Depth of foul drains under raft

[lizzieuk1]

On 11/04/2026 at 11:38, Nickfromwales said:

That’s only true if you’ve gone for a single thickness raft, on good ground, and with MUCH thicker concrete. That’s when you can say the loads will be spread out, so what is your actual foundation detail?
 

Point loads are almost unavoidable, unless it’s a small dwelling, they just become less of an issue with one beefy slab. 
 

One of the one’s I’m on currently (MBC PH TF + passiv raft) had to have 2 sizeable pads installed under the EPS level, to take the posts (x2) used to break up the weight of the ridge beam.

 

People don’t use terminology well, as raft can mean a few different methods for that one discipline.

 

An SE is absolutely necessary, please don’t assume otherwise   

It's an AFT raft so all designed by their SE, our build is in Ecobrix, again all designed by an SE. We've only got 2 drain runs out, one is app.9m straight out the other app.4m straight out. Neither are under an internal load bearing wall both obv exit under the external walls.

[lizzieuk1]

Guys I think we've gone off topic here. We have had the whole build specified by a SE, raft is by AFT so fully designed out. And the rest has been specified by a SE with experience in our construction choice. We're not going it alone in that respect! 

My initial Q was only regards bringing the (2) drain runs out from under the raft. 

 

[Nickfromwales]

To answer your original question, you need to work from the invert of the sewer back to the house, unless you're on a STP?

[lizzieuk1]

5 minutes ago, Nickfromwales said:

To answer your original question, you need to work from the invert of the sewer back to the house, unless you're on a STP?

Yes, we're installing a new STP so not too fixed with invert level. 

My original Q was really whether we had any issues with starting depth under the raft - whether there was a minimum we needed to adhere to. 

From the answers of others (including yourself Nick - many thanks) it looks like there's no set minimum. 

 

[lizzieuk1]

Can I add, I really appreciate comments & advice here. It's all taken on board and I'm always learning. I realise many of you are specialists in certain areas and some have vastly more experience of new build. 

However, this isn't our 'first rodeo' we've built 2 before so do have reasonable understanding but, it's our first one using an insulated raft & icf so plenty to learn & understand on that level.

 

Apologies if my questions may come across a bit 'novice like' I'm a bit of a belt and braces person so, when I ask its not necessarily that I've no idea, (though sometimes it is the case!) it's just my way of cross checking my thinking/understanding! 

[lizzieuk1]

Update on this :

I spoke to the raft supplier today (had a few other bits to discuss) and they confirmed there is no issue with point loading so, all fine to run foul drains wherever we need. 

Onwards & upwards!

[Nickfromwales]

7 hours ago, lizzieuk1 said:

it looks like there's no set minimum.

It took a loooong time to get there, but yes, in a nutshell. lol. 

 

1 hour ago, lizzieuk1 said:

Onwards & upwards!

🥳👍

[Nickfromwales]

7 hours ago, lizzieuk1 said:

it's our first one using an insulated raft & icf

As you’ve gone for woodcrete, I would tell you with my last breath, to get AeroBarrier in to get you airtight. 
 

Leave all the parge coating etc and just let the magic happen. You’ll thank me later. 

[saveasteading]

2 hours ago, lizzieuk1 said:

there is no issue with point loading so, all fine to run foul drains

That's not the whole answer. The whole raft could rise of fall with ground expansion. Assuming you are using plastic pipes, they will bend to suit, but avoid minimum falls.

[Nickfromwales]

5 minutes ago, saveasteading said:

That's not the whole answer. The whole raft could rise of fall with ground expansion. Assuming you are using plastic pipes, they will bend to suit, but avoid minimum falls.

 But……would the pipes not move also? 

[saveasteading]

9 hours ago, Nickfromwales said:

would the pipes not move also? 

Probably a similar amount under the building but outside could be different as more exposed to wetting, drying, trees. Depends a lots on factors we aren't party to.

When pipes were clay it mattered more, and there were standard details for articulation.

 

So just don't run at minimum grades and bed the pipes as specified and should be fine.

 

[lizzieuk1]

4 hours ago, saveasteading said:

Probably a similar amount under the building but outside could be different as more exposed to wetting, drying, trees. Depends a lots on factors we aren't party to.

When pipes were clay it mattered more, and there were standard details for articulation.

 

So just don't run at minimum grades and bed the pipes as specified and should be fine.

 

No panic we're running at 1:60 

[lizzieuk1]

15 hours ago, Nickfromwales said:

As you’ve gone for woodcrete, I would tell you with my last breath, to get AeroBarrier in to get you airtight. 
 

Leave all the parge coating etc and just let the magic happen. You’ll thank me later. 

We are certainly going to do that Nick. I haven’t forgotten our previous exchange on here about it 👍

I wasn't sure if we should parge coat as well but, from your comment, seems unnecessary?

[lizzieuk1]

15 hours ago, Nickfromwales said:

As you’ve gone for woodcrete, I would tell you with my last breath, to get AeroBarrier in to get you airtight. 
 

Leave all the parge coating etc and just let the magic happen. You’ll thank me later. 

We are certainly going to do that Nick. I haven’t forgotten our previous exchange on here about it 👍

I wasn't sure if we should parge coat as well but, from your comment, seems unnecessary?

[lizzieuk1]

15 hours ago, Nickfromwales said:

As you’ve gone for woodcrete, I would tell you with my last breath, to get AeroBarrier in to get you airtight. 
 

Leave all the parge coating etc and just let the magic happen. You’ll thank me later. 

We are certainly going to do that Nick. I haven’t forgotten our previous exchange on here about it 👍

I wasn't sure if we should parge coat as well but, from your comment, seems unnecessary?