New Concrete Slab - Advice

[richo106]

Hi all

 

We are putting a new concrete floor in our existing bungalow, the existing bungalow has a suspended joist/floorboard floor currently. 

 

The new floor make up is:

200mm Type 1 MOT

50mm Sand Blinding

150mm Concrete slab

200mm PIR insulation

50mm Screed

 

So total depth around 650mm

 

Due the slight incline in our land, one end of the bungalow is around 400mm deep below the floor but the other end is 800mm

To make up the short fall of the depth can we simply add more MOT to build up the layer flat? so it leaves 450mm depth up to the top of the blues?

 

All advice and information greatly appreciated

 

Thanks

 

[Carrerahill]

17 minutes ago, richo106 said:

Hi all

 

We are putting a new concrete floor in our existing bungalow, the existing bungalow has a suspended joist/floorboard floor currently. 

 

The new floor make up is:

200mm Type 1 MOT

50mm Sand Blinding

150mm Concrete slab

200mm PIR insulation

50mm Screed

 

So total depth around 650mm

 

Due the slight incline in our land, one end of the bungalow is around 400mm deep below the floor but the other end is 800mm

To make up the short fall of the depth can we simply add more MOT to build up the layer flat? so it leaves 450mm depth up to the top of the blues?

 

All advice and information greatly appreciated

 

Thanks

 

Before you continue, what are the reasons for this and have you assessed the impact on under house ventilation etc.?

 

I am not suggesting you should not do this, but I am suggesting you need to take into consideration quite a number of things. 

 

Even if it is cross air ducting for vent to other parts, ducts for services that may or may not go in in the future.

 

You don't mention a DPM in your make up. The answer is yes you can add more hardcore, lay it in 100-150mm layers and compact, but we aware of using a compactor inside a house as the alterations to ground conditions can damage founds and walls but at 200-400mm of hardcore you do need to compact it. 

Edited April 29, 2022 by Carrerahill

[jayc89]

Build up hardcore in layers of ~ 100mm. Scrap the sand blinding for a layer of EPS and given you have the depth, I'd use EPS rather than PIR. 

[richo106]

Sorry yes there is going to be two DPM layers.

 

What is the benefit of EPS over PIR? I am guessing cost. How much EPS would I need to match 200mm PIR, is there anywhere I could calculate this?

 

Thanks again 

[Conor]

10 minutes ago, richo106 said:

Sorry yes there is going to be two DPM layers.

 

What is the benefit of EPS over PIR? I am guessing cost. How much EPS would I need to match 200mm PIR, is there anywhere I could calculate this?

 

Thanks again 

It's just short of double, depending of its white or grey EPS, and density. It's about 1/4 to 1/3 price of PIR so wil always work out cheaper, and you'll save on hardcore and labour. You'll want 300-400mm of EPS.

[richo106]

Thanks for this Conor, I have just been look at insulation and prices (off the internet) etc...

 

My ground floor area is 120m2 so its either 42 sheets single layer or 84 sheets double layer. 

 

These are my findings:

 

PIR

 

100mm * 2 (GA4100) - 84 sheets - £45.98 a sheet - Total: £3862.32

200mm * 1 (XR4200) - 42 sheets - £106.19 a sheet - Total: £4460

 

EPS - White - Jablite Jabfloor 70

 

300mm * 1 - 42 sheets - £66.01 a sheet - Total: £2772.42

200mm * 2 - 84 sheets - £44 a sheet - Total: £3696.56

 

EPS - Grey - Jablite Jabfloor 70 High Performance

 

300mm * 1 - 42 sheets - £108.50 a sheet - Total: £4557

200mm * 2 - 84 sheets - £72.33 a sheet - Total: £6076

 

I have done this and i am still non the wiser which way to go, suppose it depends how much needs to dug up below the floor or added to etc

 

As always would welcome any advice/information on this from people with more experience than me is this 

 

Thanks again

[Chanmenie]

Why a slab and a screed ? That’s like doing it twice

put the slab on top of the insulation and use it as the floor, save the cost of the screed and less to dig out 

[jayc89]

9 minutes ago, Chanmenie said:

Why a slab and a screed ? That’s like doing it twice

put the slab on top of the insulation and use it as the floor, save the cost of the screed and less to dig out 

That's what we plan to do for the extension. I did wonder why so many decide to use a screed too. It was spec'd in our renovation and we blindly went along with it.

[richo106]

I just thought slab insulation then screed for UFH was a standard practice 

[Chanmenie]

I’m putting the UFH pipes on top of the insulation so they are in the slab, lots on here have done that or tied the UFH pipes to the mesh in the middle of the slab.

no need for screed 

[jayc89]

21 minutes ago, Chanmenie said:

I’m putting the UFH pipes on top of the insulation so they are in the slab, lots on here have done that or tied the UFH pipes to the mesh in the middle of the slab.

no need for screed 

Is there any difference in thermal conductivity between slab and screed?

[Chanmenie]

Hopefully someone more knowledgable than I will be along to answer that, but I can’t see there would be much difference, other than a 50 or 75mm screed will heat up much quicker but will also cool down much quicker. Because I’m using an ASHP which are low temp low flow the slab gives more thermal mass which reduces fluctuations once heated up.

[Jenni]

21 hours ago, richo106 said:

So total depth around 650mm

Are your existing foundations deep enough already to allow you to take this much out right next to them?

Our building control limited how deep we could go, said if existing foundations were exposed we'd have to underpin. 

[saveasteading]

18 hours ago, Chanmenie said:

Why a slab and a screed ? That’s like doing it twice

That is 2 of us think that then.

I cant see what the bottom slab is meant to be doing other than provide a prepared and clean surface.

So why 150mm?

I am in the middle of a 'discussion' on this with our Engineer (I am not on the Scottish register). He is insisting on A193 mesh s well, and resisting my proposal to use fibre mesh for crack control (which is still over the top but not costing a lot)

[saveasteading]

Other points arising, and forgive me repeating some answers.

PIR is twice the price and twice as good, so in this case use PIR for better insulation or less excavation.

50 screed is not enough. It will vary from 40 to 60 and is likely to break.

 

Exposing the walls and foundations should not be an issue for a 20thC building but needs an Engineer's say-so not BCO.

If the house is old with very shallow footings then it is an issue however.

 

Do it gently. If necessary do it in short runs, say 1m and infill with the stone and concrete as you go. Lots of bitty work but resolves that issue if necessary

 

[Chanmenie]

2 hours ago, saveasteading said:

That is 2 of us think that then.

 

It would come to a lot more than 2 If the other guys responded

there have been a few threads on discussing UFH pipes in the slab 

 

[Simplysimon]

band b floor so slightly different but - dpm, 200mm recticel, dpm and 100mm concrete, ufh pipes on top of 2nd dpm. no heat through it yet though, possibly this year.....

[saveasteading]

And what under the PIR?

[saveasteading]

4 hours ago, saveasteading said:

f necessary do it in short runs, say 1m and infill with the stone and concrete as you go

I was very unclear here. I meant expose the wall just locally for a metre, perhaps 600 wide digging-out, then backfilling with the new construction, and the existing wall strength will not be compromised. Then do the middle.

[SuperJohnG]

I have UFH pipes in my slab which is a kore insulated foundation. 250mm thick in the ring beam and 150mm everywhere else. 

 

Made perfect sense to me to put it in the slab as it cuts out so many steps. 

[richo106]

On 30/04/2022 at 15:07, saveasteading said:

Other points arising, and forgive me repeating some answers.

PIR is twice the price and twice as good, so in this case use PIR for better insulation or less excavation.

50 screed is not enough. It will vary from 40 to 60 and is likely to break.

 

Exposing the walls and foundations should not be an issue for a 20thC building but needs an Engineer's say-so not BCO.

If the house is old with very shallow footings then it is an issue however.

 

Do it gently. If necessary do it in short runs, say 1m and infill with the stone and concrete as you go. Lots of bitty work but resolves that issue if necessary

 

Regarding the concrete slab thickness that was just specified by structural engineer, i was surprised they said 150mm

 

The screed will liquid pumped screed is 50mm still too thin?

 

Yes I am there is not much need to dig close to the existing walls, however the foundations are quite deep..9 bricks and 8in concrete base

 

Another concern is that as the screed will only be 50mm over 200mm of PIR will this feel bouncy at all or will it feel solid? Planning to tile to whole ground potentially

 

[saveasteading]

16 minutes ago, richo106 said:

Regarding the concrete slab thickness

It won't be much less however good the ground is, and I can't comment without a lot more information, so I think accept this. I have argued myself for  a thinner slab, but there is little chance of the BCO accepting it without research that would cost more than the saving.*

The screed will be dead flat on top (well +/- a few mm). Getting the PIR dead level is less likely. 50mm seems thin to me, and I think you must take the advice of the installers/suppliers.

Your Engineer will advise on stability, and I expect will not be concerned.  Again , circumstances re important.

No it will not feel bouncy. Although thin, the screed is hard and spreads your load over a bigger area of the PIR. Thicker screed spreads it a lot further. The PIR is surprisingly strong ...try putting a board on it and see if it moves when stood/ jumped on.

 

* I am currently pressing to substitute fibre crack reinforcement for the specified steel mesh, which will save a lot of money...but there is resistance.

[jayc89]

Our refurb was 65m2. We have 100mm C35 conc, 100mm PIR (you could argue this isn't enough but we were battling against Victorian foundations so increasing the depth became the limiting factor) and 60mm liquid screed. It's been down nearly a year now and so far so good. 

[bassanclan]

Why not go:

 

MOT

Sand

Insulation

Structural Liquid concrete 65-75mm

 

 

 

[saveasteading]

7 hours ago, bassanclan said:

Why not go:

 

MOT

Sand

Insulation

Structural Liquid concrete 65-75mm

Because it is 'not normally done'.

No other reason I can see.

If the ground might heave (clay)  the perhaps concrete is needed.

 

7 hours ago, jayc89 said:

We have 100mm C35 conc

That is very strong concrete: any reason?  Did this have reinforcement?