Depth of foul drains under raft

[lizzieuk1]

We're prepping to dig foundation and drainage out and wondering about depths for the (approx 5m) run that is going to be under the insulated raft and how low to set the foot for the main stack which exits straight out. 

Any idea what that depth needs to be? Is it 600mm below the mot 1?

 

Also, how far out from the edge of the raft do the drains need to run - I'm reading 1m min is the part H requirement if pipe is below foundation but, as our raft is only 450mm into the ground makes interpreting the regs a bit tricky as, pretty much any drain will be below the raft level.  I'm not sure if it's interpreted the same as obv the weight is spread over the raft rather than down through strip foundations so, does this make a difference?

 

Or, do I just ask BC?!

[Nickfromwales]

On the current MBC project I'm consulting on, we have soil pipes in the 150mm MOT1 layer directly under the 50mm sand blinding, and some others like comms and landscape power are lower to avoid these. I designed all this ahead of MBC's arrival, making sure there were the least number of services crossing over/under each other.

 

As you're under a raft, there is no minimum depth THB, but for power and water you do need to drop down towards the periphery to comply with the 750-900mm that these authorities will mandate.

[Gone West]

@lizzieuk1 Our insulated raft sat on 250mm compacted type1 with 50mm granite fines on top. The insulation was 300mm thick. The inspection chamber was around 3m from the edge of the raft. The pictures show before and after sub-base was laid.

 

 

[BotusBuild]

Our soil pipes are in the compacted type 803 and sand blinding. A 1m length close to one of the stack foots comes up about 20-30mm into the 250mm  insulation.

[lizzieuk1]

12 hours ago, Nickfromwales said:

On the current MBC project I'm consulting on, we have soil pipes in the 150mm MOT1 layer directly under the 50mm sand blinding, and some others like comms and landscape power are lower to avoid these. I designed all this ahead of MBC's arrival, making sure there were the least number of services crossing over/under each other.

 

As you're under a raft, there is no minimum depth THB, but for power and water you do need to drop down towards the periphery to comply with the 750-900mm that these authorities will mandate.

Many thanks Nick, that's really helpful. 👍 

[lizzieuk1]

3 hours ago, Gone West said:

@lizzieuk1 Our insulated raft sat on 250mm compacted type1 with 50mm granite fines on top. The insulation was 300mm thick. The inspection chamber was around 3m from the edge of the raft. The pictures show before and after sub-base was laid.

 

 

Thankyou 😊  do you know if there's a reason the Chambers needed to be so far out? Or was it just 'on the way' to the mains connection?

[lizzieuk1]

30 minutes ago, BotusBuild said:

Our soil pipes are in the compacted type 803 and sand blinding. A 1m length close to one of the stack foots comes up about 20-30mm into the 250mm  insulation.

Thanks 👍  sounds like we are OK to be in or just below the MOT then. 

[BotusBuild]

I think so, especially if @Nickfromwales has done something close to what I have described 😀

[lizzieuk1]

12 hours ago, Nickfromwales said:

On the current MBC project I'm consulting on, we have soil pipes in the 150mm MOT1 layer directly under the 50mm sand blinding, and some others like comms and landscape power are lower to avoid these. I designed all this ahead of MBC's arrival, making sure there were the least number of services crossing over/under each other.

 

As you're under a raft, there is no minimum depth THB, but for power and water you do need to drop down towards the periphery to comply with the 750-900mm that these authorities will mandate.

Have you topped the pipe with pea grit then mot on top? Thinking in the 150 layer there's not much room for both!

[Nickfromwales]

17 minutes ago, lizzieuk1 said:

Have you topped the pipe with pea grit then mot on top? Thinking in the 150 layer there's not much room for both!

Yes. Shingle goes under / around / atop, and then the sand blinding goes in.

 

17 minutes ago, lizzieuk1 said:

Thinking in the 150 layer there's not much room for both!

Groundworkers will just shoot through this, but if there's any major backfill over the shingle then they may well use a bit more type 1, then the sand (usually dependant on how far they have to walk to either pile of 'stuff' ).

 

21 minutes ago, lizzieuk1 said:

Thankyou 😊  do you know if there's a reason the Chambers needed to be so far out? Or was it just 'on the way' to the mains connection?

I've set this up so the chambers are dead centre of the hard standing (walkway) around the house. Seen way too many jobs where these are half in the slab and half into the grass etc, which looks shite.

 

Have you allowed for all the requisite ducts / services etc?

[Gone West]

1 hour ago, lizzieuk1 said:

Thankyou 😊  do you know if there's a reason the Chambers needed to be so far out? Or was it just 'on the way' to the mains connection?

I put it in the most convenient position on the way to the sewage treatment plant. I don't think there are any set distances.

[Nickfromwales]

It’s just about when 2 pipes need to converge, where you’ll need IC’s at each instance or any time there’s a change in direction.

 

You can run one pipe from A>B for some distance, but getting most BCO’s to agree to it is another thing. IIRC the last one wouldn’t let us go more than 8m (in a perfectly straight line)….. so ended up with 3 IC’s between the house and the road. 

[Gus Potter]

23 hours ago, lizzieuk1 said:

Or, do I just ask BC?!

Ask your SE, they should have detailed this out already for you. BC are not Structural Engineers.

 

I might chip in as an SE but I would need to see the drawings first. There are lots of different types of raft. Until you know where the uniform loads are , where any point loads are you can't start on the drainage design. 

[lizzieuk1]

On 03/04/2026 at 12:37, Nickfromwales said:

Yes. Shingle goes under / around / atop, and then the sand blinding goes in.

 

Groundworkers will just shoot through this, but if there's any major backfill over the shingle then they may well use a bit more type 1, then the sand (usually dependant on how far they have to walk to either pile of 'stuff' ).

 

I've set this up so the chambers are dead centre of the hard standing (walkway) around the house. Seen way too many jobs where these are half in the slab and half into the grass etc, which looks shite.

 

Have you allowed for all the requisite ducts / services etc?

'Groundworkers' are me and OH! We also have a 'digger man' with plenty of experience but obv need to set out the runs etc for him. We've not got a sand blinding just peagravel for the insulation to sit on so may just fill with that above the pipe runs in the slab area.

Good thinking re the IC positioning, as you say far better to get them centrally in the paving. 

 

Yes, have considered services, ours is actually quite straightforward,  both water & electric coming into the utility just inside the outside wall and will be perpendicular to the drains so a simple cross over. I need to calculate the falls so I know what levels the drains are at to accommodate the water/electric  incoming but nothing too complex. 

[lizzieuk1]

18 hours ago, Nickfromwales said:

It’s just about when 2 pipes need to converge, where you’ll need IC’s at each instance or any time there’s a change in direction.

 

You can run one pipe from A>B for some distance, but getting most BCO’s to agree to it is another thing. IIRC the last one wouldn’t let us go more than 8m (in a perfectly straight line)….. so ended up with 3 IC’s between the house and the road. 

Yes, the architect has specified ICs on the regs drawings but he has one that is right in our paving under the portico which isn't ideal aesthetically. 

I cant understand why your bc wouldn't allow greater than 8m, isn't it far greater in part H, 22m or 45m?

[lizzieuk1]

15 hours ago, Gus Potter said:

Ask your SE, they should have detailed this out already for you. BC are not Structural Engineers.

 

I might chip in as an SE but I would need to see the drawings first. There are lots of different types of raft. Until you know where the uniform loads are , where any point loads are you can't start on the drainage design. 

I hear you Gus but I'm not sure a SE is necessary here, shouldn't really be any point loads in an insulated raft - obv the external walls & load bearing internal walls will have the most loading but by design that should be spread throughout the raft. 

[Gus Potter]

On 04/04/2026 at 13:47, lizzieuk1 said:

shouldn't really be any point loads in an insulated raft

Actually this is more common than you think. Even if it's load coming down from large openings. Now you may have a full structural and very stiff raft and that is a different animal. 

 

When we design lighty loaded  "insulated rafts" we often have point loads.

 

There is much confusion around this. Unless you are an SE or experience Civil Engineer for example with the ability to analyse these things you won't really know about this mistique and the thoery. @Nickfromwales.. has done a lot of them but many don't know the theory, how they behave and the long term implications on the structure you are putting on top. The art of design is to look at each element from the soil up and understand how all these fascinating materail interact. It's part science and part craft.. that makes design exciting.. and eh profitable!..

 

In isolation if looking for a quick fix and safe, all we do is to recognise that the insulation is a bit more "squashy" and design for that. An insulated raft just has a more flexible layer before it hits the hard core. 

 

I've copied below a bit of my conservative calculations that I present to BC for a ground bearing slab, they need to tick a box.. The calcs are for completness, so I have go conservative to head off any questions from a checking Engineer. It lets you see how I'm taking the compressive strenght of the PIR insulation and reducing deflection. I chose to set a limit of 1.0 mm on the screed deflection as BC can't argue otherwise. Normally on insulated rafts I'll pick a different value.. I'm not going to tell you what that is as it's my intellectual property and that is why I get paid as an SE!

..

 

Edited 18 hours ago by Gus Potter

[Gus Potter]

On 04/04/2026 at 13:47, lizzieuk1 said:

I hear you Gus but I'm not sure a SE is necessary here

Are you sure? This could come back to bite you unless you know your stuff! By asking the question I can see you don't!

[Oz07]

Not meant as a dig @Gus Potter but would you agree that a house can be built without SE input? Not this particular house or this particular scenario nessecarily. Or would you say that every house built needs SE input?

[Dave Jones]

On 02/04/2026 at 23:10, lizzieuk1 said:

We're prepping to dig foundation and drainage out and wondering about depths for the (approx 5m) run that is going to be under the insulated raft and how low to set the foot for the main stack which exits straight out. 

Any idea what that depth needs to be? Is it 600mm below the mot 1?

 

Also, how far out from the edge of the raft do the drains need to run - I'm reading 1m min is the part H requirement if pipe is below foundation but, as our raft is only 450mm into the ground makes interpreting the regs a bit tricky as, pretty much any drain will be below the raft level.  I'm not sure if it's interpreted the same as obv the weight is spread over the raft rather than down through strip foundations so, does this make a difference?

 

Or, do I just ask BC?!

 

tail wagging the dog.

 

work back from the invert of the manhole they connect into. That will define how deep they go.

[Nickfromwales]

On 04/04/2026 at 13:47, lizzieuk1 said:

I hear you Gus but I'm not sure a SE is necessary here, shouldn't really be any point loads in an insulated raft - obv the external walls & load bearing internal walls will have the most loading but by design that should be spread throughout the raft. 

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   

[saveasteading]

44 minutes ago, Nickfromwales said:

An SE is absolutely necessary,

Not wanting to do my trade out of work, but for a traditional construction you might not need one, or some elements are from tables now published for the world.

And highly experienced professionals and builders know a lot.

But note that even heavy structural rafts, move. They will generally move in one piece without showing distress, but drains beneath could be crushed or bent.

You don't want anything ever to go wrong under a raft.

Check that anyone doing design for you, initially or formally, has insurance for their decisions. ie not just contractors' insurance but for design liability.

 

Otoh if it us a so-called structural raft sitting on insulation, then it is only as strong as the insulation.

[Nickfromwales]

Kind of contradictory?

 

Easier said when you’re in the industry but when we’re advising first time, novice self builders then what should we actually be saying here?

 

Cautious approach, manage risk, appoint liability, measure twice, cut once.

[saveasteading]

32 minutes ago, Nickfromwales said:

advising first time, novice self builders then what should we actually be saying here?

Agreed we should advise caution, ensuring expertise where needed.

And lots of research. 

Far too many people assume building is easy. It's been my life for 50 years.

It's very few buildings, esp one-off self build, that don't need an SE.