Suspended floors

[Essex]

Victorian offshot to 14c Essex Hall House.
Considering timber suspended floor to replace concrete slab, which is cold and damp, rest of house LECA and limecrete. Therefore no DPM'S or DPC'S. lots of below floor ventilation. Do you use limecrete instead of over site concrete
Any one any experience of this?

Ta muchly

 

[AMc]

If I were in your shoes I might remove the concrete and use the same LECA & limecrete as elsewhere.

If you're going to use a suspended floor then why do you need oversite at all?
I can't see a benefit over a bare earth subfloor if the void is ventilated. If it isn't ventilated then you have problems with or without oversite.
Surely even a breathable oversite is more likely to move moisture into the walls (and rot the joists assuming there is no damp course) compared to a bare earth?

It sounds like your works are significant so make sure you have Building control on your side with any replacement floor solution.

 

[Moo]

That's most interesting, and confirms my gut reaction to our builder's insistence that we should have a cement layer beneath the new suspended floor in our kitchen.
To set the scene -
we're on good ole London clay with a very high water table;
the back third of the floor has been concreted
the rotting chipboard floor has been ripped out and they are digging down to lower the level by over a foot;
they are going to instal a healthy number of air bricks;
the ground level has also been lowered to a similar level.

My feeling is that we don't need any sort of 'oversite'. I can't see the point, and suspect it might do more harm than good.
I'm also wondering whether it might be an idea to get rid of the concrete so that the whole floor is suspended and we could get air bricks into the (short) end wall as well as the (long) side wall.

Comments, please!

 

[Penners]

If you're going to use a suspended floor then why do you need oversite at all?
I can't see a benefit over a bare earth subfloor if the void is ventilated.

I agree provided, as AMc says, that your underfloor void is well ventilated with grilles in the plinth. It is also advisable to insert insulation between your flooring joists, and ensure that any contact between a joist and its supporting brickwork is protected with a damp-proof membrane.

 

[Penners]

Comments, please!

Sounds a good plan to me, Moo. Does your builder give you any explanation of why he feels you should have concrete under your suspended floor? If he says "to protect the timbers from damp" does he understand/explain what will then happen to said damp?

As I said to Essex above, I would of course use pressure-treated timber for the joists and ensure that, at every contact point between a joist and the brickwork. the timber is isolated with a damp-proof membrane.

 

[Moo]

The boss man has just popped in and agreed that digging out the cemented bit of the floor would be the ideal thing to do, and proposes to put four air bricks in the long wall, and two in the short wall, so there should be a veritable gale blowing under the joists.
As far as the cement oversite (2" thick) is concerned, he's adamant that its necessary, to provide a stable and level base for the honeycomb dwarf walls. Not sure I agree with him about that, but at least I've convinced him that I don't want a nasty DPM. Though I will definitely insist on using it between the brick and the timber - promise!

 

[andrew gayton]

No.

the sleeper walls will require thier own footing. building them off a 50mm oversite is not a good idea. honeycombed dwarf walling off a footing with principal joisting on this with common joisting runing at 90o over if you have the depth, with boarding over. dpc to underside of principal joisting. you could - subject to relative heights of air bricks etc - loose fill between sleeper walls with the 25mm leca.

 

[middi]

Obviously Andrew is right about not erecting floors supported onto small cement screed, unless you don't mind "floating" floors,
A short and effective cut to your problem would be to use galvanised heavy duty joist hangers " slit cut and mortared" into the existing wall footings.
With adequate ventilation below these joists and insulation between and/or above the joists, with floor finish of your choise.
I would lower the floor to gain at leat 200mm clearance below joists and not bother about oversite cement.

Middi

 

[Essex]

All comments v.valuable. Have made my mind up to revert to LECA and limecrete. We are the builder, partner was looking for something faster, not on I think. It took us long enough to convince building control in first place.
Looking forward to spending more time on the forum, having had a three year sabbatical.
Again, much thanks

 

[JoceAndChris]

Welcome back Essex, and a fascinating topic that we all need to revisit as this is where the problems can begin!

 

[MatthewC]

All comments v.valuable. Have made my mind up to revert to LECA and limecrete. We are the builder, partner was looking for something faster, not on I think. It took us long enough to convince building control in first place.
Looking forward to spending more time on the forum, having had a three year sabbatical.
Again, much thanks

Presumably you have some need for a solid floor of some sort, as opposed to simply removong the concrete? I did a 21 sq metre limecrete/LECA floor here and I can't see why you want to go to that expense and trouble if you're going to cover it with a suspended floor.

My BCO was very interested to learn from me (!!) and readily accepted what I said about the limecrete slab (repeated almost parrot fashion from Mike Wye).

 

[Penners]

As far as the cement oversite (2" thick) is concerned, he's adamant that its necessary, to provide a stable and level base for the honeycomb dwarf walls.

You can, of course, rely on Andrew's view on this, with which I am presumptious enough to agree!

How many sleeper walls does your builder propose, in order to qualify for the description of "honeycomb"? I can't remember the dimensions of your building, but given sufficiently massive joists, I should have thought that a few (properly founded) longitudinal sleeper walls were all that would be required.

 

[stuart45]

I've always built sleeper walls off 4 inches of concrete on new builds, on a well compacted hardcore base.

 

[Moo]

Apparently we're going to have three longitudinal honeycomb walls.
The room is about about 9' wide.
The timbers seem pretty hefty.
The builders put DPM between timber and brick as a matter of course.

Does that seem OK?

 

[Penners]

In my ignorance I think I misunderstood the term, "honeycomb". I thought it perhaps meant a matrix of walls, whereas - having now researched it - I see it means that the bricks are laid with gaps between them. That's really good for air movement.

Three longitudinal sleeper walls sounds absolutely ample in a 9ft-wide room. Assuming that they're spaced equidistantly between the walls of the building, that means your joists only have to span 27 inches.

 

[Feltwell]

3 sleeper walls for a 9' wide room sounds like overkill to me, although it would allow you to get away with very small joists. I've ground floor rooms here of 14' joist length with no sleeper walls - and oddly enough you won't find many sleeper walls for your 1st floor rooms!

Having said that they won't do any harm for your ground floor room, and unless you invite Lady A's herd of elephants round you won't get much floor bounce.

 

[Penners]

I would agree, Feltwell, (hence my "absolutely ample" remark), except that this is a kitchen. Given the floor loadings and traffic that a kitchen tends to get, I think "overkill" may not be a bad plan.

 

[Essex]

All comments v.valuable. Have made my mind up to revert to LECA and limecrete. We are the builder, partner was looking for something faster, not on I think. It took us long enough to convince building control in first place.
Looking forward to spending more time on the forum, having had a three year sabbatical.
Again, much thanks

Presumably you have some need for a solid floor of some sort, as opposed to simply removong the concrete? I did a 21 sq metre limecrete/LECA floor here and I can't see why you want to go to that expense and trouble if you're going to cover it with a suspended floor.

My BCO was very interested to learn from me (!!) and readily accepted what I said about the limecrete slab (repeated almost parrot fashion from Mike Wye).

Just want a floor, if use LECA/limecrete (it's been down in the rest of the house for 6 to 7 years and is v successful) there will be no suspended floor.
I was just investigating whether a suspended floor was feasible because faster.

 

[Essex]

If I were in your shoes I might remove the concrete and use the same LECA & limecrete as elsewhere.

If you're going to use a suspended floor then why do you need oversite at all?
I can't see a benefit over a bare earth subfloor if the void is ventilated. If it isn't ventilated then you have problems with or without oversite.
Surely even a breathable oversite is more likely to move moisture into the walls (and rot the joists assuming there is no damp course) compared to a bare earth?

It sounds like your works are significant so make sure you have Building control on your side with any replacement floor solution.

My concern about over site is that it is in the approved doc's of the Building Regs.

 

[AMc]

My concern about over site is that it is in the approved doc's of the Building Regs.

Fair enough, I've not read that part of the docs to comment with knowledge, but you could probably argue it would be detrimental to the building. There is a specific wording in PartL1B for insulation IIRC. It will probably move ground level moisture into the walls and if there is no DPC then it would cause damage. Assuming your BCO is a reasonable person this should be acceptable, of course many people undertake work without seeking consent or informing BC but that would be your choice not my advice