In situ concrete first floor

Swan588

Hi all,

I would like to hear peoples experience if they have time to share.

I am currently building a two story house. First real step into the real world!

I am at the stage now where I am ready to begin construction of the first floor. I have decided to go with a cast in situ RC concrete floor supported with 6" x 3" permanent joists. These joists will then act as my false ceiling once the first floor is self supporting. I will be back propping the joists during the concrete pour for added support. I will be pouring a 6"/7" floor (yet to be decided) and plan on spacing the joists at 400mm centres.

I would be interested to hear how other people that used a similar approach got on and what specs did they adopt as regards floor depth, reinforcement used and joists centres? All info would be greatly appreciated!

Thanks in advance.

BryanF

Swan588 wrote: »

Hi all,

I would like to hear peoples experience if they have time to share.

I am currently building a two story house. First real step into the real world!

I am at the stage now where I am ready to begin construction of the first floor. I have decided to go with a cast in situ RC concrete floor supported with 6" x 3" permanent joists. These joists will then act as my false ceiling once the first floor is self supporting. I will be back propping the joists during the concrete pour for added support. I will be pouring a 6"/7" floor (yet to be decided) and plan on spacing the joists at 400mm centres.

I would be interested to hear how other people that used a similar approach got on and what specs did they adopt as regards floor depth, reinforcement used and joists centres? All info would be greatly appreciated!

Thanks in advance.

have you considered how you'll wrap a membrane around the joists to achieve air-tightness - i assume there staying on the inner leaf only to avoid thermal bridging? and if you have joists in your service void how will HRV ducts, wiring etc pass in the zone?

Mahogany Gaspipe

I find it hard to understand how this method would work structural.
Can you give us some information? How are the joists actually held up in place and how are they supported intermediately?

I'm sure that there are a whole load of issues to resolve before even attempting to address the thermal and air tightness concerns.

Worriedmind

Do you have a structural engineer on board?

I'm guessing you don't since you haven't even decided on slab depth and the job is on site.

If I'm correct, and you haven't already engaged an engineer, do so now before you kill someone!

Incidentally, the proposed detail sounds like absolute nonsense from a structural perspective.

mfceiling

All is well and good but have you thought about your services below your first floor? MHRV, soil pipes, electrics - have you a suitable void or do you start cutting the timbers below.

That's a serious weight on the timber joists.

Swan588

Thanks for the reply lads.

I a civil engineer myself but am working on civils work not housing so not totally familiar with it although I am liasing with another engineer friend of mine whenever possible who is involved in the game.

I havent decided on the 6/7 inch floor yet as I am doing a price comparison with the added steel required for the 6 inch as opposed to the 7 inch. In all likelihood though I will go with the 7 inch floor.

The joists will be bearing on the inner leaf of the outside wall and will span to an internal block on flat wall. The largest span will be 4 metres.

The timber joists that i will be using are for all intensive purposes going to form part of the falsework for the first floor slab and will then be used as my false ceiling once the floor has reached sufficient strenght and is self supporting. The joists will be back propped using acrows during concreteing and will be left propped for 10/14 days.

I will be liasing with my plumber and electrician prior to pouring the concrete to agree the location of the voids for services and will then be running these services through the joists where applicable. If needs be after I will core drill for any service voids that were missed although I would prefer to avoid this.

And Brian F I hadnt considered any membranes. Is this something you would advise? All I have ever seen done is the blockwork/mortar is finished tight to the joists.

BryanF

Swan588 wrote: »

I hadnt considered any membranes. Is this something you would advise? All I have ever seen done is the blockwork/mortar is finished tight to the joists.

on the structure: why reinvent? whats wrong with precast if you must use concrete at all

on the joists: joists are now generally on hangers and not built into walls

on air-tightness - start reading around boards.ie with the new regs (you should have a provisional BER done at your stage) air-tightness for HRV etc is more relevant, you need to think how you can achieve a complete air-tightness envelope (around the joists/first floor) as you'll struggle to have an efficient heating &/or ventilation system if all these junctions are not considered at this stage.

Mahogany Gaspipe

One potential problem, somewhat structural, is how you will integrate the ply, joisting and concrete floor into a single cohesive unit that will be and maintain level and straight.

Do you plan on fixing the ply back up to the set concrete floor with some type of fixing?
Also have you considered the need for bridging pieces between the joisting to stop them from twisting.

Failure to address either of these issues would likely result in your ceilings cracking, probably badly and very difficult to firstly arrest and then to repair.

Its good to see someone trying to come up with new methods; however I've worked on a couple of builds that went the in-situ pour route and IMO it is a method of construction suited to commercial and industrial projects; and not one off house builds.

Certified

There is nothing wrong with the method proposed by the OP, there is a little extra work at design stage but it has the potential to save enough in material costs to make it worthwhile in my opinion.
As already mentioned, it is good practice to place a strip of airtight membrane under the proposed floor around the external walls to ensure the airtight layer can be easily maintained.
The proposed method has to be designed and calculated by an engineer but seeing as the OP is an engineer and is also liasing with another experienced engineer, it should be fair to say that they know what they are doing.

I'm sure you have thought of this already but make sure to place void formers in the actual floor to route large services and duct work etc between the ground floor and 1st floor. If you are installing a MVHR system its best to have the duct runs planned out now so you can place the timbers in the right positions.

Just out of curiosity, were you thinking of allowing the concrete for the floor to fill in around/in between the timber joists on the supporting walls?

tred

Certified wrote: »

There is nothing wrong with the method proposed by the OP, there is a little extra work at design stage but it has the potential to save enough in material costs to make it worthwhile in my opinion.
As already mentioned, it is good practice to place a strip of airtight membrane under the proposed floor around the external walls to ensure the airtight layer can be easily maintained.
The proposed method has to be designed and calculated by an engineer but seeing as the OP is an engineer and is also liasing with another experienced engineer, it should be fair to say that they know what they are doing.

I'm sure you have thought of this already but make sure to place void formers in the actual floor to route large services and duct work etc between the ground floor and 1st floor. If you are installing a MVHR system its best to have the duct runs planned out now so you can place the timbers in the right positions.

Just out of curiosity, were you thinking of allowing the concrete for the floor to fill in around/in between the timber joists on the supporting walls?

Is this like a poured concrete deck?

Mahogany Gaspipe

Certified wrote: »

There is nothing wrong with the method proposed by the OP, there is a little extra work at design stage but it has the potential to save enough in material costs to make it worthwhile in my opinion.
As already mentioned, it is good practice to place a strip of airtight membrane under the proposed floor around the external walls to ensure the airtight layer can be easily maintained.
The proposed method has to be designed and calculated by an engineer but seeing as the OP is an engineer and is also liasing with another experienced engineer, it should be fair to say that they know what they are doing.

I'm sure you have thought of this already but make sure to place void formers in the actual floor to route large services and duct work etc between the ground floor and 1st floor. If you are installing a MVHR system its best to have the duct runs planned out now so you can place the timbers in the right positions.

Just out of curiosity, were you thinking of allowing the concrete for the floor to fill in around/in between the timber joists on the supporting walls?

Just out of interest how would address the issues in my post?

Swan588

Again thanks for all the replys.

To try and answer your question mahogany gaspipe I do not intend for the joists, plywood and slab to act as one composite structure. In the first instance the joists and plywood will form part of the falswork (along with acrow props) for pouring the slab and will then act as a false ceiling directly under the slab once the slab is self supporting....thus the joists will not be a structural element of the house just purely a false ceiling.

As regards reinforcement for the slab I will be using sheets of A393 mesh with 12mm U-bars at 400mm centres around the edge of the slab.

Bryan F - thanks for the help I will definely investigate the advantages of using hangers...seems like a better option for the integrity of the joists in the long term.

Certified - I was never intending to allow concrete to be poured in along the joists. I will be fixing plywood on top of the joist to form a deck for the pour. And cheers for the advice re ducting. I will be sure to get as much involvement from my plumber and electrician as I can prior to pouring.

I know this might not be a very conventional method but it is one that I believe I can save some much needed money on. Heres hoping anyway!

whupdedo

Why not just use a hollow core slab and a suspended metal ceiling underneath, then pour a 50 mm flow screed upstairs when everything is ready to finish

Certified

Mahogany Gaspipe wrote: »

Just out of interest how would address the issues in my post?

With correct temporary support and sizing of timbers the deck will stay level and straight.
I agree, the use of bridging pieces would be necessary.

mickdw

I dont see the benefit of going insitu concrete.
You are most likely adding significant additional weight onto the structure versus a hollowcore solution with no upside as far as I can see.
I also think that your idea of reinforcement is guesswork and without looking in detail at it, sounds insufficient.
Are you allowing costs of design and labour associated with this work when comparing this method against hollowcore or flat slab construction?

Swan588

Mickdw...as regards the reinforcement it isnt guesswork you will be glad to know. I am using a layer of A393 mesh along the bottom layer to counteract the tension bending moments. Along the edge of the slab i.e where it is bearing on the walls I will be using T12 U bars to counteract the tension bending moments on the upper face of the slab. I will also be using these T12 U bars around the cantilever void that will be left for the stairs for the same reason.

I am in no mad panic to get the house finished and dont mind losing the extra few weeks that is required for this method of construction.

I could be proven wrong but I believe having priced the various methods that this is the most cost effective...for my situation anyway. I suppose time will tell. It is also the method that I would be most familiar with from the line of work that I am involved with.

mickdw

I realise you are an Engineer yourself but I would have that checked by someone who works in the structural area if you are not a structural designer yourself.
Is this build being certified by another professional? Will they be happy with your design? Do you carry insurance for this type of design work or is your engineer friend certifying the design.
Are there wall loads mid span above? loadings from roof supports?
It just seems like madness to me when you can have a precast supplier in and provide you with a certified system.

mfceiling

I'd have to agree with mick. The sheer speed of setting in slabs then starting upstairs rising walls is hard to beat in my own opinion.