DIY Insulated Raft

[Conor]

I'm committed to using a passive raft, have prices coming higher than expected. Best quote is £6.5k supply only, and I've priced up using standard EPS 100 and 300 sheets and it comes in at around £4k.

 

I've seen it mentioned on here before that people have done their own insulated rafts using standard EPS sheets and made their own edges.

 

Has anybody done this and how do you make the raised edges of the raft to take the concrete pour? Need to figure out if the passive raft "L" pieces are worth the £2k premium.

[joe90]

I forget where I saw it but someone who DIYed their passive slab made shuttering from timber to support the outside from “blowing”. I suppose it depends on your ground and your ability to guess the loads imposed, could be expensive and messy if you got it wrong! ?

[Cpd]

Possibly have the site nice and clean, build up the shuttering and then put loads of aggregate / hardcore  around the shuttering to support it and after the pour spread out the aggregate so that you have a nice clean environment to carry out the rest of your build. I think someone has done it and posted about it before but can’t remember who..... 

[Conor]

Thanks guys.

 

It's a basement build... So we will be backfilling around the slab, so I'm thinking we can get the forms in place, put perimeter drain in place, then backfill with stone until we get to the top of the forms. Should hold them in place... May rethink my plan to use self compacting concrete tho!

[willbish]

Pretty sure it was @Triassic who made his own raft.

How much of the £2k saving will be eaten up by timber formwork and moving aggregate around?

 

When I looked at pricing the savings from DIY was a lot more. Have the raft foundation systems held their pricing whilst the price of EPS has creeped up?

[K78]

AFT provided the best supply only quote for me. 

[Dreadnaught]

I have a supply-only quote from AFT as well (plus a full-installation quote).

 

I am following this with discussion with interest. If I thought I was up to a DIY-install, I would be tempted.

[K78]

When you consider that the supply prices include SE, a £2k saving would be nowhere near enough to tempt me to diy. 

[Thorfun]

9 hours ago, Conor said:

Thanks guys.

 

It's a basement build... So we will be backfilling around the slab, so I'm thinking we can get the forms in place, put perimeter drain in place, then backfill with stone until we get to the top of the forms. Should hold them in place... May rethink my plan to use self compacting concrete tho!

We are planning a basement and I was initially looking at an insulated raft but can’t figure out how that would work with the basement underneath? 
 

I fully admit that I have a ton of reading and research to do for our self build but I think not asking how you’re planning to do it would be a wasted opportunity! ?

[Conor]

1 hour ago, Thorfun said:

We are planning a basement and I was initially looking at an insulated raft but can’t figure out how that would work with the basement underneath? 
 

I fully admit that I have a ton of reading and research to do for our self build but I think not asking how you’re planning to do it would be a wasted opportunity! ?

 

Not quite sure what you mean about the basement being underneath? The insulated slab is the foundation to the building. Basement level goes on top of the insulated slab, with tanking system continuing from under slab up along basement walls. Then you've your ground floor slabs (usually precast hollow core concrete), then first floor (of you have one). The only insulated floor is the basement floor slab, everything above is then within the thermal envelope. 

 

However, if your basement is just plant, storage or utility, then you might want to keep it uninsulated, and instead insulate the underside of your ground floor slabs.

[Thorfun]

2 hours ago, Conor said:

 

Not quite sure what you mean about the basement being underneath? The insulated slab is the foundation to the building. Basement level goes on top of the insulated slab, with tanking system continuing from under slab up along basement walls. Then you've your ground floor slabs (usually precast hollow core concrete), then first floor (of you have one). The only insulated floor is the basement floor slab, everything above is then within the thermal envelope. 

 

However, if your basement is just plant, storage or utility, then you might want to keep it uninsulated, and instead insulate the underside of your ground floor slabs.

Yeah, sorry. I didn’t make it very clear! Our basement will only be under half of the house and will be a music and games room so needs to be cosy. 
 

as it’s only under half the house I guess I’d need 2 x separate slabs. One for the house sitting on the ground and one for the basement as the one for the part of the house with no basement won’t be able to extend to the part over the basement. 
 

I hope that makes more sense!

[PeterW]

8 hours ago, Thorfun said:

Yeah, sorry. I didn’t make it very clear! Our basement will only be under half of the house and will be a music and games room so needs to be cosy. 
 

as it’s only under half the house I guess I’d need 2 x separate slabs. One for the house sitting on the ground and one for the basement as the one for the part of the house with no basement won’t be able to extend to the part over the basement. 
 

I hope that makes more sense!

 

That is going to need some serious structural engineering as you’re going to create a pair of load planes for the slab and connect them with a vertical wall. Partial basements can be done, but it wouldn’t surprise me if your steelwork and concrete for then wall/ slab junctions and the

second slab would cost more than digging a full basement. It would probably be quicker and easier. 

[Thorfun]

24 minutes ago, PeterW said:

 

That is going to need some serious structural engineering as you’re going to create a pair of load planes for the slab and connect them with a vertical wall. Partial basements can be done, but it wouldn’t surprise me if your steelwork and concrete for then wall/ slab junctions and the

second slab would cost more than digging a full basement. It would probably be quicker and easier. 

Hi Peter,

that is very interesting as we're only doing half a basement to save money! I will speak to our architect and a structural engineer friend and see what they say.

 

thank you so much for the input.

[PeterW]

The whole point of rafts (which an insulated slab is) is to be able to cope with differential movement as the whole of the slab and house are a single unit. 
 

If you’ve got a split level then you will not be able to install the insulation for the upper slab until the lower slab and adjoining wall are complete. This will create a shear plane in the second slab, and unless you can work out a way of creating a shutter to pour a full monolithic top slab for the ground floor / basement ceiling. 
 

Ask your architect and engineer how they plan for it to be constructed to meet the passive slab you’ve asked for - I think they may struggle with the answer ....

[Thorfun]

1 hour ago, PeterW said:

The whole point of rafts (which an insulated slab is) is to be able to cope with differential movement as the whole of the slab and house are a single unit. 
 

If you’ve got a split level then you will not be able to install the insulation for the upper slab until the lower slab and adjoining wall are complete. This will create a shear plane in the second slab, and unless you can work out a way of creating a shutter to pour a full monolithic top slab for the ground floor / basement ceiling. 
 

Ask your architect and engineer how they plan for it to be constructed to meet the passive slab you’ve asked for - I think they may struggle with the answer ....

We don’t have an SE yet but I have spoken to 3 friends who are an SE, a builder and a surveyor and they have given me some good ideas but basically the outcome of the conversation was that we need to wait until we’ve got the Geotechnical survey to know what we’re dealing with. 
 

so, for now, we’re going to submit the planning application to get that ball rolling and then get the geotechnical survey done and engage the SE to design the foundations. If at that point we determine it’ll be cheaper to extend the basement And slab the entire thing then we can go back to the council for an amendment to the plans. 
 

the whole thing is complicated by the fact we have an annex, a garden room and a boot room that all branch off the main house that there’s no way a basement will be going under. 
 

once I have the geotechnical survey and structural engineer report I’ll start my own thread. 
 

sorry to hijack this one @Conor!

[Conor]

11 hours ago, Thorfun said:

Hi Peter,

that is very interesting as we're only doing half a basement to save money! I will speak to our architect and a structural engineer friend and see what they say.

 

thank you so much for the input.

 

You'll find there will be a negligible price difference. We started out with a partial basement, but when I looked in to it in more detail, doing a full basement made so much more sense. Doing a partial basement makes the foundation and ground works for the ground floor so much harder. You'll end up the massive, deep foundations for the rest of the ground floor as you'll be working with disturbed ground and inevitable differential movement issues. 

 

We're doing a full basement, but will only have one habitable room as we don't have the money to fit it out. But the construction, foundations, tanking, insulation will be so much easier and faster. Yes, I'll have a few more loads of earth to get rid of, but worth it.