Ground bearing or Raft? (Insulated foundation - Kore)

[SuperJohnG]

I have been gathering prices from the usual suspects (Kore, isoquick, Izodom) for my insulated raft foundation. I thought i was up to speed however Kore has just thrown me off a little with the engineering costs sheet they supplied. (See attached) and below for the two specific paragraphs. So when we are talking Kore type - insulated foundations, when is a ground bearing insulated foundation, not a insulated raft? 

 

 

 

Insulated Foundation Engineering costs (One off).pdf

[nod]

Looks expensive just for design 

I would gave thought calcs would be in the first figure 

[PeterW]

They mean if they have to calculate complex loads across mixed substrates such as made ground or different gravels etc.

 

TBH that is a very good price anyway - £1000 all in, even if you need a raft..??

[SuperJohnG]

It costs for engineering hence it is calcs. Cheaper than what I had been told to budget at £2500 by everyone.  

 

But still nobody any difference between  the two things.? 

[PeterW]

A raft is normally a thickened edge with steel

etc that is designed to work as a stressed member. An insulated slab is just that - a slab of concrete with mesh in it and pretty much that’s it. 

[Sandybay]

A question about insulated raft foundations,  why do some have the dpm bellow the raft and some have it inside the raft? And are the Eps rafts not waterproof anyway? 

[SuperJohnG]

3 hours ago, Sandybay said:

A question about insulated raft foundations,  why do some have the dpm bellow the raft and some have it inside the raft? And are the Eps rafts not waterproof anyway? 

I have questioned this also. 

 

DPM is usually placed inside the raft from whay I have seen. When you see it below the raft it is actually a radon barrier (common in Ireland). This is what I have picked uo anyway.  I have have seem some without any DPM inside the raft  which I found strange and have previously questioned. 

 

See this thread. 

 

Edited June 14, 2020 by SuperJohnG
added link

[PeterW]

4 hours ago, Sandybay said:

A question about insulated raft foundations,  why do some have the dpm bellow the raft and some have it inside the raft? And are the Eps rafts not waterproof anyway? 

EPS isn’t fully waterproof as there are potentially thin capillary gaps between the EPS blocks. Not unusual to see radon or other thick barriers added as BCOs “expect” to see them 

[Dan F]

On 14/06/2020 at 19:10, PeterW said:

EPS isn’t fully waterproof as there are potentially thin capillary gaps between the EPS blocks

 

If this is the case, and EPS thermal performance is reduced when it absorbs moisture, isn't there an argument for always using a layer of DPC below the EPS?

[SuperJohnG]

It wouldn't seem to make sense to me. As if you have a DPM below the EPS then water could get inbetween the two and never be able to drain out. Having the EPS sitting on the compacted base allows any water below to easily drain down to the perimeter drain. 

[Dan F]

1 hour ago, SuperJohnG said:

It wouldn't seem to make sense to me. As if you have a DPM below the EPS then water could get inbetween the two and never be able to drain out. Having the EPS sitting on the compacted base allows any water below to easily drain down to the perimeter drain. 

 

If the base is type3 + fine gravel, yes  If it's type2 + sand (like ours) then you could potentially argue that DPM here is a good idea.

[Gus Potter]

Hello all.

Rafts.

To quote Peter W “EPS isn’t fully waterproof as there are potentially thin capillary gaps between the EPS blocks. Not unusual to see radon or other thick barriers added as BCOs “expect” to see them”

I’m just getting the hang of this so not sure on the etiquette; how to quote others etc, compliment , reference others, length of response and so on. Any guidance will be much appreciated.

Peter - Good point. To compliment you points (please feel free to add or question, again not yet sure of etiquette).

DPM - It’s not just a bit of plastic and should it be more appreciated?

The DPM on top of the insulation stops any concrete directly laid on top of the insulation from bleeding water/ the fine content of the concrete  down into the gaps or forcing the insulation apart and potentially creating thermal bridges. Your concrete (assuming ready mix rather than hand batched) is mixed assuming a certain amount of water content, so maybe you don’t want to lose it into the gaps as it can make not least hard to float / polish off, especially on a hot day and when you have run out of steam after a hard day. The water is also required for the curing process as the batching plant measure this very carefully. Don’t be tempted to add water to a ready  mix concrete, your won’t need to if you plan and you prepare for the pour. That includes food, coffee etc, leave the Sherry / Buckfast tonic wine (choose your region / tipple) for later when you have finished and cleaned up.

Concrete slab design is still very much an art, although there has been some progress in terms of analysis, but we are talking about domestic projects here. In that vein one of the keys is to control cracking in the slab as it cures and then dries out. You don’t want to lose your wallet / money container into the cracks.

What you often look to do is to allow the slab to shrink and you then dictate (with a fair wind) where you want this movement to take place. The DPM acts as a slip membrane so when the slab shrinks it slides on the plastic. If you have gaps the DPM sinks into these and starts creating keys which grip the slab, a bit cautious to be honest but more importantly if you have a badly prepared substrate this can lead to problems. Take your time and prepare well what is going under the slab / insulation.

If  your sub base (type one etc)  is all wavy (flatness) and not level  (there is a difference between flatness and level funnily... for another day, but worth finding out a bit more about so that you don’t have an issue with your flooring etc later) then you are inviting problems. Essentially if you have not prepared the surface the slab sits on properly then slab starts to “get a grip of the ground” and this causes unwelcome stresses which exacerbate the cracking, if your substrate is  too high then your slab will be too thin in places and this too will cause problems. Take your time.

Last but not least the DPM can be designed to work in both directions. The one we think about most is to stop water coming up from the ground. It can also work the other way. Very broadly speaking (comments welcome )  on a hot day the house gets warm, you do some cooking say and the moisture content in the air rises further. We know water vapour condenses on cold surfaces. You have a nice cool slab so it starts to attract moisture. It may start to raise the moisture content in the slab. Fine it will dry out but you don’t want it to go further and start forming puddles under the insulation. That is one good reason for considering putting the DPM on top of the insulation and under the concrete as it work in reverse as a vapour barrier.

Next time you buy a roll of DPM and maybe feel and you paid a tenner too much  look at what it may be doing for you. Perhaps there is a free lunch after all!

 

The dreaded raft foundation?

Again to quote (sorry) Peter W “A raft is normally a thickened edge with steel etc that is designed to work as a stressed member. An insulated slab is just that - a slab of concrete with mesh in it and pretty much that’s it.”

Good summary. To expand a bit, some food for thought and a bit of background.

Start with a light domestic strip foundation. This is usually a strip of concrete laid in the ground with some light steel mesh in  it. The mesh is really intended to control cracking rather that to turn the strip of concrete into a reinforced concrete beam which is a different animal. You may see this type of light crack control mesh mentioned as an A142 sometimes A193 mesh.

Often the house walls will sit over the centre of the strip foundations unless there is a significant mismatch between the load on the outer and inner leaf. Remember we are dealing with domestic projects here so perhaps look at the practical and buildability side of things to suit your circumstances. All other things being equal the pressure the strip found puts on the ground is fairly even.

If you locate the wall at the edge of a strip foundation then it puts more weight on one side This can cause the foundation to overstress the soil (formation) on one side and the foundation can start to rotate/ twist  (settle on one side!) more than you want.

There are lots of ways of designing a raft /basement structure but in the spirit of things I’ll continue with one option. You can have what (depending on where you live ect) is called an edge thickened raft as Peter describes. One way of making this work (broadly) is to put heavier (more than that usually required for simple crack control) steel reinforcement in the top of the slab all the way to the edge. The edge of the slab is thicker (thickened edge) and can if need be designed to act as a beam if you have concentrated loads say each side of a big set of bifold doors or under a column.  The steel in the top of the slab stops the foundation rotating but often it needs to be heavier (thicker bars) than that required just for the crack control.

But how?

The detailed  explanation is very lengthy and best left for now. Essentially you decouple the two forces – the downward load is resisted by the soil under the thickened edge (just the same as a strip foundation)  and you use the slab and it's top steel to stop the foundation from rotating. Essentially this part of the slab works a bit like a cantilever beam, like a balcony on a building (being very simplistic). Often you can use the same techniques when designing a basement.

Real life and rafts?

If you’re a self builder, extending or just doing some stuff then is there not merit in keeping it simple, easily buildable and approaching the structural type work pragmatically?

Discussions can revolve around a few millimetres of slab thickness (an inch or two) , quantity of rebar and so on as that is something that is more easily quantifiable an easy to price when you are doing it yourself. You can nail the material costs to a good extent but the labour costs are more flexible. Thinner more heavily reinforced slabs are harder to do (conjested reinforcement etc) and pour and compact properly hence the labour cost goes up if you want the same quality of workmanship? What about the unknowns such as variable ground conditions that Peter mentions?

Is it worth while starting with the “simple stupid” but proven to work type of design and work up from there? You reduce the risk perhaps and that allows you to spend money on the exciting things like heat recovery systems, kitchens and the things that float your boat? After all once it's all finished you don't see the "structural" design" unless it's a feature of the build.

One pragmatic solution to rafts whether you are self building or extending etc is to look at the worst layer of ground, below the hard core, look at the thickness and depth of this layer  and decide if you want to design for that. The zone of pressure that a raft type foundation imposes on the ground often goes much deeper (well beyond the standard 150mm hard core layer) than a strip foundation anyway.

If you have a simple design you will probably get more realistic quotes. More builders will have the capability to take on the job as they will be in their comfort zone. Perhaps you will feel better able to manage your project. Maybe it’s worth maybe sacrificing a bit of extra concrete here and there?

How common is it for a builder to look at a set of drawings and think.. that looks hard so I’ll bung on 50 – 100 % and if I get the job then I’ll then work out how to do it as I’ll be well covered anyway?

In the interests of fairness that last comment was slanted. There are lots of very experienced builders about who have spent many years learning their craft and will give good sound advice on the dreaded rafts and point out that while the material cost may be a bit more the reduction in labour cost for the" simple stupid" will more than offset the material savings.

 

 

 

[MikeSharp01]

8 hours ago, Dan F said:

If the base is type3 + fine gravel, yes  If it's type2 + sand (like ours) then you could potentially argue that DPM here is a good idea.

Rafts tend to have peripheral drainage so working all the time to dry out the makeup below the slab thus reducing the likelihood of water up between the blocks.