Why construct a sub floor?

Rambowes · 16-01-2009 2:25pm #1

I'm about to start building a new bungalow and would like to know from experienced people why I can't install the finished screeded floor once the rising walls are built to the DPC. The advantage from my point of view is that I would not need a sub floor hence cost and reduced floor to ceiling height. My builder says he needs a stronger floor cause the walls are built from inside and the blocks are stored internally!

Construction method 804 binding - sand binding - Radon / DPC - 100mm insulation - underfloor heating buried in 75mm sand cement screed.

sydthebeat · 16-01-2009 3:04pm

a sub floor is required for structural reasons.

a 75mm screed on insulation on hardcore will most likely crack, either due to settlement or due to point loading by furniture etc.

in your case just reduce the hardcore by 75mm and add it onto the screed.

Rambowes · 16-01-2009 3:36pm

The ground is very well compacted its next to a quarry for slig or sleak or something sounding like that. The ground we have excavated is hard but I suppose you dont take the risk. Regarding point loading I would have thought 75mm screed could take your average sofa would you not have this issue anyway with insulation directly below even with a sub floor below that?

Slig · 16-01-2009 5:01pm

A concrete subfloor should be a reinforced slab. its not just to take the weight of the furniture above but also to provide a platform. The area around your exernal walls is compacted hardcore right down to the foundations and going nowhere but the centre of the floor is hardcore laid on the subsoil. If there is any shrinkage in this your floorslab shouls still stay level. A non reinforced slab might crack.

Rambowes · 16-01-2009 5:18pm

What the h*ll do I pay my engineer for I ran it past him not to have a sub floor just lay the insulation and screed onto the DPM / Radon and he said that would be fine. If I knew I would have to pay for a load of concrete and steel I would have just built a raft foundation because thats basically what you end up with and probably cheaper in the long run.

sydthebeat · 16-01-2009 5:37pm

Rambowes wrote: »

What the h*ll do I pay my engineer for I ran it past him not to have a sub floor just lay the insulation and screed onto the DPM / Radon and he said that would be fine. If I knew I would have to pay for a load of concrete and steel I would have just built a raft foundation because thats basically what you end up with and probably cheaper in the long run.

i wouldnt put the engineer down at all.

If he is quite happy to stand over this construction, then fine. I would assume has has done impact tests, or bearing ratio tests on the ground to be 100% happy that there will be absolutely no settlement...

,... if he hasnt, then he would be taking a risk in signing it off.

Is the build registered with homebond?

And dont say you would just have poured a raft foundation. A raft foundation, properly installed can cost 100-150% extra on top of what a traditional strip might cost.

If a trad strip will suffice, great.

Also, if this is a new build, then the incorporation of a subfloor is not going to reduce your floor to ceiling height!!!.. how could it, it doesnt exist yet!;)

AMIIAM · 16-01-2009 5:45pm

Rambowes, Does your drawing/ Planning permission not have any specifications? If not, why not? If so, you should adhere to them, you will have recourse to redress if you follow to the letter. Beware of allowing water pipes being buried under concrete though.

Rambowes · 16-01-2009 5:46pm

Correct it would not affect my floor to ceiling height and no the engineer has not carried out that kind of a test unless that’s when you go to site look around and say that ground is going nowhere.

I have decided to go with the sub floor as my finished floor will be adversely effected by weather and pallets of blocks laid onto it and I have just spoke to my Chartered Engineer and no steel is needed. I think I'll send him an email to confirm the same just in case...

Slig · 16-01-2009 5:53pm

AMIIAM wrote: »

Rambowes, Does your drawing/ Planning permission not have any specifications? If not, why not? If so, you should adhere to them, you will have recourse to redress if you follow to the letter. Beware of allowing water pipes being buried under concrete though.

All that is required for planning permission is finishes no spec is needed. Dont build of planning permission drawings it really isnt worth the hassle and if they are clearly marked as planning drawings there will be no comeback if they are wrong.

mrgaa1 · 16-01-2009 5:56pm

The main reason is that if you put in a finished floor the mess you get with building block, plastering the walls etc.. would be a nightmare to fix. Also you'll find that heating pipes are laid on top of the sub-floor.
So you have the sub-floor - make all your mess etc... and then when all else is finished put your finished floor in so that you can skirt, skim etc....

In a timber frame house a finished floor is applied as internal works are skim etc...

Rambowes · 16-01-2009 6:02pm

Amiiam the house was designed by an Arch Tech you are not obliged to construct to the construction method shown on the drawings from a planning prospective once you dont change outside agreed finish / appearance the level and height of the house which I am not.

AMIIAM · 16-01-2009 6:12pm

Is there no specification of quality of blocks, insulation damp courses etc? Also if your house has sub-standard foundations it goes without saying you will have a sub-standard house also. Builders and plumbers are notorious for taking the easiest way out (for them of course). The client is then left with the pending problems

Rambowes · 16-01-2009 6:24pm

AMIIAM wrote: »

Is there no specification of quality of blocks, insulation damp courses etc? Also if your house has sub-standard foundations it goes without saying you will have a sub-standard house also. Builders and plumbers are notorious for taking the easiest way out (for them of course). The client is then left with the pending problems

I am the client I will not be building sub standard foundations that for sure. Everything is specified right down to the specific wall ties to be used just querying the sub floor and to put this thread to bed I would always recommend a sub floor for all of the reasons discussed above.

Thanks for all the comments..

RKQ · 17-01-2009 5:37pm

150mm thick concrete floor slab is a structural requirement, see Building Regulations & Homebond manual, for ground supported slabs. A 75mm screed is soft and can move / crack quite easily compared to a 150mm conc slab.

It would be crazy to try to omit the slab. The most expensive faults to rectify are foundations and ground floors! Both structural elements must be professional instpalled.

Seek Professional advice before trying to make such drastic changes.

sinnerboy · 17-01-2009 8:48pm

you MUST have a subfloor

defective ground floors are the single biggest item homebond pay out to correct . Before you go much further order the homebond manual AND the set of right on the site leaflets

100 insulation may not be enough - min U value of 0.15 is required with UFH - ask your "singing off" professional to calculate , taking into account area /perimeter ratio

if using a foil faced insulation- make sure you place 500 gauge polythene over the insulation so the screed does not attack the foil face

make sure that the UFH pipes are pressure tested before you screed over

don't screed untill all heavy works are done - it wont survive undamaged otherwise

Why construct a sub floor?

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