900mm raised insulated slab

[Fly100]

Currently looking at getting ahead of some planning conditions. Looks like I will need to come out of the floor 900mm or 1200mm to resolve the flood issue. So the build is likely to be SIP construction, I dont like the idea of a 900mm concrete faced slab, is building the external walls in brick and then filling the area in the middle with hardcore, insulation and then screed up to FFL ? Im guessing the slab would have to be divided up into smaller areas.

Im yet to instruct an Architect, just thinking about the challenges I will be coming across.

 

Fly

[ProDave]

Beware of floating insulation if the predicted flood ever happens

 

https://www.bbc.co.uk/news/uk-england-london-37600533

 

 

If I remember from the time, the slabs of insulation was used as a cheap infill to build up the floor level and the flood in this case was a burst water main.

[saveasteading]

38 minutes ago, Fly100 said:

building the external walls in brick and then filling the area in the middle with hardcore, insulation and then screed 

Yes this is normal. You have to compact it as you go (150mm layers) and yet not knock over the walls. 

Best to use crushed hard-core for density and to avoid voids. 

Then for the top layer a better quality with more fine stuff. Then probably a sand blinding for level control.

 

Slab into smaller areas? Do you mean crack control?

 

[Fly100]

18 hours ago, saveasteading said:

Yes this is normal. You have to compact it as you go (150mm layers) and yet not knock over the walls. 

Best to use crushed hard-core for density and to avoid voids. 

Then for the top layer a better quality with more fine stuff. Then probably a sand blinding for level control.

 

Slab into smaller areas? Do you mean crack control?

 

Was thinking that a big pour like that will take some managing and its a big area with cracking potential and alot of force on the

perimeter

walls.

[saveasteading]

No problem with the right prep. What size  will it be?

If an architect can't deal with this then they need changing, so a good chance to test some.

[Fly100]

10 minutes ago, saveasteading said:

No problem with the right prep. What size  will it be?

If an architect can't deal with this then they need changing, so a good chance to test some.

 

200m2 region as a ball park at the min. I havent spoken to one yet hence my question. Wanted to get ahead of a few issues.

[Russell griffiths]

29 minutes ago, Fly100 said:

 

200m2 region as a ball park at the min. I havent spoken to one yet hence my question. Wanted to get ahead of a few issues.

You will pour that as one. 

However your idea of an insulated raft sort of goes away if you build the walls up and pour the concrete inside that structure, where will the sips walls sit, on the brick walls or on the concrete. 

You need to do a section drawing really to get a grasp of where all the elements combine. 

[saveasteading]

1 hour ago, Fly100 said:

Wanted to get ahead of a few issues.

Let's say it is 20m x 10m.

Build the wall to whatever construction is appropriate. Up to your concrete level makes it easy.

Fill with stone in layers, compacting as you go. Use good stone on top, and blind.

Then polythene dpm horizontal and turned up the wall face. This provides a break between concrete and wall, as well as waterproofing. Then insulation including between wall and slab and another polythene layer.

Then your concrete. 200m2 is quite a big pour: one day if access is easy. 

Sometimes 150mm is specified but 100mm is enough, so 20m3 which is 3 x the biggest wagons of readymix......not insignificant. 

 

This slab should have contraction joints to control cracking, ie have tidy joints where you want them. All concrete cracks but it can be controlled with these joints, steel mesh or microfibre and minimising water content.

 

As someone else will be detailing it, that is enough for background.

 

If access was difficult then you might consider beam and block instead if fill.

[Iceverge]

If flooding is really a significantly risk I would be drawn towards shuttering and pouring a reinforced concrete perimeter wall.  Especially if it's a simple shape.  

 

Much stronger than blockwork and no more expensive. Hollow core planks on top and a void underneath may be cheaper too depending on how much it costs to import hardcore. There is a risk of differential settlement of such a large mass of compacted stone if not done really well. 

 

 

 

 

Edited March 1, 2023 by Iceverge

[saveasteading]

1 hour ago, Iceverge said:

Much stronger than blockwork and no more expensive.

Sorry Iceverge, I disagree with this.

Unless a raging river, masonry will not be at risk from flooding. It is supported by internal fill and anchored by the house weight.

 

I would hazard that a concrete beam to the perimeter will be 4 x the cost of masonry. RC walls to 1m are specialist and expensive. 

1 hour ago, Iceverge said:

not done really well. 

 

So do it well. Why wouldn't you?

1 hour ago, Iceverge said:

Hollow core planks on top and a void underneath may be cheaper too 

I don't disagree . Round about 1m and more and it becomes worthwhile, but may need extra footings. Access is the other differentiator  or building on a slope.

[Russell griffiths]

ICF perimeter walls, slab floor, but make sure your flood risk height is the bottom of the slab floor not finished floor height. 

[saveasteading]

15 minutes ago, Russell griffiths said:

ICF perimeter walls, slab

Preferable to RC,  but not unless the whole house is ICF.  A brick and block cavity wall or solid wall will do just fine, will be much cheaper and easy to resource.

[Iceverge]

2 hours ago, saveasteading said:

Unless a raging river, masonry will not be at risk from flooding. It is supported by internal fill and anchored by the house weight

I was hazarding that the R/C would be inherently waterproof, keeping any fill  dry and less lightly to expand/contract? 

 

 

I'm thinking farm prices maybe. I don't know for non agri projects. 

 

2 hours ago, saveasteading said:

So do it well. Why wouldn't you?

 

Ideally yes. Mind you it might only take 1mm of settlement to crack a tile at a threshold. That's a miniscule shrinkage over 900mm. Our builder put rebar from the floor slab into the walls at the thresholds of doors because he had had callbacks from sinking floors and cracking tiles.  

 

 

[saveasteading]

39 minutes ago, Iceverge said:

R/C would be inherently waterproof,

Yes that would be correct, if well made, as reservoirs can be waterproof without any tanking.

 

It is just cost really, Brick would stop water flowing in and through, even if it wasn't static water, which most floods are. Any dampness would be under the dpm and would dry out eventually. If the fill is clean stone it won't subside due to wetness. 1mm settlement should not occur.

 

Agreed though that not all construction is well done. So this does need supervision or a trustworthy contractor who looks at drawings (2 big Ifs)

 

I had an old project subjected to about 300mm of flooding, and it was at a fast flowing major river, so not just static water. It was surprising, and pleasing, how well it performed. The walls up to slab were single block footings, simply to contain the slab.

 

Using steel mesh instead of fibres does give some more security but again at a cost. Fibres would save 200m2 x £10 here.