Raft slab - buildability issues

[maxdavie]

Hello all, 

 

We are building an insulated warm raft slab for a timber framed building. 

My groundworker is very good, dedicated, hard working etc. But, he has never built a raft foundation like this (image found on google and indicative only) : 

 

 

Here's a site image of the prepared ground before the shuttering, DPM, insulation, mesh and concrete go in.  

 

 

He's suggesting its much easier to do this without the 45degree slope on the inside and instead just have a right angle there even if he has to shutter both sides of the trench. He says, he wouldn't have to bend any mech or rebar. 

He's also worried about having to cut the insulation at various angles and doesn't really have a plan yet for how he cuts the insulation at the corner - which will have to be mitred in two planes! - the 45 degree sectional slope and the 90 degree turn in plan. 

I have two questions for the Guru's: 

 

1. Can anyone with direct experience of building this type of foundation offer any tips on how to install the insulation with all the awkward cuts etc ? 

 

2. This seems to be a pretty standard way of doing a raft slab. Yet, my groundworker is having real difficulty with it.

What do others with experience think of this method, should it be this tricky?

 

 

Thanks for any comments! 

 

 

[Thorfun]

mine was designed by TSD (Tanners Structural Design) and had 90^o insulation. in fact, where you have the raised area of hardcore/sand we just used more insulation. here's ours for reference.

 

 

that above is using the Kore preformed insulation but our groundworkers just bought EPS300/EPS100 without the sloping former and cut it to fit on-site. photo below:

 

 

hope this helps and it's posted with the caveat that any changes to your foundation/slab design should be ratified by your structural engineer.

[Gone West]

We used the Isoquick system which is preformed and slots together with vertical sides. The specific design is dependant on the site and your structural engineer should have provided drawings for your groundworkers.

 

 

[maxdavie]

Great - thanks - this is useful. 

 

FYI - of course we have SE drawings - sorry I didn't make that clear.

 

The issue is that my groundworker is finding the task tricky. 

 

Anyway, thanks all

[Roger440]

7 hours ago, maxdavie said:

Hello all, 

 

We are building an insulated warm raft slab for a timber framed building. 

My groundworker is very good, dedicated, hard working etc. But, he has never built a raft foundation like this (image found on google and indicative only) : 

 

 

Here's a site image of the prepared ground before the shuttering, DPM, insulation, mesh and concrete go in.  

 

 

He's suggesting its much easier to do this without the 45degree slope on the inside and instead just have a right angle there even if he has to shutter both sides of the trench. He says, he wouldn't have to bend any mech or rebar. 

He's also worried about having to cut the insulation at various angles and doesn't really have a plan yet for how he cuts the insulation at the corner - which will have to be mitred in two planes! - the 45 degree sectional slope and the 90 degree turn in plan. 

I have two questions for the Guru's: 

 

1. Can anyone with direct experience of building this type of foundation offer any tips on how to install the insulation with all the awkward cuts etc ? 

 

2. This seems to be a pretty standard way of doing a raft slab. Yet, my groundworker is having real difficulty with it.

What do others with experience think of this method, should it be this tricky?

 

 

Thanks for any comments! 

 

 

 

Thats exactly how i did my garage foundation. 

 

I cant say it was difficult. An adjustable square and a saw with adjustable angle blade was all i needed. Any slight inaccuracies i filled with foam afterwards.

 

The most difficult bit was getting the sand on the 45 degree slopes right.

 

Im no groundworker, though when the concrete guys turned up, they asked who did it as it was the best insulated raft prep they had ever seen!

 

I gather the standard approach is not to worry too much on the accuracy of whats under the unsulation as it cant be seen. To my mind, thats a nonsense. If there are voids under it, that might cause long term trouble. If i recall the insulation called for a +/- of 10mm per meter accuracy.

 

[SuperJohnG]

If its for your house, use kore, isoquick, econekt (izodom) or similar. Uiull get a custom made system  with support from the supplier and likely better end result. 

 

I used Kore, cheap and designed by TSD above ( search tanners) worked fine and much easier than you library image. 

[George]

You could square it up, as least in the corners.

 

I would normally say keep the chamfer but put the reinforcement in square (if the SE is happy with it, can't see a good reason why not on a domestic scale build), but I haven't designed a raft slab with rigid nsulation below the slab itself before.

 

The non-structural reason for a chamfer makes laying the DPM easier and doesn't create and tricky to compact locations internally.