Raft comparison

[deuce22]

Hi.

 

I've never constructed a raft before and will get the exact specs from the SE at some point. I'm just trying to plan out which method I'm going to use. I've been told how the raft gets constructed, so I'm working off that.  

I'm estimating that the depth will be around 500mm poured in 2 separate amounts. The insulation is then laid and screed finish on top. The other method is to used the insulated forms (such as isoquick. From what I have watched the forms are laid and then concrete poured once for a finished floor. I have been quoted £7700 for the forms from Isoquick and the concrete depth is then 250mm.

I've done a quick calculation and it has worked out around £1500 cheaper to use the forms. I'm wondering if I've missed something out as I would have thought this would have been more expensive.

 

Thanks.

[Mr Punter]

500mm sounds really thick.  Maybe wait for the spec from the SE.

[Carrerahill]

1 hour ago, deuce22 said:

Hi.

 

I've never constructed a raft before and will get the exact specs from the SE at some point. I'm just trying to plan out which method I'm going to use. I've been told how the raft gets constructed, so I'm working off that.  

I'm estimating that the depth will be around 500mm poured in 2 separate amounts. The insulation is then laid and screed finish on top. The other method is to used the insulated forms (such as isoquick. From what I have watched the forms are laid and then concrete poured once for a finished floor. I have been quoted £7700 for the forms from Isoquick and the concrete depth is then 250mm.

I've done a quick calculation and it has worked out around £1500 cheaper to use the forms. I'm wondering if I've missed something out as I would have thought this would have been more expensive.

 

Thanks.

500? What are you building? Multistory carpark?

 

250mm sounds more sensible, I think what you have missed is the cost in 500mm of concrete vs. 250mm!

 

 

 

 

[Conor]

So, here's my story. We originally wanted an insulated raft for our build (2.5 story ICF build Inc 100m² basement). Loads of promises etc from the sales people. When the SE started her calcs, it turned out we'd need massive ring beams (1200mm and 1700mm wide and 400mm deep in places) as well as loads of steel. It was all because of the limits of the EPS300. The option of making the entire raft with eps300 was silly money, £11k I think.

 

If you are doing a concrete build (ICF or block, with concrete floors) over 1.5 stories you might find that a traditional raft will work out simpler and cheaper. Our design now is a simple 200mm thick raft (250mm at edges) with only one layer of mesh, so much faster and cheaper to build. We'll then have 150mm insulation and 100mm screed.

 

Don't get me wrong, an insulated raft is a great idea, just be aware of the limits of loading onto EPS.

 

Be wary of the insulation form people saying it's a simple matter of a xxxmm thick raft and a bit of steel...

Edited April 23, 2020 by Conor

[Bitpipe]

We built an insulated RC basement with a MBC timber frame on top.

 

Substrate is clay on river gravel on chalk. Chalk is not good as it's soluble, soft and can have 'solution features' - fancy word for holes. So the raft needed to be self supporting.

 

We did two passes of ground investigation and SE specced a 300mm thick RC flat slab sitting on 300MM EPS200. Point loading was within published limits.

 

As we went for an 'open box' with suspended timber floor on top, the walls needed to be 300mm thick as they acted like cantilevers. 16t of steel rebar in the finished basement which is 11.5m x 10.5m (and a small pokey out bit for plant room).

 

49 blocks of 300mm EPS 200 to cover the footprint was £3.6k ex VAT. 200mm EPS 70 for the basement walls was about £1.2k I think (and I ordered too much, had about 10 sheets left over, some went under garage slab, some is under the kids summer pop up pool).

 

To match up with the passive spec MBC design, we needed a continuously insulated envelope.

 

If your slab sits direct on ground then  you will have an uninsulated cold bridge where the concrete in your ICF walls meets the slab. Not sure what this means in practice.

[Oz07]

@LA3222 just did it diy might be worth checking thread out

[LA3222]

You can DIY the insulated raft fairly easily. I did most of the raft myself, the only outside help I had was:

 

1. Operator for the excavator 

2. Groundworkers to do the concrete pour and powerfloat.

 

Everything else was me with a little help from the missus and father in law. Altogether it took me about 6 weeks, the main reason being the excavation/stone subbase took 2 weeks and it was peak storm season which was a nightmare to work in and I made a couple of mistakes which cost me time.

 

My slab is 180m2, if it had been good weather and I'd planned the lorries how I thought rather than what the plant operator said then it could probably have been done in half that.

 

I made mistakes aswell which cost me time, think I covered them in the thread previously referred to.

 

 

Edited April 24, 2020 by LA3222

[deuce22]

Ok. So as I said, I haven't constructed a raft before (only strip foundations). Can somebody give me an idea of how it is laid? I was told it was shuttered and poured once, then shuttered on top to create the step cavity tray and then poured again. Insulation is the laid and then a screed finish.

Is this correct?

 

Thanks. 

[Jeremy Harris]

7 minutes ago, deuce22 said:

Ok. So as I said, I haven't constructed a raft before (only strip foundations). Can somebody give me an idea of how it is laid? I was told it was shuttered and poured once, then shuttered on top to create the step cavity tray and then poured again. Insulation is the laid and then a screed finish.

Is this correct?

 

Thanks. 

 

 

Some photos in this link showing how our insulated raft was laid: http://www.mayfly.eu/2013/10/part-sixteen-fun-and-games-in-the-mud/

 

 

[deuce22]

34 minutes ago, Jeremy Harris said:

 

 

Some photos in this link showing how our insulated raft was laid: http://www.mayfly.eu/2013/10/part-sixteen-fun-and-games-in-the-mud/

 

 

 

That was very interesting Jeremy. I have watched a video of the Isoquick system, but I am trying to find something for traditional rafts. Is the Kore system similar to Isoquick? Why did you choose Kore? Why did you have a 200ft bore hole?

What was the reason to choose this over a traditional raft, it does seem a lot quicker and more efficient.

[Jeremy Harris]

42 minutes ago, deuce22 said:

 

That was very interesting Jeremy. I have watched a video of the Isoquick system, but I am trying to find something for traditional rafts. Is the Kore system similar to Isoquick? Why did you choose Kore? Why did you have a 200ft bore hole?

What was the reason to choose this over a traditional raft, it does seem a lot quicker and more efficient.

 

I don't think there's much difference between the various insulated slab systems.  I had quotes from Kore, Isoquick and Supergrund and all were pretty close in price and spec.  We went with Kore only because the company we used for the foundation and house (MBC Timber Frame) offered it in their package, and it seemed to me that having the foundations and house from the same company meant there was one less risk to worry about, as it meant we didn't have to take responsibility for ensuring that all the dimensions, tolerances etc matched.  I'd have happily used either the Isoquick or Supergrund systems, as there's nothing much to choose between any of them, IMHO.

 

The main reason we chose an insulated slab like this was to get good thermal performance, as, with the insulated upstand all the way around, that lines up with the insulation in the walls, it virtually eliminates thermal bridging at the foundation to wall junction.  It was a bonus that the whole system was very quick to lay, and the house was pretty quick to get to watertight.  This speed was a real advantage, as we were building late in the year, during a pretty wet spell, so getting the frame up and watertight so quickly protected the inside of the frame from too much wet weather exposure.

 

The borehole was because the mains water supply was 140m away, up a single track lane, and it would have cost us around £23k to dig up the lane and run a pipe, plus it would have caused major disruption to three or four houses that have drives from that lane, we'd have pretty much cut their access off for a fair time whilst digging up the lane.  The borehole, with all the hassle it caused, water treatment kit, etc, was less than half the price, plus we don't have a water bill.  In practice we pay around 1/4 the price for water, allowing for the running cost of the pump, treatment stuff, etc, so it's worked out OK.

 

 

[deuce22]

Thanks Jeremy for the explanation.