To Screed or Not To Screed

[ultramods]

I was thinking about our slab insulation and UFH today. I noticed that our architect specified screed for the UFH. The slab make up is below.

 

GROUND FLOOR: Ground floor to be constructed of 70mm self-levelling concrete screed incorporating underfloor heating system, on 150mm KOOLTHERM K3 Kingspan insulation, on 125mm thick structural C35 grade concrete slab, to be laid with A252 mesh reinforcement as per engineers details at 50mm bottom cover, poured in equal sections and placed onto 1200 gauge polythene damp proof membrane - DPM to be returned vertically up around edges and lapped with horizontal DPC through wall a minimum of 150mm above ground - on a minimum of 50mm sand blinding and well compacted 150mm layers of Type 1 well graded granular fill.  25mm perimeter insulation to be provided at all external edges of concrete slab. U-Value to be no more than 0.11W/m²K. 

 

The house will be Timberframe with block+render and granite cladding rain screen.

 

From reading the various posts on the forum to achieve a better u-value and similar UFH operation for around the same cost of the current slab should I ask to architect to change the spec from 150mm PIR to 200mm PIR and use concrete instead of screed for the UFH pipes?

 

Also would this spec also be OK for the garage as I would like to have the flexibility of the garage being a habitable space at a later date.

 

The quotes we have received for the foundations/slab are:

 

Foundations
Excavate trenches for foundations, pour
C20 concrete (no mesh)				11.00	m3	£185.00	£2,035.00
Backfill foundation with suitable material
and 150mm of type 1 sub base compacted
in layers (inc garage)				64.00	m3	£30.00	£1,920.00
50mm sand blinding,  Radon DPM and 125mm of C35
to house sub floor. 1 layer A252 mesh				152.00	m2	£32.00	£4,864.00
150mm Celotex inc 25mm perimeter insulation				101.50	m2	£24.50	£2,486.75
65mm UFH screed				152.00	m2	£26.50	£4,028.00
150mm concrete to garage floor, I layer A252 mesh				37.00	m2	£26.00	£962.00
Backfill around the perimeter of the found
to accommodate the scaffold				32.00	m3	£26.00	£832.00

Edited April 30, 2018 by ultramods

[PeterW]

Are they the material costs or including labour costs on the right..??

 

if so, MoT is £11-14 a tonne delivered bulk and your standard C20 should be no more than £90 dependent on location

[ultramods]

4 minutes ago, PeterW said:

Are they the material costs or including labour costs on the right..??

 

if so, MoT is £11-14 a tonne delivered bulk and your standard C20 should be no more than £90 dependent on location

 

That's the complete cost of the completed foundations, so supply and install

[PeterW]

Ok - odd that some are cubic and the others are square metres

 

can you not dispense with the structural slab and screws, make it all slab and put 200mm of insulation and then a 150mm slab over the top ..?

[ultramods]

@PeterW what do you mean by screws?

[PeterW]

21 minutes ago, ultramods said:

@PeterW what do you mean by screws?

 

Screed !! Sorry ..!!

[ultramods]

2 minutes ago, PeterW said:

 

Screed !! Sorry ..!!

 

Ah, that's what i was proposing, isn't it?  ?

 

Edited April 30, 2018 by ultramods

[PeterW]

3 minutes ago, ultramods said:

 

Ah, that's what i was proposing, isn't it?  ?

 

 

Not sure..? Thought you were suggesting concrete for the top rather than S&C. 

 

It it would be bottom up :

 

150mm MoT

sand blinding

DPM

Insulation

slip membrane

120-150mm concrete with rebar and ufh pipes

 

 

[ultramods]

1 minute ago, PeterW said:

 

Not sure..? Thought you were suggesting concrete for the top rather than S&C. 

 

It it would be bottom up :

 

150mm MoT

sand blinding

DPM

Insulation

slip membrane

120-150mm concrete with rebar and ufh pipes

 

 

Yes i was proposing concrete instead of screed as it will be cheaper and dry faster and we will be tiling the floors.

[DreamingTheBuild]

Thanks for the figures Ultramods, would you mind saying what your footprint is please?

 

[PeterW]

22 minutes ago, ultramods said:

Yes i was proposing concrete instead of screed as it will be cheaper and dry faster and we will be tiling the floors.

 

Ok so that’s still a big cost and additional complexity that you don’t need. Is this TF..?

 

Ground bearing structural slab would save you £4K plus the additional cost of excavating less 

[ultramods]

3 minutes ago, DreamingTheBuild said:

Thanks for the figures Ultramods, would you mind saying what your footprint is please?

 

Size 10

 

the ground floor is 145 sqm internal floor space and the garage is 36 sqm internal floor space

[ultramods]

@PeterW when you say ground bearing slab are you meaning having no strip foundations? I have attached the current plans for the foundations.

565-15 Foundation and sections.pdf

[PeterW]

34 minutes ago, ultramods said:

@PeterW when you say ground bearing slab are you meaning having no strip foundations? I have attached the current plans for the foundations.

565-15 Foundation and sections.pdf

 

Sorry - try this as way of explaination. This is one I did recently that gave insulated ground bearing slab and also a filled cavity below ground which helps for UFH losses. 

 

[Tin Soldier]

@Ultramods

 

I changed my floor makeup at the last minute to save on screed after getting quotes for 6k plus for it, plus the cost of the 150 PIR, plus the drying time etc.

 

Changed the floor from 100mm c25 with 150mm PIR on top, then ufh pipes, then 70mm screed

to 125 c25 mm with 300mm of eps70 underneath, with my underfloor heating pipes ziptied to the mesh held halfway  in the concrete.

I only needed 230mm of eps to make up the same U Value, but I went with 300mm as it allowed me to get a decent bulk discount on 100mm sheets.

 

I installed all the ufh pipework and insulation myself over the course of a few days

Edited April 30, 2018 by Tin Soldier

[ultramods]

7 minutes ago, Tin Soldier said:

@Ultramods

 

I changed my floor makeup at the last minute to save on screed after getting quotes for 6k plus for it, plus the cost of the 150 PIR, plus the drying time etc.

 

Changed the floor from 100mm c25 with 150mm PIR on top, then ufh pipes, then 70mm screed

to 125 c25 mm with 300mm of eps70 underneath, with my underfloor heating pipes ziptied to the mesh held halfway  in the screed.

I only needed 230mm of eps to make up the same U Value, but I went with 300mm as it allowed me to get a decent bulk discount on 100mm sheets.

 

I installed all the ufh pipework and insulation myself over the course of a few days

 

Thanks @Tin Soldier. So did you still use screed instead of concrete or is that a typo? What did you use to hold the mesh halfway before the screed/concrete was poured?

[Tin Soldier]

typo, sorry

 

Concrete meshmen, got them from Travis perkins, cheap as chips

 

they support at either 40mm or 50mm, dependent on what you need.

[ultramods]

@Tin Soldier Thanks. I am wondering if I should look at doing the insulation and UFH myself as well. Will need to get a further breakdown from the builder to see what he would be charging for this part.

[ultramods]

13 minutes ago, Tin Soldier said:

typo, sorry

 

Concrete meshmen, got them from Travis perkins, cheap as chips

 

they support at either 40mm or 50mm, dependent on what you need.

 if you are using 40mm concrete meshmen then the pipes on top of the mesh is 70mm deep enough, approx 14 mm of concrete on top of the pipes

[JamesP]

1 hour ago, ultramods said:

Size 10

 

the ground floor is 145 sqm internal floor space and the garage is 36 sqm internal floor space

Just checking the amount of celotex, they quoted for 101 sqm yet your ground floor is 145 sqm. 

[ultramods]

2 minutes ago, JamesP said:

Just checking the amount of celotex, they quoted for 101 sqm yet your ground floor is 145 sqm. 

I was wondering about that as well

[JamesP]

1 minute ago, ultramods said:

I was wondering about that as well

Only mentioned because I bought 70 sheets in December as the price went up 30% in 6 months.....

[Nickfromwales]

On 30/04/2018 at 15:27, ultramods said:

 if you are using 40mm concrete meshmen then the pipes on top of the mesh is 70mm deep enough, approx 14 mm of concrete on top of the pipes

No, but you'd be having 120mm concrete not 70mm screed layer  

Im all for the proposed approach by others above, as in ditch the concrete subfloor and have the concrete as your top layer over insulation. The way its proposed in the OP is archaic and id rather have more insulation than concrete in my given floor construction ( and given depth ). 

Yes, do the insulation yourself, but the sand blinding layer need to be spot on flat as that'll leave voids under the insulation otherwise. Id recommend :

 

MOT1 then whack that flat,

then sand blind,

then 25mm of EPS as a sacrificial layer to protect the DPM,

then DPM,

then insulation,

then light gauge membrane,

then 120mm reinforced concrete floor, ( including mesh and UFH pipes as required ),

then your chosen floor covering including any surface prep required. 

Edited May 1, 2018 by Nickfromwales

[ultramods]

Thanks @Nickfromwales it's all starting to make sense now.  

 

 

[Miek]

One thing to think about is the quality and flatness of the finished floor. Screed is easier to work flat whereas a concrete pour will need an experienced hand to get it as flat and level as a screed finish. What is the final floor going to be, and what are the tolerances for level?