Will increasing the thickness of a concrete floor affect the foundations and base?

[andrews]

My property has a single extension that was built in the 1970's and the walls were constructed using 140mm light weight blocks with polystyrene insulation in between batons, covered with plasterboard and externally rendered.

 

In an ideal world, I'd have the whole thing re-built to today's standards but for various reasons (cost being the main one), I can't do this.

 

From an insulation point of view, the building is fine but I'd like to raise the concrete floor. At the moment, there is a wooden floor floating on wooden joists that were laid directly onto the concrete base. For years this has survived but as you can imagine, it's noisy to walk on and 'feels' cheap.

 

I would like to remove the wooden joists (they are floating) and raise the concrete floor (about 70-80mm) so that I can lay some ceramic tiles.

 

Would this be better done in concrete with a final top screed layer or is it better screeded that thick?

 

A big concern of mine is the additional weight the aggregates will add to the base and the surrounding foundations. Should I be concerned about this?

 

Having had a peek at the foundations around the extension by digging deep down, they do go down further than 800mm and appear to be constructed with concrete slabs at the base and block-work further up. The internal floor area is approximately 5.1 x 2.8m.

 

Any advice would be welcome.

[Russell griffiths]

There are a lot of other things you need to consider 

 

Damp, you will need to protect the upper floor from anything rising up. 

 

Cold. The timber floor offers a modicum of cold protection from the concrete below, if you chuck another slab on top you will transfer everything from the lower slab to the upper. 

 

It would be just a token gesture but you could put 25mm of insulation on top of a damp proof membrane and then a thinner screed with a light weight mesh to give it a bit of strength. 

 

 

[jack]

Edited to add: Welcome to Buildhub!

 

Could you dig out the old concrete, lay a decent amount of insulation, and drop a screed on the top, maybe with underfloor heating? It's a fair amount of manual labor, but it wouldn't be hugely expensive, and the result would be much better than just dropping more concrete on top of what's there. It'd also be a lot lighter.

 

@Onoff did something like this and documented the process. I'll see if I can find the thread later when I'm at my computer.

[Onoff]

Wade through this from the beginning, enjoy!

 

https://flic.kr/s/aHsk23FYzd

[andrews]

3 hours ago, Russell griffiths said:

There are a lot of other things you need to consider 

 

Damp, you will need to protect the upper floor from anything rising up. 

 

Cold. The timber floor offers a modicum of cold protection from the concrete below, if you chuck another slab on top you will transfer everything from the lower slab to the upper. 

 

It would be just a token gesture but you could put 25mm of insulation on top of a damp proof membrane and then a thinner screed with a light weight mesh to give it a bit of strength. 

 

 

 

Russell, the damp and cold will be dealt with. My main issue is the additional weight. I'm looking into using insulation with a screed layer. According to the FeRFA, a floating floor with light domestic traffic and a polymer modified sand/cement screed can have a thickness of 33/55mm of screed over high compressive strength insulation. I've worked out that should add about a ton and a half of extra weight to the extension. Do you think this would be an issue?

[andrews]

2 hours ago, jack said:

Edited to add: Welcome to Buildhub!

 

Could you dig out the old concrete, lay a decent amount of insulation, and drop a screed on the top, maybe with underfloor heating? It's a fair amount of manual labor, but it wouldn't be hugely expensive, and the result would be much better than just dropping more concrete on top of what's there. It'd also be a lot lighter.

 

@Onoff did something like this and documented the process. I'll see if I can find the thread later when I'm at my computer.

 

Hi Jack, digging out the old concrete is just too much of a big and messy job. I'd rather work with what I've got.

[jack]

10 minutes ago, andrews said:

Hi Jack, digging out the old concrete is just too much of a big and messy job. I'd rather work with what I've got.

 

Fair enough, just a thought!

[PeterW]

22 minutes ago, andrews said:

 

Russell, the damp and cold will be dealt with. My main issue is the additional weight. I'm looking into using insulation with a screed layer. According to the FeRFA, a floating floor with light domestic traffic and a polymer modified sand/cement screed can have a thickness of 33/55mm of screed over high compressive strength insulation. I've worked out that should add about a ton and a half of extra weight to the extension. Do you think this would be an issue?

 

If its slab on slab, the 1/2 tonne will be spread over the whole floor area and it will be ground bearing - no different than 6 people stood on the existing floor.

 

I would try and get as much insulation in as possible and crack on with the floor as is.

[Carrerahill]

Assuming the concrete oversite was laid properly, i.e on compacted sub base etc. then the additional weight will not be an issue, the additional weight will be nothing over that footprint. I would be looking very carefully at the existing concrete, you want to look for cracking, separation, sinking etc. if any of these exist then you should not start to add more weight without investigation works or taking precautions.

 

So here are your options:

 

If the oversite concrete looks undisturbed and in good condition then I would be happy to pour over site, don't let the new concrete touch the existing wall, create a expansion gap with thin (10mm) EPS or dense foam (you can buy rolls specifically for this) this will stop the new concrete potentially causing damage to the walls. I don't know what your current oversite has in terms of a DPC but if it is already sealed then you don't need to add any more, simply roughing up the old concrete surface, cleaning it, removing all dust will then give your new concrete something to bond to, alternatively you can use an agent to ensure good bonding. Then pour to required FFL. 

 

If there is no DPC (but what is stopping damp coming up into the floor area now anyway?) then I would be putting a DPC in much like you would when putting it over a new site ready for the dwarf walls to be flooded, again use a EPS or foam around the edges to minimise contact with the wall. Then pour.

 

If the oversite concrete is showing signs of cracking, sinking etc. then I would cut out 350mm squares of the existing concrete every 1200-1500mm, and dig down till you reach compacted hardcore or if none, soil, repack properly, and pour strong piers with re-bar, your new concrete will then essentially sit on these piers, go for about a C35 and this concrete will then be self supporting on your new piers. You can do this pour all in one and I would strongly advise just getting the concrete delivered rather than mixing by yourself.

 

 

Edited August 7, 2018 by Carrerahill