Installing new suspended timber floor - fixed to walls?

[mrjoet]

Hi,

I’m digging up some of the ground floor in my house, it was just concrete on top of soil with tiles on top. I’ve done this to put in a new suspended timber floor to match the rest of the house, and then insulate under all the floors (PIR boards between joists).

In any case I was a bit unsure as to whether the new floor / joists need to be actually be connected to the walls. In the rest of my house, the joists are just sitting on sort of shoulder walls, they‘re not embedded into the walls (like on the first floor).

Having looked on the internet the standard practice seems to either build joists into the walls or use joist hangers by either building them into the walls or by bolting a ledger board to the walls and the have the joist hangers attached to the ledger board. Obviously my house is already built so I can’t be building into the walls, so I’m left with the ledger board and joist hanger option.

My question was whether it is actually necessary to secure the floor / joists to the wall? Is it also acceptable to build a wall (or pillars with a wall plate) close to the walls and have the joists rest on those so the floor won’t be in contact with the walls, similar to how the timber floor is in the rest of the house? The reasons I ask is that it’s an old house and the brickwork on the inner side of the external walls is damp and I’m worried that trying to drill into the bricks and bolt a ledger board to it would be a problem.

 

[Dereekoo]

From a stuctural point of view it is feasible ( similar to sleeper walls) as long as your support walls are bearing on ground sufficiently strong enough to take the design load, although with existing walls you can be pretty sure there would be no future settlement. What do BC say as you should be running past them if you are replacing a whole floor. After excavation you are required to cover the underlying ground with a concrete skin. Bolting a ledger board to the existing brickwork shouldn't be an issue and IMHO is the way to go

 

[^woody^]

My question was whether it is actually necessary to secure the floor / joists to the wall?

How else is there if you are digging up existing the concrete floor?

Also, are you aware that you can't just have a timber suspended floor over soil?

 

[^woody^]

Bolting a ledger board to the existing brickwork shouldn't be an issue and IMHO is the way to go

And the damp/rot prevention measures ....

 

[mrjoet]

Hi,

Thanks for the replies.

I was planning on speaking with building control next week.

After digging up the concrete and soil underneath I have laid down hardcore, followed by blinding sand. I’ll shortly be putting a DPM down and then putting in a layer of concrete. I’ve planned it so as to have a minimum 150mm between the new concrete floor and underside of the joists. Regarding how else apart from fixing the floor / joists to the wall, as I mentioned in my original post, the proposal was to build pillars and use a wall plate for the joists to sit on or use like shoulder walls similiar to the rest of the house but I wasn’t sure whether this would be acceptable as per the building regulations.

 

[oldbutnotdead]

Yes you can use dwarf walls to support your joists. You should have put your building notice on before you started...so do that now, get an inspection booked in, have a chat with the bco about dwarf walls

 

[^woody^]

Hi,

Thanks for the replies.

I was planning on speaking with building control next week.

After digging up the concrete and soil underneath I have laid down hardcore, followed by blinding sand. I’ll shortly be putting a DPM down and then putting in a layer of concrete. I’ve planned it so as to have a minimum 150mm between the new concrete floor and underside of the joists. Regarding how else apart from fixing the floor / joists to the wall, as I mentioned in my original post, the proposal was to build pillars and use a wall plate for the joists to sit on or use like shoulder walls similiar to the rest of the house but I wasn’t sure whether this would be acceptable as per the building regulations.

I'd suggest you check your detailing and clarify the requirements for a suspended timber floor.

The floor will need insulating, and the insulation should pass beneath the joists. The 150mm distance would be between the oversite and the insulation, not the joist.

The 150mm may need to be greater in certain soils.

150mm means the DPC and wall plate is only 75mm above the oversite. How is any excess water going to be dealt with?

Is a 75mm single course going to allow sufficient ventilation across the floor void?
How are the air bricks to be sited - on the oversite?

If the oversite is below external ground level, how is it to be drained?

Otherwise to avoid the risk of the new timber floor settling if built on the oversite, you are better off hanging the joists off the wall with suitable damp precautions. All timber should be pressure treated.

 

[Dereekoo]

The floor will need insulating, and the insulation should pass beneath the joists.

Normally between the joists

The 150mm may need to be greater in certain soils.

Why?

If the oversite is below external ground level, how is it to be drained?

Unless you are next to a river with a high water table level water ingress should not be an issue?

 

[^woody^]

Normally between the joists

Why?

Unless you are next to a river with a high water table level water ingress should not be an issue?

Thermal bridging

Soil heave

Gravity and Pascal's Law

 

[Dereekoo]

Thermal bridging

Soil heave

Gravity and Pascal's Law

Of little issue in suspended floors

If it aint heaved by now it never will

Gravity takes flow downwards

Pascals law , I assume you are in to artesian well theories now as opposed to lifting the house up

 

[^woody^]

I don't make the laws of England or nature, just obey them .... especially the latter.

 

[Dereekoo]

I don't make the laws of England or nature, just obey them .... especially the latter.

God for Harry, England and St George!!

 

[^woody^]

God for Harry, England and St George!!

I will thank you for not mentioning Harry.