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How should I construct my ground floor? Can a timber floor be made to feel like concrete?


Dreadnaught

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I have for the last year or more been assuming that I would have an insulated-concrete-raft floor in my modern bungalow, but I am now having second thoughts. 

 

The alternative is to have a timber-cassette floor *. But I have a constraint which is making me re-consider things: height: a ridge-height limit **. Anything that I can do to lower my dwelling's height is attractive as I am struggling to get under the limit. A timber-cassette floor filled with PIR could easily be 200mm thinner ***.

 

The reason I favoured a concrete raft was that I felt a timber-cassette floor may well feel insubstantial, hollow and flex. 

But… am I wrong? 

Can a timber floor be made or feel as solid as a concrete floor? For example, would something as simple as a biscuit screed do the job (I have never seen one)? 

 

I would be grateful for your thoughts.

 

- - -

(* By the way, my foundations will be screw piles. The floor will have no direct contact with the ground surface.)

(** And I have a no-dig requirement too.)

(*** The build-up of the concrete-raft floor will be 775mm for a U-value 0.10 W/m².k, as follows: small pea shingle 50 mm, Cellcore HX-S heave protection 225, EPS 300 mm, reinforced concrete raft 200 mm.)

 

 

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I would stick to an insulated concrete floor.  just dig deeper and start the insulation deeper down so your ground floor level is lower.

 

It will be a LOT harder making (in effect) a sunken timber floor.  that will need ventilated space underneath it and air bricks etc so is not the normal way to go if you are trying to get your finished floor level lower.

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  On 01/02/2020 at 15:49, ProDave said:

just dig deeper

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Oh but I cannot dig down. I have a no-dig requirement because of protected tree roots.

 

I have learnt that "no dig" really means "don't dig very much" so I can probably dig down 200mm or so when I am levelling the site. 200mm will take the heave-protection CellCore HX-S at least.

 

A timber floor would start just above ground level.

Edited by Dreadnaught
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  On 01/02/2020 at 15:54, Dreadnaught said:

 

Oh but I cannot dig down. I have a no-dig requirement because of protected tree roots.

 

I have learnt that "no dig" really means "don't dig very much" so I can probably dig down 200mm or so when I am levelling the site. 200mm will take the heave-protection CellCore HX-S at least.

 

A timber floor would start just above ground level.

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What is the span the floor has to bridge?  If going timber I would suggest sleeper walls, but that will need digging.  How are you supporting the house with no dig? I presume screw piles and a ring beam?  Could you add to that to make sleeper walls for intermediate support of a timber floor?

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  On 01/02/2020 at 16:12, ProDave said:

What is the span the floor has to bridge?

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Max span is about 5.8 metres. If that span was problematic, I presume it would be quite straight forward to specify an extra screw pile or two.

 

  On 01/02/2020 at 16:12, ProDave said:

I presume screw piles and a ring beam?

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Yes screw piles. The floor will have no direct contact with the ground surface.

 

  On 01/02/2020 at 16:12, ProDave said:

Could you add to that to make sleeper walls for intermediate support of a timber floor?

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Yes, I presume so.

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We have timber floors spanning 5 metres, JJI joists at 400 centres downstairs and posi joists at 600 centres upstairs.  Both about 300mm thick.  If you could get an extra sleeper wall mid span on some extra screw piles that would bring the span down to 2.9 metres and should let you get a thinner floor make up.  A timber floor spanning 2.9 metres should be quite solid.

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  On 01/02/2020 at 16:52, Dreadnaught said:

Thanks @ProDave, as always. Good thoughts.

 

Why do you have a timber floor downstairs?

Do you get much underfoot flex?

Do you have a screed? I am wondering if that would change how it feels.

 

 

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Opposite problem to you. Sloping site. Ground floor level determined by garage floor level plus step up into the house. This meant at the far corner, FFL is over a metre above finished ground level and that is after raising the ground level at that side of the site.

Then add it we already had to dig off a lot of soft organic top soil, it would have been a LOT if inert fill to bring onto site to build it up that much to make a solid floor.

 

I actually "made it worse" by specifying to the SE that I did not want sleeper walls (I was trying to simplify the ground works and foundations)

 

The floors have turned out very well. Yes they do bounce, like any timber floor but not excessively.  Mostly engineered oak with some slate downstairs.

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  On 01/02/2020 at 17:31, Dreadnaught said:

 

Now that's an unexpected suggestion, worth exploring! Thank you. 

 

Supported on the screw piles?

How to insulate,  PIR insulation above somehow?

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You will have problems with depth as you’ll end up with a floor of about 360mm or more to do that span and still have a bounce. Concrete beam and block floors are not as strong as some think. 
 

Go with a JJI / Posi at 300 centres and you’ll have no bounce. It will be 245mm joists IIRC and you’ll need to insulate between and possibly above to get to BRegs levels of insulation. Bigger issue may be the bottom of the beams being exposed to damp/cold. 
 

Have you considered making some sort of cassette type floor with 300mm Posijoists and OSB and then pumping it full of cellulose ..? Add 25mm battens / timbers and UFH in sand/cement and you’ll get the best of both worlds in terms of feel under foot and insulation 

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  On 01/02/2020 at 17:47, PeterW said:

Have you considered making some sort of cassette type floor with 300mm Posijoists and OSB and then pumping it full of cellulose ..? Add 25mm battens / timbers and UFH in sand/cement and you’ll get the best of both worlds in terms of feel under foot and insulation 

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Thanks @PeterW. Interesting suggestion. Will explore. One question: why "Add 25mm battens / timbers", what are they for? (Its a ground floor.)

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  On 01/02/2020 at 18:22, Declan52 said:

The 25mm battens will allow you to lay ufh pipes and then cover it in a biscuit mix.

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Ah, I'm slow on the uptake. So a 1:8-biscuit-mix screed does not form the final floor surface. It is battened out and a final floor surface (e.g. Egger Protect) is added above it.

 

[Bottom] Deck 1 → Joists (filled with insulation) → Deck 2→ Battens, biscuit screed & UFH pipes → Deck 3 [Top]

 

Is that right?

Edited by Dreadnaught
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  On 01/02/2020 at 18:35, Dreadnaught said:

 

Ah, I'm slow on the uptake. So a 1:8-biscuit-mix screed does not form the final floor surface. It is battened out and a final floor surface (e.g. Egger Protect) is added above it.

 

[Bottom] Deck 1 → Joists (filled with insulation) → Deck 2→ Battens, biscuit screed & UFH pipes → Deck 3 [Top]

 

Is that right?

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nope missing a layer of OSB..! 
 

OSB > Joists (with insulation) > OSB > Battens and pug mix > DPC > Floor finish 

 

Top finish could be engineered wood or 9mm tile backer plus tiles. 

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Our ground floor is supported on steel beams connected to steel posts with the whole thing between .5m and 2.4m above the sloping ground. Our build up is concrete board-battons for ventilation-vpl-joists with insulation - plywood floor - DPL - battons,UFH,biscuit mix and then whatever we decide for final floor

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