raft foundations

[deuce22]

Hi.

 

I've just had a Mining risk assessment report completed and I'm almost positive that I'm going to have to use a raft foundation. I'll confirm this with the SE and he will then tell me exactly how he wants it constructed.

 

In the meantime, I would like to get some information on rafts, so that I have an idea of the costs involved and exactly what needs to be done. I've always used strip foundations, so don't know how much more rafts cost or exactly how they are constructed and formed.

 

Could someone please give me some advice on this, so I at least know what the SE is talking about.

 

Thanks.

[ProDave]

If you are doing a raft anyway, then the logical extension to that is a passive insulated slab foundation instead. Similar in many ways but very much better.

[Jeremy Harris]

+1 to @ProDaves comment about a passive slab.  Loads of advantages, like you can integrate the insulation under the raft, fit UFH pipe inside the slab, powerfloat the slab to get a ready-to-lay-flooring top surface, plus it's dead quick.  Ours took four days to put down, from the start to the power-floated finish, including all the UFH pipes (see here: http://www.mayfly.eu/2013/10/part-sixteen-fun-and-games-in-the-mud/ , although the last photo there was taken before the slab was power floated dead flat and smooth).

[Alexphd1]

Our took slightly longer than 4 days but was a DIY effort (got in a company to pour the concrete). 

[deuce22]

Thanks ProDave.

 

I've just had a look at Kore systems. It looks a lot easier than the standard way of constructing rafts.

[PeterW]

@deuce22 Where in the UK are you based..? There are 3 or 4 major suppliers of the components - some do DIY supply too - but very few SEs that are experienced with designing.

[deuce22]

Hi JSHarris.

 

I just had a quick look through the images, looks pretty straight forward. Where does the outer leaf sit? It's normally lower with a cavity tray between the two.

 

How much did it cost per m2?

[deuce22]

I'm in south Wales.

 

I'll speak with the SE later. I would be doing it DIY style.

[Triassic]

1 hour ago, Alexphd1 said:

Our took slightly longer than 4 days but was a DIY effort (got in a company to pour the concrete). 

I’m one of those DIY insulated raft installers, like @Alexphd1, I got a local guy in to help pour the concrete. 

 

This is is what I’m doing this morning, tying the steel work ready for the next concrete pour.

 

 

Edited August 7, 2018 by Triassic

[Alexphd1]

@Triassic That picture looks very similar to our next build. 3 sided icf basement with bigger raft footprint area on 1st floor.

 

[Jeremy Harris]

30 minutes ago, deuce22 said:

Hi JSHarris.

 

I just had a quick look through the images, looks pretty straight forward. Where does the outer leaf sit? It's normally lower with a cavity tray between the two.

 

How much did it cost per m2?

 

 

The outer skin on our house is larch cladding, so doesn't sit on anything.  The twin stud passive frame sits with the inner structural frame on the reinforced concrete ring beam, then the outer, non-structural frame rests on the DMP that's wrapped over the 200mm wide EPS upstand.  The big advantage is that the under floor and wall insulation become contiguous, as the hollow walls are pumped with cellulose that sits down on the EPS upstand, with no thermal bridging at the wall/floor junction.

 

I don't know the slab cost, as it was included in the foundation and frame package, but the weathertight insulated frame and foundation, including the UFH pipes but excluding the windows, doors, slates or external cladding came to around £415/m² (bear in mind that this was nearly 5 years ago now and prices have increased a fair bit).

[jack]

Bear in mind that there's a big difference between a timber frame and a brick/block with cavity on a raft. Brick and blockwork adds a lot of weight, so the slab needs to be engineered accordingly.

 

Do you have a construction type (eg, timber frame, ICF, blockwork, etc) in mind yet?

 

Here's a section through the raft we have (which is the same as Jeremy's):

 

[Jeremy Harris]

If you want chapter and verse on the variations of the Kore design for different forms of construction, this booklet covers most of them, including conventional block and brick cavity wall:

 

http://www.mayfly.eu/wp-content/uploads/2017/12/Kore-Insulated-Foundations-Report.pdf

[deuce22]

I have always used a double block construction when building extensions and the 2 new builds I've done, so I don't have experience in other types of construction. A friend built his with a 6x2 frame, covered with 100mm fibre board and then cladding. I am really interested in this because it takes away a large amount of work for bricklayers (which is something I don't do). I'd be able to do all of that myself with a labourer. I just don't know if it would be approved by planning because the SE said the planners still prefer traditional builds.

 

Jack.

 

I'm assuming that your construction is similar to a traditional build with a cavity between walls, but timber used instead of brick/block. How does it work at the bottom where you'd normally have a cavity tray for moisture, either building up inside the cavity or coming through the outer leaf?

 

Thanks. 

[jack]

The equivalent of the brick/ block "cavity" is actually the ventilated space between the outer stud and the cladding or render board.

[deuce22]

The cladding or render board has to be fitted to something though, so surely there's not just a complete gap between the cladding and the outer stud? How does the cladding get fitted, so it is not bridging to the outer stud?

[jack]

Vertical battens, which fix through the outer board (of the outer stud) and into the outer stud.

[deuce22]

A friend of mine (from England) did something similar about 18 months ago and Planning/BC were fine with it. However, when I spoke with a BCO about 12 months ago about doing the same thing, he said it wouldn't be allowed. If the cladding broke down in some way and damp got into the battens it could easily travel through into the main structure. I'm from Wales, so maybe building regs are different. Ill ask again again because it may have changed now.

 

I'm guessing you had the same construction as JSHarris and used the twin stud method. What is the benefit of that over a single 6x2 stud?

 

Thanks.

[Jeremy Harris]

Just now, deuce22 said:

A friend of mine (from England) did something similar about 18 months ago and Planning/BC were fine with it. However, when I spoke with a BCO about 12 months ago about doing the same thing, he said it wouldn't be allowed. If the cladding broke down in some way and damp got into the battens it could easily travel through into the main structure. I'm from Wales, so maybe building regs are different. Ill ask again again because it may have changed now.

 

I'm guessing you had the same construction as JSHarris and used the twin stud method. What is the benefit of that over a single 6x2 stud?

 

Thanks.

 

 

No problems at all with BC - take a look here and you can see all the documents I submitted to them for approval: http://www.mayfly.eu/2013/09/part-fifteen-the-site-is-finally-ready/

 

The building regs are the same in England and Wales, AFAIK, it's only Scotland and NI that have slightly different regs, so there should be no problem at all with using this system in Wales, AFAICS.

 

The massive benefit over a single studd is that there is no thermal bridging plus you have 300mm of relatively high decrement delay insulation filling the entire wall - makes a significant difference to the comfort of the completed house I've found, it tends to be cool in summer and warm in winter, due to the decrement delay primarily.

[Mr Punter]

9 minutes ago, JSHarris said:

The building regs are the same in England and Wales, AFAIK, it's only Scotland and NI that have slightly different regs, so there should be no problem at all with using this system in Wales, AFAICS.

 

 

The regs are different in Wales.  For example they require sprinklers to meet Part B.  Not sure about foundations though.

[Jeremy Harris]

Just now, Mr Punter said:

 

The regs are different in Wales.  For example they require sprinklers to meet Part B.  Not sure about foundations though.

 

 

Thanks for that, I'd assumed that building regs was one of those things that hadn't been devolved to the Welsh Assembly, but hadn't checked to be sure.

[deuce22]

5 minutes ago, JSHarris said:

 

 

No problems at all with BC - take a look here and you can see all the documents I submitted to them for approval: http://www.mayfly.eu/2013/09/part-fifteen-the-site-is-finally-ready/

 

The building regs are the same in England and Wales, AFAIK, it's only Scotland and NI that have slightly different regs, so there should be no problem at all with using this system in Wales, AFAICS.

 

The massive benefit over a single studd is that there is no thermal bridging plus you have 300mm of relatively high decrement delay insulation filling the entire wall - makes a significant difference to the comfort of the completed house I've found, it tends to be cool in summer and warm in winter, due to the decrement delay primarily.

 

 

Thanks.

 

No Thermal bridging or reduced? They must be connected together in some way, so there should be some.

 

I'd have to calculate the costs as I'm planning to construct the timber frame myself and i'm guessing these are manufactured. I'll also be adding 100mm fibreboard which should reduce the TB.

 

Has anybody used this insulated raft system by DIY? I've just spoken with Isoquick and they quoted me £75 m2 roughly. Seems really expensive.

[Jeremy Harris]

22 minutes ago, deuce22 said:

 

 

Thanks.

 

No Thermal bridging or reduced? They must be connected together in some way, so there should be some.

 

I'd have to calculate the costs as I'm planning to construct the timber frame myself and i'm guessing these are manufactured. I'll also be adding 100mm fibreboard which should reduce the TB.

 

Has anybody used this insulated raft system by DIY? I've just spoken with Isoquick and they quoted me £75 m2 roughly. Seems really expensive.

 

 

There are small noggins connecting the inner and outer frames, but they are relatively long, surrounded by insulation (so can be considered to be 2D thermal transmission paths) and make up such a small part of the overall wall area that their effect is very small.

 

It's a system that was first invented as an "add on" for conventional timber framed houses by a chap called John Larsen, in Canada around 30 or more years ago, and such walls are often referred to as Larsen truss walls.

 

Others here have done various forms of DIY insulated raft - not that hard to do, as several people have come up with ways to get around having to use the EPS/XPS pre-forms for the edges, and fabricate them using EPS sheet.

[Alexphd1]

The £75m2 from isoquick, what exactly is that for ( Design supply or install etc) ?

[Ian]

33 minutes ago, deuce22 said:

If the cladding broke down in some way and damp got into the battens it could easily travel through into the main structure.

There's also a breather membrane between the battens that support the cladding and the main structure of the building