Suspended timber floor structure (old house)

[Kaymo]

Hi,
Rebuilding a suspended timber ground floor (200 year old house). Previous joists were about 170mm depth but were resting on soil so were completely rotted.

I am planning on replacing with 95mm depth (by 45mm) C24 treated joists at 400mm centres. The room width is only 3m and the allowed clear span is 1.73 to 1.54 in the span tables so I plan to support the joists in the middle with brick or concrete block, giving a maximum clear span of about 1.3.

This seems quite flimsy compared to what was there before, but it seems to meet the requirements of the span tables and shallower joists are necessary to get 100-150mm gap for ventilation, which was completely lacking before. I can't dig down any further as ground level is already at level of footings.

Does this sound okfrom a structural perspective?

DPM on joist ends and piers, batten along bottom of joists and large staggered sheets of plywood on top for stability before screwing my solid wood boards back on. Vapour barrier and insulation between joists will also be added. Vents were blocked previously but this is all sorted now.


P.S. Before anyone suggests a solid floor I did consider this but have discounted for a number of reasons, mainly due to the lack of DPM in the 200 year old walls.

Regards,
Kenny.

 

[blup]

4x2 at those centres is fine IMO, as is 150mm below. Put a strip of dpm under the new support walls to prevent rising damp

 

[Notch7]

What are you going to bed the blocks on?

If you are going to dig out for a small trench foundation or pads, is the ground nice and stable?

 

[Kaymo]

What are you going to bed the blocks on?

If you are going to dig out for a small trench foundation or pads, is the ground nice and stable?

To be honest I was just going to rest the blocks on the ground. That's the way it was on the few joists that were already raised on bricks and the ground is quite well compressed.

 

[Kaymo]

4x2 at those centres is fine IMO, as is 150mm below. Put a strip of dpm under the new support walls to prevent rising damp

Thanks. I was thinking that I would put DPM at both the bottom of the blocks and the top. The bottom to stop the wall getting damp and the top in case the bottom DPM gets comprised over time as debris tends to build up in solums. Also on joist ends. Heard a tip somewhere to use flashband on the joist ends, both as a DPM and a bit of a pad.

 

[catlad]

A concrete lintel might be useful to rest the joists on in the centre, you can get them 100mm x 65mm seen them up to 2.4m in length.

 

[Kaymo]

A concrete lintel might be useful to rest the joists on in the centre, you can get them 100mm x 65mm seen them up to 2.4m in length.

Thanks. That's a good idea. I actually have a bunch of leftover dense and aerated blocks that I can use. My preference is to use individual blocks rather than a continuous wall to help maximise airflow which is pretty poor.

 

[ree]

OP,
Presumably you are seating joist end tails in brick pockets in walls?
The joist ends will need capping with DPC material. Dont use Flashband anywhere under a floor.
As a damp precaution, & before anything else, the whole solum can be covered with a sheet of plastic.
Insulate between joists before covering them with floor boards - & run all cables & pipes clipped to the bottom edges of the joists: then test them.

 

[Notch7]

To be honest I was just going to rest the blocks on the ground. That's the way it was on the few joists that were already raised on bricks and the ground is quite well compressed.

It might be ok if the ground under your house is always bone dry

But blocks resting on soil is not the most stable

 

[Notch7]

As a damp precaution, & before anything else, the whole solum can be covered with a sheet of plastic

I’m not terribly sure that is good practice in the UK

I think it’s best to leave the soil exposed and allow ventilation to do its thing

 

[ree]

Poster #10,
Its very common practice in the UK.

 

[Kaymo]

OP,
Presumably you are seating joist end tails in brick pockets in walls?
The joist ends will need capping with DPC material. Dont use Flashband anywhere under a floor.
As a damp precaution, & before anything else, the whole solum can be covered with a sheet of plastic.
Insulate between joists before covering them with floor boards - & run all cables & pipes clipped to the bottom edges of the joists: then test them.

No, there are no pockets for the joists. The original joists were sitting on bricks or cut out to fit around the footings. Its an old house, probably from the very early days of suspended floors.

I don't agree about covering the solum with plastic, that could potentially force moisture into the walls which don't have a DPC. My understanding is that the reason for ventilation is to carry away the moisture that evaporates from the soil.

Whats the reason for not using flashband under the floor. Isn't it effectively a DPM and also has a layer of bitumen in the middle?

 

[Kaymo]

It might be ok if the ground under your house is always bone dry

But blocks resting on soil is not the most stable

Thanks. I will give this some thought. After raking out the surface debris the ground did seem quite solid, and where I did try to dig down a little I needed a pick to help break it up. I had been thinking of maybe using a little sand to help bed them flat and had been thinking that the fact its only one block it should be ok, but I can see your point so will take a look at this again.

 

[Nige F]

Whats the reason for not using flashband under the floor. Isn't it effectively a DPM and also has a layer of bitumen in the middle?

The clue is in the name - flashing

 

[Kaymo]

The clue is in the name - flashing

That's not really an answer to the question. B and Q sell 'loft' insulation, but that doesn't mean you can't use it for floors.

I saw someone once before on another forum who said they primed the ends of the joists and stuck flashband on as the DPM. I think the reasoning was that it provided a bit of padding when resting on sometimes uneven stonework in old houses, I don't remember exactly.

To be honest a roll of DPM is about a fiver, so its cheaper and even if I wanted to try the flashband it would be as well as not in place of, but in theory flashband should work as a DPM too as it is two layers of plastic with bitumen in the middle.