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?

[Nickfromwales]

18 hours ago, lizzieuk1 said:

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?

Totally. 
 

For woodcrete I’d get them in before you have boarded, as that will allow you maximum exposure for the magic goop to do its thing.

 

Follow on trades will all need inductions and in agreement to maintain the integrity of that airtight layer. 
 

On an Isotex job the client tried a liquid membrane, spray and brush applied, but the woodcrete simply has too open a pore for this to work well on bare woodcrete. Another parged first and got 0.66ACH, but neither were ‘cost effective’ (they were expensive as feck). 
 

AB is your friend here, but obvs a certain amount of prep needs doing ahead of their arrival. 

[lizzieuk1]

1 hour ago, Nickfromwales said:

Totally. 
 

For woodcrete I’d get them in before you have boarded, as that will allow you maximum exposure for the magic goop to do its thing.

 

Follow on trades will all need inductions and in agreement to maintain the integrity of that airtight layer. 
 

On an Isotex job the client tried a liquid membrane, spray and brush applied, but the woodcrete simply has too open a pore for this to work well on bare woodcrete. Another parged first and got 0.66ACH, but neither were ‘cost effective’ (they were expensive as feck). 
 

AB is your friend here, but obvs a certain amount of prep needs doing ahead of their arrival. 

I was thinking the same Nick re the use of AB before boarding, am assuming also better to do 1st fix before to allow the AB to seal everything up?

Think we will also have to use something like parge or poss some membrane behind our pole plates as they will obv go in before and not sure the AB will penetrate behind those once they're in.

We were graduating towards float and set plaster as think it would go well with the woodcrete but, a. I'm not sure we will easily find a plasterer who's skilled in that, as d&d has been the preferred for so long now, and b. feels a bit like overkill using that and the AB.

Still so many choices to make.....