Depth of compacted hardcore for new floor slab?

oranjeboom · 5 Apr 2016

I will be removing current floor slab in the house and replacing with an insulated slab with UFH. I'm also aiming to do without a separate screed layer - simply powerfloated concrete as top layer.

Intend to have a powerfloated concrete slab which will contain the UFH pipes (DPM to add still):

- Tiles
- Concrete (with UFH) 75mm
- PIR insulation 150mm
- sand blinding or EPS 25mm
- Hardcore ??mm

If I did down to the correct depth without disturbing the ground too much, can I get away with 50mm of hardcore? Current floor has been insitu for the past 70yrs so I would like to think that the ground underneath is pretty compacted over time and would perhaps allow for minimal hardcore?

It's a domestic ground floor (bungalow).

TIA

r896neo · 6 Apr 2016

75mm of concrete is not enough. You'd be much better having 125mm of concrete with no hardcore than having 50mm of hardcore and 75mm of concrete.

Hardcore less than 3 inches thick does nothing for strength as a sub base needs to knit together to spread the load.

noseall · 6 Apr 2016

What's the deal with the UFH pipes and the the aggregate from the concrete? Will the concrete need pokering if you are going to power-float it? Not sure you are going about this the right way.

oranjeboom · 7 Apr 2016

r896neo said:
75mm of concrete is not enough. You'd be much better having 125mm of concrete with no hardcore than having 50mm of hardcore and 75mm of concrete.

Thanks. Not having to go any deeper that way. I will have to set the pipes a bit higher in the concrete, but that is do-able.

noseall said:
What's the deal with the UFH pipes and the the aggregate from the concrete?

You mean risk of puncturing the pipes with aggregate? Hopefully not, but good point.

noseall said:
Will the concrete need pokering if you are going to power-float it? Not sure you are going about this the right way.

Not sure if it would need pokering - could mess up my pipes!

Main reason for this approach was to save a bit of time, money and hassle by not having two seperate layers of screed and concrete.

r896neo · 7 Apr 2016

I think noseall is on about the possible voids under the pipes. You would need to vibrate the concrete in some way to make it spread around the pipes well or you could lose the effectiveness of the pipes to transfer heat properly.

Are building control involved in this? Worth checking with them they are happy to sign this off done in your spec.

Also what is the final floor finish. If your wanting to tile direct onto the concrete, with large format tiles for example beware, concrete can be a cruel mistress to get level without bumps or troughs.

mointainwalker · 7 Apr 2016

oranjeboom

From your comments, I'm not sure how many mm above PIR ( hell of a thickness there !)you aim to locate the piping and how you propose to do it. My experience was that plastic ties that spike into the insulation tear out fairly readily so I laid panels of 3 mm mesh and wired the piping to this. You can then put bits of slate/stone under the mesh to choose your distance off the floor to get better overall coverage of the pipe with your flooring mixture.

Will.. · 7 Apr 2016

Why have a powerfloated finish if you're just going to tile over it? A bull float finish is fine for tiling.

Also with that set up you'll be heating up your concrete slab before heating the room above. The idea is to have insulation under the heating pipes then a screed on top.

From bottom to top I'd go hardcore, blinding, dpm, slab, insulation, ufh, flowscreed.

Are you notifying building control with replacing a thermal element? They will stipulate minumum concrete/insualtion thickness.

oranjeboom · 16 Apr 2016

Coming back to this now after almost completing the roof. Thanks for all your comments/replies.

mointainwalker said:
From your comments, I'm not sure how many mm above PIR ( hell of a thickness there !)you aim to locate the piping and how you propose to do it. My experience was that plastic ties that spike into the insulation tear out fairly readily so I laid panels of 3 mm mesh and wired the piping to this. You can then put bits of slate/stone under the mesh to choose your distance off the floor to get better overall coverage of the pipe with your flooring mixture.

I'm aiming for high levels of insulation everywhere, and well above building regs. Plenty of slates to use now to elevate the pipework/mesh - thanks for the tip.

r896neo said:
I think noseall is on about the possible voids under the pipes. You would need to vibrate the concrete in some way to make it spread around the pipes well or you could lose the effectiveness of the pipes to transfer heat properly.

Also what is the final floor finish. If your wanting to tile direct onto the concrete, with large format tiles for example beware, concrete can be a cruel mistress to get level without bumps or troughs.

Okay, best to poker the concrete then. Better mesh and fit pipes to that too then.

Floor finish not decided yet, mix of carpet and tiles. That is also my fear with concrete....uneven surface for tiling.

Will.. said:
Why have a powerfloated finish if you're just going to tile over it? A bull float finish is fine for tiling.

In some rooms (e.g. utility and my study I was just going to go for a concrete polished floor finish. Never heard of bull float, will look into that.

Building inspector is here next week to look at roof, so will discuss floor options too. I may have to go down the traditional route of having a screed and concrete layer.

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Depth of compacted hardcore for new floor slab?

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