First Post - Foul Drainage Layout.

[DTB1985]

Hi Guys,

First post so please be gentle.

Doing an extension and looking to get a rough idea of the new foul drainage setup before building regs comes round.

We have a joint foul/grey water system where it all goes into one outlet.

I've attached a couple of drawings. One shows the original layout and the second shows the new extension and outlets we need. The second drawing is just drawn with no knowledge so is full of mistakes I'm sure. Having done a little research it seems most of the joints should be outside of the property possibly via an inspection hatch or at least have some rodding points. Really the second drawing just shows where we need things to get to and now I am working on the best routes to achieve this.

Ideally I'd like to keep using the existing connection to the vertical soil stack as would be a massive bit of work to remove but if its required we will move it.

Any thoughts would be appreciated and if you have any questions please just leave them and I will try and get back to you.

Thanks,

James.

 

[George_engineer]

Oh dear. I'll be gentle but don't build what you've drawn - recipe for blocked drains and no hope with BC.

I'll note a few principles... sometimes drainage is left up to the builders to sort out with a bit of help from BC, but if I was designing a layout, thesea re the general principles.

even with combined sewers you need to separate SW (surface water) and FW (foul water) for new build
- these stay separated until the last chamber, before rejoining and going out to the main combined sewer
- this can be bent a bit to fit in with exg drainage, but that is the expected standard
- SVPs (soil vent pipes) need to go direct to an external IC (inspection chamber).
- runs must be straight between ICs (except for a small bend in order to directly join into the ICs, so that...)
- in a IC, no change in direction less than 90 degrees
- if you must go below a structure, then you would want to collect all the drainage at one side and go straight under to another chamber
- if an additional chamber seems excessive, for example at the very head of a drainage run, you can use a rodding eye
- land drains need to go somewhere too. Would have to check but don't think they can go into combined sewers
- you should be lowering / grading the external ground to slope away rather than relying on land drains (guessing french drains) - they're great in the right circumstances but are at the bottom of the list.


Have you got room along the side of the building to put ICs?
Have you got room for a soakaway?

 

[^woody^]

You will be required to put the rainwater into a soakaway in the garden.

Your proposed layout seems overly complicated, with risk of blockages and no means of venting the drain - so flushing a toilet could pull all the traps out of the other parts of the system and the house filling with a smell of sewerage.

All drainage should be designed and sorted out by whoever draws the plans. The builder should price on the plan design, so you don't get a nasty shock after you have been quoted one figure and then builder starts work and then says "Oh I have to change the drains and you owe me an extra £5000"

 

[DTB1985]

Hi guys,

George, thanks for the advice, really helps clear things up as to what's expected.

- The path to the side is 1m wide so there is room for some inspection hatches
- Regarding a soak way the garden is big enough to accommodate one. Is this in relation to the land drains/French drains? Having done more research the only one we need is small one at the front of the property. At both sides we will slope the path away from the house and our site naturally slopes down towards the back to surface water will head towards the garden.

Woody, again thanks for the comments.

So probably going to say the fateful words here but we are self-building / project managing. If it helps we have family who have self built before and are in the construction industry but obviously we have some gaps such as drainage and I want to do all you research before we put anything in the ground and I make a mistake. We are using trades for many areas of the build and will be doing a lot of work ourselves as well.

The architects did draw the plans and I will go back to them re the design of the drainage but whilst they have been very good in many ways this area doesn't seem to be one they are ken to commit to as they are not the principal contractor on the job.

I do totally agree about my original drawing hence why I've come here looking for some advice from those that know better.

Updated Plan

I have attached an updated plan that I'm sure isn't the finished article but hopefully is more on the right track. I have moved the s/w drainage to a soak way in the garden
I have moved the main line to outside under the path with I/C's at each point

A Few Questions

- On the revised plan there are a couple of red boxes. One is where the main line has to bend to intercept the line from the street. I haven't added on but do I assume I need a I/C there as well due to the direction change?
- The second red box ins on the sink. Not sure what is the best way to connect this so I just wanted to get you opinion.
- I've still left the existing hose stack in place which vents at the top. Is this ok and as the downstairs toilet isn't directly connected to it will that cause a problem?
- Currently all the I/C's I've dawn are all small inspection chambers as I dont think we will need to go deeper than 600mm. Does this seem appropriate.


So I think that is everything. As said, thanks for the advice, its been really helpful and whilst I dont want to take up to much of your time anything else you can help me with is greatly appreciated.

Thanks

James.

 

[George_engineer]

That's better

- On the revised plan there are a couple of red boxes. One is where the main line has to bend to intercept the line from the street. I haven't added on but do I assume I need a I/C there as well due to the direction change?

Yes you'd need an IC. I'd shift things a bit (see below). The sink drain just has a little y-junction onto the toilet drain, this is is OK so long as it's immediately adjacent to the chamber.

- The second red box ins on the sink. Not sure what is the best way to connect this so I just wanted to get you opinion.

'Above ground' in a box in. You can sink them into the floor (i was forced to do that in my downstairs WC) but it's much better to just attach to a wall and box it in.

- I've still left the existing hose stack in place which vents at the top. Is this ok and as the downstairs toilet isn't directly connected to it will that cause a problem?

Potentially - it probably exceeds the allowable distance so you may need an internal air admittance valve if you can't get to atmosphere. If this is boxed in then it needs to be accessible via a hatch as they do give up after 15 years or so.

- Currently all the I/C's I've dawn are all small inspection chambers as I dont think we will need to go deeper than 600mm. Does this seem appropriate.

Yeah. If possible keep it shallow until you get to the last manhole. Reasons not to is if you're driving over it a lot. You'll need to find the outgoing invert level and work backwards. AD H has the information on cover depths, pipe diameters etc.

 

[DTB1985]

Hi George,

Thanks for the update and the drawing.

Just to confirm, the IC you've added that does the downstairs bathroom and sink near the outer wall. Are you suggesting and IC and then imidiantly attaching a y split to it. The Y goes to the sink and the straight trough to the bathroom. If so one question. I like the idea but the straight through as drawn would need to bend up to the toilet/shower in the bathroom. Is this down with a shallow bend close to the IC. Just checking if thats what your getting at.

Secondly I see you have drawn the soil stack being connected to the outside drain. Just checking I'm reading that right.

Thanks,

James.

 

[George_engineer]

Just to confirm, the IC you've added that does the downstairs bathroom and sink near the outer wall. Are you suggesting and IC and then imidiantly attaching a y split to it.

Yes

The Y goes to the sink and the straight trough to the bathroom. If so one question. I like the idea but the straight through as drawn would need to bend up to the toilet/shower in the bathroom. Is this down with a shallow bend close to the IC. Just checking if thats what your getting at.

Vertical to horizontal bends are OK (you get 'sweeping' bends made for this purpose - https://www.screwfix.com/p/floplast-push-fit-87-5-double-socket-rest-bend-110mm/12719 as opposed to a 90 degree bend), it is plan bends are to be avoided.

The Y-junction would be at the bottom at the IC invert level then once under the wall and into the house, would have a sweeping bend up to create a vertical pipe through the floor.

Secondly I see you have drawn the soil stack being connected to the outside drain. Just checking I'm reading that right.

Thanks,

James.

Yeah I'd take the shortest route out to the drain. A SVP bending once as it comes down to ground is OK, but what you drew was a bend vertical to horizontal, then a plan bend to head downwards. A single sweeping bend to the nearest IC is better.


This is more about surface water drainage but has some good background info https://www.pavingexpert.com/drain02

 

[DTB1985]

Hi George,

That makes sense and thanks for the links. Think I'm nearly there with a lot of help from your good self. Thanks for the product link and I'll have a good look at the surface water plan as you suggested.

Speaking of surface water I have spoken to our BC. He has confirmed that our surface water / rain water drains can go into the shared grey/foul water system. It does surprise me but he said as the system in where we live in Leeds is setup that way they have no problem us using it as such.

I've attached a plan having taken everything you have suggested into account and it has the land drain and supper drains linked in. If you don't mind commenting on it I would appreciate it but I know I've already taken up a lot of your time so no worries if not.

Again many thanks for all the help, much appreciated.

James.

 

[George_engineer]

I would check they're happy for you to have it combined all the way through. Like I say you usually need to keep the FW and SW separate until the final MH when they get combined again.

(This might seem pointless but the thinking is that if/when they update the sewers in the road, they can separate the householder's drains a lot easier. This is a 100 year plan.)

As the eventual destination is a combined sewer, all the SW drains will need a u-bend / p-trap to prevent sewer gases coming up the rainwater pipes/scupper drains. Ensure you get roddable versions, these usually have a removable cover inside the gulley pot which bypasses the trap.

Something I always add is a gulley next to the bins which drains to the FW - again this is something from the commercial world and might not be needed, but can be useful for washing bin area to have a FW connection.

 

[DTB1985]

Hi guys,

So one final update to this. Been considering moving the existing soil stack inside the house and upto the existing backroom. There are a number of good reasons of this and either way we will be replacing the existing one with a new one as its quite old so we will already have to do half the work.

My one question is can a new vent stack be put anywhere in the chain? My research seems to indicate it can be but I just wanted to check what I have drawn would work. I appiracte the new vent stack I have drawn is essentially as far away as it can be from the inlets but it would be the most straightforward to install here.

Any thoughts would be apppricated.

Thanks,

James.

 

[Hugh Jaleak]

Ideally vent needs to be at the head of the drain, but anywhere is better than nowhere! Looking at your proposals though, I would be inclined to continue the stack serving the upstairs bathroom up into the roofspace and through the roof. Your halfway up already at first floor level, should be easy enough to box into a corner.