as a cost saving approach then I think you need to change your plans.

 

Either its dead simple and easy for the contractors or it's not happening. In other words I think it's only a viable option if you have an insulated slab or have designed around an uninsulated/badly thermally bridged floor (which I'm not sure would meet current regs).

[flanagaj]

1 hour ago, saveasteading said:

In what way? It's standard construction and means all your structural walls are sitting on the same strength of ground.

 

The only issue I can see is if you wanted a power floated slab. But it's a house not a warehouse. 

 

What am I missing,?

All aspects are complicated.

1)  More trenches to dig and concrete to pour.
2) More blockwork to build up to DPC
3) More time spent levelling multiple areas with hardcore / sand blinding
4) More insulation cuts and perimeter insulation against said block work
5) DPM membrane is more fiddly as opposed to just laying over a single large area.

6) Added hassle of now having multiple floor areas to level as opposed to a single slab

 

Edited Friday at 12:37 by flanagaj

[saveasteading]

47 minutes ago, flanagaj said:

All aspects are complicated.

To me these are normal things and not complicated. More importantly the same applies to any builder.

 

Rafts are expensive. Not to be confused with a simple slab.

 

There is nothing wrong with bearing your structural walls on a standard slab IF it offers the same resistance as as the deeper footings. That is for your SE to agree and then for you to impose strict quality control. Is the exposed ground undisturbed and compacted? Is your sub base properly compacted? Concrete thickness, reinforcement and quality of course.

 

The slab will likely be thicker too, and I'd forecast, more expensive than with  footings.