Insulated Raft Slab Foundations - Finished Flooring

[TBlew]

Hi All,

 

We are using an insulated raft foundation system with a 250mm concrete slab with mesh. We are going to install our UFH into this slab and will be building our walls with ICF using the same product as the foundations. 

 

I have seen people use this slab as their finished floor for the ground floor with a polished concrete finish. Has anyone done this that can offer some information? 

I am struggling to find any details for door frames/thresholds etc. as usually the internal floor height is above the cill/frame of the external doors.

We also have steel columns with baseplates wider that our internal walls. These were drawn to be hidden under a floor screen or similar. However, it seems pointless putting another screed on top of a slab. Is there anyway to hide these baseplates?

 

Image shows our slab make up and the steel column and baseplate under the 'finished floor' height. 

 

Thanks in advance! 

[saveasteading]

15 minutes ago, TBlew said:

this slab as their finished floor for the ground floor with a polished concrete finish.

Beautiful if it works, but very risky.

All concrete cracks. If poured to perfection the cracks will be invisible but will still be there, to collect dust, and to show up whenever there is a spill.

so it needs a lacquer finish.

Look closely at the floor in a b&q for example and there are cracks everywhere.

 

If you mean a ground, polished finish, with the stones visible then the same applies.

 

The cracks will be severe if you don't create a box-out around the columns to let it move as the concrete shrinks.

 

I recommend separating the functional concrete floor from the finished screed in a domestic situation. They cannot float it to kitchen specifications.

The workers pouring your slab are not thinking about an internal finish. And what if it is raining? 

 

You can later screed in many finishes, including very expensive, polished, exposed aggregate.

[TBlew]

16 hours ago, saveasteading said:

All concrete cracks. If poured to perfection the cracks will be invisible but will still be there, to collect dust, and to show up whenever there is a spill.

so it needs a lacquer finish.

Look closely at the floor in a b&q for example and there are cracks everywhere.

 

I recommend separating the functional concrete floor from the finished screed in a domestic situation. They cannot float it to kitchen specifications.

The workers pouring your slab are not thinking about an internal finish. And what if it is raining? 

Thanks for the info. 

Not so keen on exposing all the aggregates. From speaking with a company about the finish, it would need to be power floated and then polished rather than a full grind.

Then 2/3 layers of penetrating sealer.

They've suggested minimum 100mm concrete to be able to polish it. Seems like a lot of extra costs for a second floor. 

Is it highly likely that you will get cracks all over the floor even with expansion joints? 

What do you mean by kitchen spec? Just having a perfectly level finished slab? 

[Tom]

Don't forget to think about how you are going to brace the ICF - we are using Nudura and it needs bracing against the floor with props which need to be bolted down. If this is in to your final floor finish (albeit before the final grinding perhaps) could mess things up a bit. We are having the slab as the final floor finish in the main living are of our build so have poured small strip footings within the walls to take the bracing. Obviously not an option if you are doing a raft. 

[markc]

36 minutes ago, Tom said:

Don't forget to think about how you are going to brace the ICF - we are using Nudura and it needs bracing against the floor with props which need to be bolted down. If this is in to your final floor finish (albeit before the final grinding perhaps) could mess things up a bit. We are having the slab as the final floor finish in the main living are of our build so have poured small strip footings within the walls to take the bracing. Obviously not an option if you are doing a raft. 

You can use concrete blocks or weighted timber battens to fix bracing to if you don’t want to drill the floor

[saveasteading]

4 hours ago, TBlew said:

What do you mean by kitchen spec?

I was guessing what room might want a polished concrete floor. it is fashionable to have an exposed aggregate, or a sprinkle finish, in kitchens.

 

For a polished concrete floor you need a big machine with blades that scrubs the surface with blades a few  hours after it was poured. (picture  at end) and it really should be done in perfect weather..no rain , warm, and no wind, so indoors. Your concrete will have to more than 100 anyway so that isn't the issue.

Otherwise it is a grinding a machine much later.

 

4 hours ago, TBlew said:

extra costs for a second floor. 

you mean a second layer on the ground floor? then yes, but it isn't an option anyway as you wouldn't get the machine in and out of the building.

 

Joints. You don't need expansion joints as the concrete shrinks. It is a term misused for all joints. If you control the slab size and ensure that there are no barriers to shrinking (such as columns) and control the concrete mix to be drier than the workers will like, then there will be no big cracks. But there are still millions of tiny (possibly invisible) cracks.

4 hours ago, TBlew said:

2/3 layers of penetrating sealer.

The normal concrete sealer reacts with any cement that has not been chemically converted into the concrete. If the concrete is well made and poured then this doesn't really do anything and sits on the top.

If the concrete is poor, then it penetrates the surface and makes it a bit harder. It will also help a bit in closing miniscule cracks.

 

If I have wrong assumptions again, please say. You will be gathering that I can't see the point, as I see a polished concrete floor as a high quality, economic, but industrial, finish.

 

[TBlew]

Thanks all for your comments and advice. 

 

Although we weren't after a premium glossed polished concrete floor, more a dulled down grey finish rather than exposed aggregate. There seems to be a lot more reasons not to go down this route. After speaking with my structural engineer and they resonated your comments regarding environment for curing, cracking, bracing etc.

 

I am still keen to put the UFH into the structural slab as this will be the main thermal mass of the house and feel into the concrete of the walls. 

We are now thinking large tiles downstairs would be the way to go. 

 

Does anyone know what the best practice is for the bathroom plumbing is in a concrete slab? 

Mainly the shower trap as we would like to have a wet room shower on the ground floor.

Would it be a case of getting the drainage in as best we can the cutting out the concrete to sink the shower trap down and connect into the pipework set in the slab? 

Struggling to find details of this in an insulated slab. 

[Gus Potter]

Great info from all.

 

All I can add is great to see raft foundations.. and the base plates need holding down bolts so the nuts protrude a bit too.

 

In terms of drainage this can go above and below the slab. For me.. best way to make either option work is to leave yourself plenty leaway in terms of pipe position and fall. When the drains are below the slab even the best contractors get the odd pop up holes a bit off.. the pipes get a knock when concreting. Cut yourself a bit of slack on positioning and it will pay dividends later when the pressure builds, lots of trades on site and you have to make on the spot decisions.

 

For the shower trays I would try and leave almost a partial depth generous box out in the slab, check with the SE re rebar positions. In some cases you can omit the top rebar locally but you need some extra bars and diagonal bars round the edge of the boxout to prevent the slab cracking at the corners of the box out.. very important if you are having polished concrete floors. Mention this to your SE and it's a two minute job to add these bars to the reinforcement drawings.

 

Partial depth box outs.. the seasoned contractors / addicted self buiders etc will be quick to point out that this sounds great on paper but in practice hard to execute if not sometimes impossible on site. The easy way is to know where you want the box out with some easy reference points to measure from,  jump back onto the slab 18 - 24 hours after is has been cast, cut out the depth you need with a sthill saw and gently break out the green concrete. This is the easy way especially if you have omitted the rebar.

 

TBlew keep us posted!

 

 

[Onoff]

15 minutes ago, Gus Potter said:

box outs

 

I believe a few on here have put a section of EPS in, cast level with the slab, then dug it out later. 

[SuperJohnG]

On 28/10/2021 at 15:20, TBlew said:

Is there anyway to hide these baseplates?

I changed the design of mine so they were hidden in the wall thickness. 

 

2 hours ago, TBlew said:

am still keen to put the UFH into the structural slab as this will be the main thermal mass of the house and feel into the concrete of the walls. 

No Brainer that's what all of us have done and that the brucey bonus with an insulated slab.

2 hours ago, TBlew said:

Mainly the shower trap as we would like to have a wet room shower on the ground floor

I put a bit of EPS in there around  110mm soil pipe so I can dig it out later and do exactly as @Onoff says. 

[mvincentd]

My power floated raft is my finished floor throughout my ground floor.  The concerns being raised are all valid and it's good to be aware of them and know what you're getting into, but I believe they are all merely aesthetic.  There's no reason why it won't work, but you need a shift in aesthetic sensibilities, and a pretty open mind to your precise outcome.

The visual outcome of a powerfloat is a pretty individual thing dependent on numerous uncontrollable variables.  Protecting that surface for the remainder of your build can have implications.  Sealing powerfloat has problems associated with it, but (limited) solutions exist (look at Watco).

I have one crack in 160sqm after 3.5 years, maybe 0.3mm at widest, I like it , whats the problem.  I like the rust I sealed in from nails hanging around on the surface during the build, and the footprint.

I'm ok with the steel baseplates and the protruding bolts on show, but it could have been avoided using the same principle as for a wetroom.

I have 52 holes in the floor that have been grouted....from the bolts that supported soldiers holding wall shuttering up...there's no hiding them...so what.

I appreciate 98% of people dont want a floor like this...but i didnt want to create a floor like 98% of peoples.

[Jilly]

I get that. Trades keep telling me my ideas won't look good (to them!). 

[saveasteading]

18 hours ago, Gus Potter said:

it's a two minute job to add these bars to the reinforcement drawings.

 

Clearly Gus and others have done this in real life.

Theory is great but everything changes when a big concrete wagon turns up. The pressure to pour in 30 minutes, the force of the concrete pushing most things away, the groundworkers start shovelling like mad and standing on the wrong places and your bits of eps and diagonal bars end in the wrong place.

Therefore I suggest a timber boxout for each pipe, held with a few spikes, and laid when there is calm and a chance to check it. 

And the pipe that is in the boxout should be closed off with a polythene bag and sticky tape or string, so it doesn't fill with concrete.

 

For all the above, double the risk if the weather is foul. Miserable people with hoods up, mud on the site, water getting in the concrete. Design and manage with bad weather in mind, and it doesn't rain.

 

The diagonal bars are very important, but the groundworkers don't understand, never will and will not put them in (or may even take them out if they are not tied.) (Because they know better than any Engineer).  Therefore this is a diy  (or commercially, senior management) job , with tying wire to secure the bars to the main mesh.

[Nick Laslett]

We have a power floated insulated slab with embedded UFH and ICF walls. We are having engineered timber for our finished floor. Be warned the slab takes a battering during the ICF wall build. The bracing needs to be bolted to the floor and makes a right mess. We had our main internal walls ICF too. I posted about the issue of the bolt depth and avoiding the UFH pipes elsewhere on the forum. 
 

 

Edited November 8, 2021 by Nick Laslett

[TBlew]

Thanks for all your comments. Very helpful for the waste in the slab.

 

Still undecided for the floor finish. Everyone keeps telling me that I need to put a screed over the slab but this seems stupid. 

Does anyone have and door threshold details that they could share? 

In particular, we would like to have a fairly smooth transition from inside to out through our bi fold. If we want to use the slab as finished floor, or a small floor make up (an uncoupling membrane and tiles for example) how would the external doors sit with the slab? Do then sit on the concrete or on the insulation? 

 

Your responses are much appreciated 

[Highland build]

Hi @TBlew 

I,m on an insulated raft 200mm reinforced structural slab with the UFH on the top layer of reinforcement mesh (before anyone asks no I'm not building concrete multistorey) a lightweight timber frame but hey ho I absolutely thing this has been belt and braces possibly overengineered by the structural engineer but I'm obviously no the expert so I'm going with it! Anyway I'm just trying to workout thresholds for doors and sliders. I too though about power floating and polishing the structural slab as the final floor finish but decided too risky cracks expansion gaps could look pretty rough possibly meant cutting out edge module to get flush thresholds so amd just going to aim for a flat level slab and go with tiles or wood flooring straight onto slab. So apparently my door thresholds will be 25mm above the slab a tile plus adhesive will ( @nod ) just told me will take up that space and I know you can get ultra thin low profile 25mm shower trays so I am thinking phew we may not have to try all this complicated boxing out walk in shower area or digging out the depth of tray after concrete pour to achieve a flat tile to shower tray junction. Anyway just though it might help you @TBlew as when your having the cost of the raft and concrete slab and reinforcement a levelling screed is definitely something you could do without.

Anyone got any idea of a build up 25mm with wood how would you make up the space with a laminate to me an underlay layer will get in the way of the thermal transfer of heat from UFH pipes in slab.

Any of you insulated raft folk got any input, that won't work that will work gratefully received! Starting on site March.

 

[IanR]

8 minutes ago, Highland build said:

So apparently my door thresholds will be 25mm above the slab

 

Why don't you choose the floor finish you want and then set the threshold at the height above slab that matches the build up of your chosen finish?

 

I've got just 4mm of poured resin flooring on my power-floated slab, and the thresholds are set down to match this.

[Highland build]

2 minutes ago, IanR said:

 

Why don't you choose the floor finish you want and then set the threshold at the height above slab that matches the build up of your chosen finish?

 

I've got just 4mm of poured resin flooring on my power-floated slab, and the thresholds are set down to match this.

well I would love to do this it, would make life very easy but i presumed with the structural slab setting the level for the soleplate? and then window frames? set depths?  I might be wrong, first build I've never seen a kit go together, soleplates, window sliders doors being fitted so i presumed these ontop of slab would create a height of the door thresholds above the slab. 

[SuperJohnG]

I put rebates in my slab as part of the pour so I could have flush thresholds. So there are 50mm bits where the my 5m wide and 3m wide sliders sit. It was all then packed up to suit a 4mm LVT flooring, plus I made an allowance for a 3mm latex screed as my slab wasn't power floated. 

 

Worked out well. 

 

My underfloor heating is below my mesh attached to the insulated foundation with mesh on top ( but castles keep it off the pipes.)

[Highland build]

  Do you mean you cut away a depth of the edge of the concrete to sink the window thresholds lower? Has anyone on here used and isoquick base, @Gus Potter, @PeterW @Jeremy Harrisis what @IanR suggested make the threshold fit the flooring not the other way round possible with the isoquick raft? Thanks @SuperJohnG i need building by pictures to get my head round this bit! 

[gravelrash]

Be wary of power floated floor, it will not be as you expect. My recent slab was power floated and while most of it is reasonably flat the slab runs out of level across the 15m length with some 8mm high spots around utility pipes. I had it explained that the heavy slump required in cold weather can cause compacting issues when floated.

[IanR]

4 hours ago, Highland build said:

 i presumed with the structural slab setting the level for the soleplate? and then window frames? set depths?  I

 

Yes the sole-plate height is generally set on top of the Slab, but windows/doors are best outside of the slab, sitting over the insulated perimeter, or at least partially sitting over the insulated perimeter (to avoid a cold bridge), with the height adjusted to meet your Finished Floor Level, not the top of the slab, unless you are have polished concrete. 

 

Something along the lines of

 

 

This actually has a mat well recessed 20mm into the slab, and the door frame is set over the EPS upstand on a 9mm thick GRP "L" Profile. The door threshold is set at a height that brings it flush with the FFL, which is a 5mm thick poured resin over the power-floated slab.

 

Edited to add:

What's you planned wall build up, and does it insulation layer of the wall sit over the insulated perimeter of the slab?

 

Edited February 4, 2022 by IanR

[saveasteading]

 

58 minutes ago, gravelrash said:

I had it explained that the heavy slump required in cold weather can cause compacting issues when floated.

That's a new one.

'Compacting issues", so some of the 200mm of concrete levels nicely but some won't?

Concrete moves when shovelled, but it contains lots of stones and is not easy to level.

For precision the price goes up a great deal, and for ultra precision they have to grind the top off again.....and it is usually unnecessary.

 

What look like flat slabs  in retail warehouses are up and down just like yours, but nobody notices other than the racking installers (who adjust for it).

It is not a bowling rink so shouldn't matter.

 

4 hours ago, Highland build said:

i presumed with the structural slab setting the level for the soleplate

It is remarkable that we are all still reinventing how to build  a house, but we are.

I agree with drawing your situation to scale, starting with floor finishes and working down.

[SuperJohnG]

2 hours ago, Highland build said:

  Do you mean you cut away a depth of the edge of the concrete to sink the window thresholds lower? Has anyone on here used and isoquick base, @Gus Potter, @PeterW @Jeremy Harrisis what @IanR suggested make the threshold fit the flooring not the other way round possible with the isoquick raft? Thanks @SuperJohnG i need building by pictures to get my head round this bit! 

I'll see if I can look out a few pictures. 

[Gone West]

15 hours ago, Highland build said:

Has anyone on here used and isoquick base

We used an Isoquick raft foundation. We had an I-beam frame with an I-beam sole plate. The installers fitted the sole plate around the whole perimeter and then when the frame was up they cut out all the openings in the sole plate at ground level. Even though the slab was power floated it had lots of dips and we used a latex self levelling compound. A picture paints a thousand words as they say.