Foundation / subfloor concrete slab, yay or nay ?

MackDeToaster

My builder and engineer are recommending a foundation/subfloor structure as follows..


SR21 hardcore well compacted with blinding layer of sand, radon barrier, then insulation, underfloor and screed.


I had expected this but with a slab of concrete under the insulation - i.e. SR21 hardcore, sand, radon barrier, then concrete slab followed by the insulation etc.


Apparently it's quite common to leave out the slab now, but I'm thinking it'd be extra insulation and a bit of protection from any possible future subsidence. Would I be wasting money putting it in or what's the views on this ?

BryanF

It would definitely not offer any insulation benefit. The only one who can answer this is your engineer with support from who ever is specifying your insulation / thermal bridging detailing.

MackDeToaster

Thank you. The engineer doesn't think it's necessary, maybe I'm just old school in this as I'd always thought a slab was an integral part of the structure.

sydthebeat

Is the foundation a raft?

If not, a concrete subfloor is most definitely required IF the screed mentioned isn't structural and is just a typical 75mm screed.

The make up of the floor is fine, if the "screed" is a 150 concrete slab

MackDeToaster

It's a strip foundation. There's maybe a foot or more of sr21 well compacted. The screed would be the 75mm covering the underfloor heating pipework so wouldn't be structural ?? The builder and engineer say the sr21 is sufficient which is what I'm dubious about.

sydthebeat

MackDeToaster wrote: »

It's a strip foundation. There's maybe a foot or more of sr21 well compacted. The screed would be the 75mm covering the underfloor heating pipework so wouldn't be structural ?? The builder and engineer say the sr21 is sufficient which is what I'm dubious about.

while i am very slow to question an engineer who may have carried out calculations to come to this conclusion, but i will anyway.

if this is the case then its one of the biggest secrets in the domestic build industry. A structural concrete slab is always required in my opinion, and i havent heard anything about sr21 graded hardcore being an alternative. T1 struc is typically used in most domestic situations and is pretty much the same as the old 804 hardcore that was specified .... so i cant see why all of a sudden its deemed an alternative to a structural slab.

MackDeToaster

Thank you for the advice, much appreciated. Concrete slab it is so !

Still waters

MackDeToaster wrote: »

Thank you for the advice, much appreciated. Concrete slab it is so !

Will the builder have priced for this, just keep in mind if you push for a slab it will probably be extra, it does seem an unusual way of doing it, have you consulted another engineer to see what their view is on it, I'd be interested to see if theres any engineer on here to see what they think

MackDeToaster

I know another engineer so I'll ask him tmrw what he thinks. It's a self build so I'm managing it myself, I just haven't come across this before but speaking to a few other self-builders, their engineers and builders are doing the same thing and leaving out the slab so I thought I'd ask here as it doesn't seem right to me.

The chan chan man

Building Regulation TGD C Section 3.1.4 (a). You’re not complying with it unless the “screed” is 150mm thick - which I suspect its not...

Can you ask the builder/engineer about this and post the response/justification here?

Calahonda52

MackDeToaster wrote: »

My builder and engineer are recommending a foundation/subfloor structure as follows..


SR21 hardcore well compacted with blinding layer of sand, radon barrier, then insulation, underfloor and screed.


I had expected this but with a slab of concrete under the insulation - i.e. SR21 hardcore, sand, radon barrier, then concrete slab followed by the insulation etc.


Apparently it's quite common to leave out the slab now, but I'm thinking it'd be extra insulation and a bit of protection from any possible future subsidence. Would I be wasting money putting it in or what's the views on this ?

OP,
Your concern was with a slab of concrete under the insulation

My guess is that the builder wants
50/75mm screed with UFH
150mm slab
Insulation
radon
sand
hardcore

Screed is non structural

I suppose they want to use the UFH pipes as rebar

Calahonda52

The chan chan man wrote: »

Building Regulation TGD C Section 3.1.4 (a). You’re not complying with it unless the “screed” is 150mm thick - which I suspect its not...

Can you ask the builder/engineer about this and post the response/justification here?

Not just 150 mm
https://www.housing.gov.ie/sites/default/files/migrated-files/en/Publications/DevelopmentandHousing/BuildingStandards/FileDownLoad%2C1642%2Cen.pdf

Ground Supported Floors
3.1.4 A ground supported floor, unless it is liable
to be subject to ground water pressure, should be constructed of dense concrete laid on a hardcore bed and incorporate a damp proof membrane as follows (see Diagram 4):
(a) The concrete should be at least 150 mm thick (but thicker if the structural design requires) and be composed of cement (type CEM1) to I.S. EN 197 – 1 2001 and fine and coarse aggregate conforming to I.S. EN 12620: 2002: Aggregates for Concrete; and be
(i) reinforced concrete of a grade of concrete in accordance with I.S. EN 206-1:2002 (including the Irish National Annex) or
(ii) unreinforced concrete with a characteristic 28 day cube strength of 20
3.1.1
(a)
(b)
3.1.3 The following paragraphs (3.1.4 to 3.1.10) give some guidance on good practice insofar as it relates to non-complex buildings of normal design and construction.
A floor next to the ground should:
prevent ground moisture from reaching the upper surface of the floor (see Diagram 3); and
not be damaged by moisture from the ground.
Guidance in relation to dealing with moisture from the ground is contained in clause 11 of CP 102 : 1973 and in BS 8215 : 1991.
3.1.2
16
(b)
The hardcore bed should be at least 150 mm thick. Hardcore should conform with I.S. EN 13242:2002 and meet the specification as outlined in Annex E of the accompanying guidance document to this standard, SR21: 2004+A1: 2007. The layer of hardcore should be well compacted, clean and free from matter liable to cause damage to the concrete. Specific guidance is given in 3.4.2 of SR21: 2004+A1: 2007 on limiting the presence of a reactive form of pyrite which may give rise to swelling or sulfate attack on concrete.
N/mm2, a minimum cement content of 200 kg/m3 and a maximum water cement ratio of 0.85 (when volumetric mixing is required for small projects, a 1:7 cement aggregate mix may be used).

Metric Tensor

Don't forget the mandatory 200mm layer of T2 as well as your T1.

MackDeToaster

Calahonda52 wrote: »

OP,
Your concern was with a slab of concrete under the insulation

My guess is that the builder wants
50/75mm screed with UFH
150mm slab
Insulation
radon
sand
hardcore

Screed is non structural

I suppose they want to use the UFH pipes as rebar

Not quite, my concern was that they were saying no slab at all.

autumnalcore

What was the end result here out of interest. I don't know why you would go against the engineer here. The technical guidance documents are just that - guidance which is why everything in it is 'should' and why it says "However, the adoption of an approach other than that outlined in the guidance is not precluded provided that the relevant requirements of the Regulations are complied with."

There is nothing in the regulations I can see that would prohibit what the engineer was suggesting. If the hardcore and blinding was sufficiently compacted and screed was fibre reinforced whats the fuss?

EDA27

I am in the same boat now, except it is for remodelling job. Old house with suspended floor, now removed. Very dry underneath.
Just today I was given the same recommendation: 804 packed, sand prevents the plastic from damage, 1200 Gauge plastic/Radon Barrier, 3 no 50 mm layers rigid insulation, 500 Gauge Plastic Taped and sealed, UFH with 16 mm pipe clipped to insulation and scree.
I cant find many references to this approach thus keep to find the outcome for you?
Thanks in advance?

chooseusername

Calahonda52 wrote: »

OP,
Your concern was with a slab of concrete under the insulation

My guess is that the builder wants
50/75mm screed with UFH
150mm slab
Insulation
radon
sand
hardcore

Screed is non structural

I suppose they want to use the UFH pipes as rebar

That slab would take some heating up!

monseiur

EDA27 wrote: »

I am in the same boat now, except it is for remodelling job. Old house with suspended floor, now removed. Very dry underneath.
Just today I was given the same recommendation: 804 packed, sand prevents the plastic from damage, 1200 Gauge plastic/Radon Barrier, 3 no 50 mm layers rigid insulation, 500 Gauge Plastic Taped and sealed, UFH with 16 mm pipe clipped to insulation and scree.
I cant find many references to this approach thus keep to find the outcome for you?
Thanks in advance?

Are all internal walls removed or are you doing rooms individually ?

EDA27

they are removed - all bar one as it would be a large room + office

monseiur

EDA27 wrote: »

they are removed - all bar one as it would be a large room + office

The reason I asked is that the less internal wall you have the easier it will be to seal the radon barrier, indeed it may be next to impossible to fit it to required standard in your case.
I assume all internal walls will be stud partitions - so I would recommend, if you have the depth required, well compacted 804 blinded with sand- vapour barrier,
125mm concrete slab reinforced with A142 mesh - 150mm PIR insulation - 100mm screed.
Ensure that you fit a strip of insulation between finished floor screed and external walls to avoid cold bridging.

EDA27

Thanks,
My biggest concern was the lack of concrete sub flooring.
We decided to dig a bit deeper and do the following:
200-250mm 804+ sand compacted
1200 Gauge plastic/ Radon barrier (we are actually in low radon area and also essentially sitting on a huge granite geology), rigid insulation 150 mm in 3 no 50 mm layers 500 Gauge Plastic Taped and sealed, UFH with plastic trays, 16 mm pipe clipped to insulation screed provided by easy screed 35 mm to 50 mm Maximum.

One of the inside walls is granite and brick, and one is stud partially and partially brick.House is very old build from 1861 mostly granite.

I hope we are taking the correct steps

monseiur wrote: »

The reason I asked is that the less internal wall you have the easier it will be to seal the radon barrier, indeed it may be next to impossible to fit it to required standard in your case.
I assume all internal walls will be stud partitions - so I would recommend, if you have the depth required, well compacted 804 blinded with sand- vapour barrier,
125mm concrete slab reinforced with A142 mesh - 150mm PIR insulation - 100mm screed.
Ensure that you fit a strip of insulation between finished floor screed and external walls to avoid cold bridging.