suspended timber ground floor

[Gaima]

Hey all,

I was hoping someone could clear up some confusion on the construction of suspended timber ground floors.

Does the ground surface underneath *have* to be concreted? If so, what are the depths required?

My builder has put in this floor to our extension, with the correct sized joists cut into the inner block skin, and is today knocking out the morter to put DPC around the ends at my request as they breach the DPC. However at the moment only the new block edge.
The existing house has a solid concrete floor.
He says that concrete isn't needed, I think it is.

Searching hasn't yet brought up any definitive answers, only more confusion.
Stuff like not needing a vapor control layer underneath, but a recommendation for one over the joist tops and insulation under the chipboard floor.

Also, is 50mm kingspan on battens between joists enough?

Thanks
Mike

 

[Gaima]

My builder has put in this floor to our extension, with the correct sized joists cut into the inner block skin,

Also, is 50mm kingspan on battens between joists enough?

Thanks
Mike

Err, 6x2 joists 400mm centres, 3m span.

 

[JohnD]

you do not have to have oversite concrete, but it is preferable. A DPM is also preferable as it will reduce dampness rising from the ground. Many older houses in some parts of the country have strip foundations, no slab, no DPM and no oversite. They need plenty of air bricks to prevent dampness.

Did you have plans approved by Building Control for this extension? Did they specify oversite concrete or not?

If the extension is being built without plans it is a bit late to be deciding on how to build it now.

p.s. The BCO will be a good source; he will know what is required and what is usual practice in your area (including familiarity with soil, water table etc).

 

[Gaima]

Ooh, quick response, thanks John!

you do not have to have oversite concrete, but it is preferable. A DPM is also preferable as it will reduce dampness rising from the ground. Many older houses in some parts of the country have strip foundations, no slab, no DPM and no oversite. They need plenty of air bricks to prevent dampness.

DPM straight onto the ground?
The extension is 3m out from the building, 6m along it, with 3 airbricks on the single new 6m wall. Is that enough?
No airbricks in the existing house due to solid concrete floors.

Did you have plans approved by Building Control for this extension? Did they specify oversite concrete or not?

If the extension is being built without plans it is a bit late to be deciding on how to build it now.

Yep, plans show a solid concrete floor, builder built suspended timber.
From at least one thread here I understand building control can specify suspended timber despite plans to the contrary.

p.s. The BCO will be a good source; he will know what is required and what is usual practice in your area (including familiarity with soil, water table etc).[/quote]

May have to call them out then.

Thanks for the info.

 

[jeds]

There is no absolute requirement for a concrete oversite but you must have a barrier to prevent both moisture ingress and plant growth. So bare earth will certainly not do the trick. Easiest way in fact is a concrete oversite - usually a 75mm mix laid fairly roughly on a membrane does the job.

You must also have a minimum 150mm void beteen the underside of the joists and ground. And of course the void must be ventilated.

Ground bearing slabs are not always appropriate so It's not impossible that BC insisted on a suspended floor. Personally though I would have gone for a beam and block solution.

 

[Gaima]

There is no absolute requirement for a concrete oversite but you must have a barrier to prevent both moisture ingress and plant growth. So bare earth will certainly not do the trick. Easiest way in fact is a concrete oversite - usually a 75mm mix laid fairly roughly on a membrane does the job.

You must also have a minimum 150mm void beteen the underside of the joists and ground. And of course the void must be ventilated.

Ground bearing slabs are not always appropriate so It's not impossible that BC insisted on a suspended floor. Personally though I would have gone for a beam and block solution.

Thanks Jeds
Currently we have 160-170 of clear gap, so I'll pull up all the bits of various building rubbish, to dig it down a little for DPM and concrete. Would need some levelling off for DPM anyway.

Mike

 

[freddiemercurystwin]

http://www.planningportal.gov.uk/uploads/br/BR_PDFs_ADC_2004.pdf page 27 of the pdf

 

[andywatson]

That PDF gives me (potentially) a massive headache.

I am going to convert my garage into a bedroom, and the floor is currently concrete, but 6" below the level of the rest of the house. My simple plan was to suspend 4*2 joists across the 4.5m span- supporting them in the middle (and including a dpm layer above the prop).

This should leave me a clear 1" under the joists for air flow but also to run any cables as necessary. Unfortunately the PDF linked above shows a minimum of 150mm between the concrete & thebottom of the joists. This obviously makes it impossible to raise the floor 6".

How do I get around the problem?

 

[JohnD]

solid floor

 

[andywatson]

Don't really think laying a solid floor (I assume you are talking about concrete) is something I am comfortable with taking on myself.

Are you saying you can't raise a floor 6" with a suspended timber floor?

 

[JohnD]

you could put down a DPM, insulating slab and then screed or floating timber floor. Insulating slabs are usually 100mm but you can get then in 50mm and (I think) 20mm so could assemble the depth you need.

Would give a nicely insulated and damp-proof floor without the use of tonnes of concrete.

You could use engineered or T&G flooring as the wearing layer, or ply (I recently got some Spruce flooring ply and found the surface very shabby, so would suggest far-eastern WBP ply.

There may be a diagram somewhere in that PDF

Put your queston in the Flooring forum and you will find an expert.

 

[andywatson]

That looks like a much more workable solution- thanks

 

[jeds]

Personally I would lay 130mm polystyrene insulation, vapour barrier over and then finish to level with 18mm T&G chipboard. If the levels don't work out perfectly you could pack out with a layer of hardboard.