Strip, Trench or Insulated structural raft for timber framed house with Brick plinth

[IanR]

1 hour ago, zzPaulzz said:

 He says the free draining nature of type 3 will allow water to percolate beneath the raft.

 

Type 1 won't stop water "percolating" beneath the raft, it will just drain more slowly and wash out the fines as it drains away. For an insulated raft the EPS would typically sit on a "no fines" pea shingle or granite chippings and there'd be a perimeter drain to aid water clearing.  If you have a high water table, or it's temporarily high through surface water flooding then the water will breach the EPS, up to the DPM. Any EPS sitting in water is no longer acting as an insulator.

The section I included in your other post shows this.
 

 

Has your SE got experience of insulated rafts? There should be no issue contracting out the Engineering of the raft to a specialist and have your SE do the other details you've hired him for.

Do you know that you can get the formers you need (or at least make it much easier) to create the ring beam detail with the specified insulation? It can be a bit of a closed shop where the insulation providers will only supply the "approved" formers for an Insulated raft that's been Engineered by a company they have a relationship with.

Are you hoping to have a building warranty? Will your provider accept the raft design? And will Building Control accept it?

Edited February 7 by IanR

[saveasteading]

1 hour ago, Nickfromwales said:

there’s a decent argument both ways, (type 3 vs 1),

And the main thing is...most builders would happily use cheap harcore, poorly compacted and full of voids, then covered with a load of sand, some of which will gradually disappear into the voids.

 

So any spec is better than that.

 

As I said above, I'm used to designing and building for slabs with forklifts and racking. That fails if badly done. For houses the worst load is a sofa or the grand piano.

Observation of new mass house building shows me that they get away with a lot.

[zzPaulzz]

22 hours ago, IanR said:

Has your SE got experience of insulated rafts? There should be no issue contracting out the Engineering of the raft to a specialist and have your SE do the other details you've hired him for.
Do you know that you can get the formers you need (or at least make it much easier) to create the ring beam detail with the specified insulation? It can be a bit of a closed shop where the insulation providers will only supply the "approved" formers for an Insulated raft that's been Engineered by a company they have a relationship with.
Are you hoping to have a building warranty? Will your provider accept the raft design? And will Building Control accept it?

I've put the questions posed here to him. His responses have been confident and reflect the debate here so I don't believe he's out of his depth. I hear your points about the formers. If Building Control do not accept the SE's design then he doesn't get paid. 

 

[Nick Laslett]

On 06/02/2025 at 14:00, Alan Ambrose said:

That's my guys . One on the rake. And one standing and one digger driver watching .

 

V helpful content though guys.

 

@Nick Laslett - what did you source for MOT3? - I'm not finding local suppliers.

@Alan Ambrose, pretty mundane answer, my groundworks contractor organised and procured all the materials, they were supplied from Jewson.