Concrete needed under suspended timber floor

[1974stephen]

We're about to start on a single storey rear extension which will have a suspended timber floor. I've read that (despite the rest of the house not having this) we will need to have concrete poured on the ground under the floor to stop weed growth, but it doesn't mention a thickness or whether some kind of DMP is required as well.

Is this right, and as a layman, I can't see why a weed fabric wouldn't suffice, or even nothing being as we don't have a weed problem anywhere else under the house, or any other house I've seen with a suspended timber floor for that matter.

I'm just wondering how much soil I might need to remove to allow this thickness of concrete and still maintain the minimum 150mm clearance between joists and ground.

 

[motorbiking]

If you don’t have a suspended floor in the rest of the house, why are you doing it here?

You will need ventilation in the air space and proper insulation anyway. Unless the builder is using a system to build the floor with a built in insulation, I’d be questioning if an insulated slab would work better ?

 

[1974stephen]

I may not have made myself clear; it's that the rest of the house doesn't have concrete poured under the suspended timber floor, but it does have a suspended timber floor.

 

[motorbiking]

Does the rest of the house have an insulated floor or is this a typical block and beam 1920s (etc) setup. There is no reason you can't duct the existing vents and go for a concrete insulated base.

 

[1974stephen]

House was built in 1962. Existing is 3"x2" timbers on dwarf walls with T&G boards and no insulation. I was thinking of doing it in the same way albeit with PIR insulation in between the new joists (which will be 8x2 due to span - we're using the existing conservatory wall as a single dwarf wall halfway across the width of the new extension rather than building three new dwarf walls to match existing and using smaller joist) because I can happily carry out this work myself, rather than paying someone to do concrete base, plus access for getting concrete in is tricky.

 

[motorbiking]

How will that comply with building control? (single dwarf wall).

 

[1974stephen]

No idea, it's what's on the architect's drawings. The existing joists are sat on walls that support the centre of the joists which are just single honeycomb brick walls - I thought that was how it was still done. Is there a need for anything more substantial to support the midpoint of a joist under the floor?

 

[motorbiking]

I've misunderstood.

I thought you were re-using an existing dwarf wall left over from your conservatory and that wont comply with insulation requirements. I.e a dwarf wall like this (only single skin).

 

[motorbiking]

from https://www.planningportal.co.uk/info/200130/common_projects/17/extensions/4
Suspended timber floor
As a requirement of the Building Regulations the structure should be protected against the growth of weeds and other plant-life. The ground should have a layer of concrete poured across and there should be a ventilated gap of at least 150mm between the underside of the timbers and the concrete, to prevent moisture gathering and affecting the condition of the joists. The timber floor joists should be sized correctly depending on their span (length between supports) and are normally laid across the shortest span from wall to wall with a gap underneath.

An intermediate wall with a small foundation may be needed to reduce the span and keep the thickness of the floor joists to a minimum. A damp proof course (DPC) should be placed between the timber and the wall. Insulation is then placed between the joists (thickness depends on the product used). Air vents should be placed underneath to provide ventilation to the void and the air should be able to travel from one side of the building to the other.

Is this not shown on the architect drawings?

 

[1974stephen]

The thickness of that concrete layer you describe is not mentioned on the drawings, in fact any concrete - I think they were drawings designed for planning, not for construction. It just shows a gap that's one block high between the soil and underside of the joists. The text you have copied is the exact same I came across, it mentions concrete, but how much?!!! 2mm, 20mm, 200mm?

 

[motorbiking]

75mm - 100mm + 100mm of hardcore. All takes you toward the idea of a solid floor. Are you using LA building control? Have you asked them?

 

[1974stephen]

LA building control yes - but an extended permitted development decision isn't due for a week so not had any contact yet. I'm just forward planning! Wow, 175-200mmm all to stop plant growth.

 

[motorbiking]

How long do you think 50mm of concrete will last on mud?

How deep is the floor to base currently?

 

[1974stephen]

Unless I'm missing something, my understanding is that the concrete under the timber floor is to stop the growth of weeds or plants. It just seems odd to have to put 200mm of hardcore and concrete to stop this in an area that will be perfectly dry and have no light, two things plants need to grow. In any case I can see other ways of achieving this that will be easier, cheaper and more environmentally friendly, plus the fact that doing nothing about stopping plant growth hasn't resulted in plants growing under the rest of the suspended timber floors in the house.

The existing house ground floor has a void of between 500 and 700mm between the ground and the underside of the joists as the site slopes slightly.

 

[freddiemercurystwin]

https://www.diynot.com/diy/threads/oversite.541530/