Concrete raft foundation for timber frame annex

[StevieT]

Hi,

 

Can anyone offer advice on installing a concrete raft foundation to reg's.

This would be for a timber framed single storey annex, lightweight roof.

Thanks

[the_r_sole]

really you need to start with what foundations are suitable for the ground conditions and work from there - a structural engineer would always be my first shout for foundation design

[StevieT]

Ground is good and would be suitable for most foundations. Basically we are wanting to keep the build low and not install on a timber frame with plinth system.

Was really after the method of installation for an insulated raft to regs.

 

[Russell griffiths]

You need to look at the ground it’s sitting on and how you will provide insulation to the building, under the raft or on top. 

Is this to provide accommodation or just a games room. 

 

[StevieT]

Can the insulation be installed on top with a screed or is it required within the raft and say 50mm PIR over DPM internally as a floating floor?

[Russell griffiths]

Depends how low you need to go, bear in mind you will need 150mm above the finished ground level up to the damp course to provide adequate clearance. 

So any system will be that high before you add 2.4m walls and a roof. 

[Russell griffiths]

1 minute ago, StevieT said:

Can the insulation be installed on top with a screed or is it required within the raft and say 50mm PIR over DPM internally as a floating floor?

Depends what regs you are complying with 50mm doesn’t comply with anything. 

Whats the room for??

[StevieT]

"Depends how low you need to go, bear in mind you will need 150mm above the finished ground level up to the damp course to provide adequate clearance. 

So any system will be that high before you add 2.4m walls and a roof". 

 

 --Do you have any diagrams of how this would be constructed ? - residential

 

[Conor]

14 minutes ago, StevieT said:

Can the insulation be installed on top with a screed or is it required within the raft and say 50mm PIR over DPM internally as a floating floor?

 

Not the same scenario but here's what we're doing:

 

200mm ground bearing slab (or raft) with one layer of mesh on top of 100mm compacted stone. Rising walls built up from this level to 150mm above the outside FGL.

DPM (tanking in our case) is placed under the slab and under the the externall walls.

150mm insulation is placed on top of the concrete slab, followed by 100mm of screed.

 

If you call your surrounding ground level "0m". You'd dig down until you hit suitable stiff soil, lets say 400mm. You'd then add 200mm stone, 100mm concrete + mesh, DPM, 150mm PIR, 100mm screed to give you a FFL of 150mm above your exterior ground level.

 

You can also use an insulated raft foundation - basically the concrete is poured inside an EPS tub you make out of formwork. Check out the Kore webdite for typical drawings.

 

Whatever method you use, you'll have to have your FGL/thresholds 150mm higher than your surrounding final ground level.

Edited July 22, 2020 by Conor

[StevieT]

Thanks thats really helpful.

[StevieT]

Thanks Conor...what was that website?