Foul drainage under slab

[BenGillyHills]

I am currently working on a 5 x 11m Annex/Cabin. It will be single storey, one bedroom, timber frame with strip foundations built to comply with UK building regulations. I have a private BC  company on-board. We have the footing poured so the next stage is block work below DPC and services laid under the slab. The project is self design and self build and it’s size and scale is well within my comfort zone although I am not a professional builder. (I’ve done lots of renovations and built smaller cabins)

 

I’ve been doing research to get an idea of rules and regulations for underground drainage, and I can see that a good design and rodding access is vital but there doesn’t seem to be any hard and fast rules about underground bends and direction changes as far as I can tell.

 

can anyone give any feedback of the drainage plan I have come up with?

 

Look forward to hearing anything anyone has to offer.

[Mr Punter]

The long radius horizontal is a no-no.  There are no inspection chambers.  Why not take the pipework outside from the bathroom and run around the building?  That way you can add proper access for inspection and rodding.

[JamesP]

I agree with @Mr Punter. Have 2 runs.

Not a professional but I exited pipework out of the building to avoid long runs within the slab.

Bit of extra external excavation but all accessible.

[BenGillyHills]

Thanks for your helpful comments. The pipe has to exit the building where shown on the diagram as this is the only place where the trenches are deep enough. When it became evident that these trenches would be stepped down, I earmarked this area for pipe to exit the building but never considered the location of the bathroom in relation to it. Some head scratching required…As there will be a service room behind the shower, Initial thoughts to solve the problem would be to put an inspection chamber in the service room, running all bathroom drainage to this point. Then it will be a straight run under slab to the existing exit point. There will need to be a bend here just before the pipe exits, not sure if that’s going to fly but will amend the design accordingly and post back here for more feedback. 

[Mr Punter]

After the pipe crosses the foundation it can immediately drop down, so unless your external finish level is on the foundation you should be able to accommodate this.  At this stage it is not too late to create a channel in the foundation and run the pipework a bit lower.

[BenGillyHills]

Thanks again, this is how the ground floor make-up will be in the area where you were suggesting the pipe could exit (outside bathroom). Do you suggest the 4 inch pipe is channelled through the concrete underneath the course of trench blocks? I haven’t laid trench blocks yet so at this stage the opening could be cut and broken out rather than cored through. Which would be straightforward. Am I interpreting what you’re saying correctly?

[BenGillyHills]

This was the other option I was considering, which includes an inspection chamber within the service room. Although there is a  90° bend before the exit, the access from both inspection chambers is very good so I’m wondering whether this a valid solution.

[Oz07]

Stop having 90s under your slab. Just get the straightest runs you can

[Oz07]

Something like this. You can take thru the opposite wall put a 45 on then rodding eye at ground level. I would say be better if wc comes in with a 90 bend to the invert rather than a swept 90 poking upwards so put a horizontal y in. Make the run inline with basin point and join wc into it with a y. If you cant take shower drain out right where it is on external wall then join with a y just before the drains pass thru wall. Also prob take shower drain out with rodding access. 

 

I'm just thinking on my feet. There are probably better ways

[BenGillyHills]

4 minutes ago, Oz07 said:

Something like this. You can take thru the opposite wall put a 45 on then rodding eye at ground level. I would say be better if wc comes in with a 90 bend to the invert rather than a swept 90 poking upwards so put a horizontal y in. Make the run inline with basin point and join wc into it with a y. If you cant take shower drain out right where it is on external wall then join with a y just before the drains pass thru wall. Also prob take shower drain out with rodding access. 

 

I'm just thinking on my feet. There are probably better ways

Thanks, I appreciate the thought. I’m going to digest what you said and see if I can improve the layout. I think I need to establish if I’m able to break through my foundation concrete to run pipe the in which case I will make logical exits near the bathroom and run pipe outside the building accordingly. I’ve made a rod for my back by limiting where I can easily

exit pipe through foundations. 

[BenGillyHills]

Hi everyone, my original query was getting opinions on the layout of drains going under the slab, from comments it became evident to avoid major changes of direction under the slab, pipe exit should ideally be brought out of the building close to the bathroom and run around the outside of the property.

 

After much thought, rather than change my pipe exit location (and create a number of other problems specific to the site), I have adapted and simplified the design, still taking all pipework across and under the slab, but in a way which I am hoping is good practice and compliant with approved document H.

 

My questions are:

 

1. Are the joints and bends under slab acceptable?

 

2. Any problems with having x2 adjacent linteled openings in foundation with 450mm long trench block in between.

 

3. Overall, is there anything I’ve missed or is this layout acceptable?

Edited 11 hours ago by BenGillyHills

[BenGillyHills]

Plan view

[Russell griffiths]

Put the bends as close to the chamber as possible not under the slab. 

[-rick-]

Can you do something like this (red line):

 

[BenGillyHills]

20 minutes ago, Russell griffiths said:

Put the bends as close to the chamber as possible not under the slab. 

Thanks for your input. The pipe exit openings cannot go any further to the right when looking at the plan view. So the angles just don’t work to allow the bends closer to the chamber. The 30° and 45° bend along with non-perpendicular exit angles are the best I can achieve. Is there a definitive rule to say that those bends under the slab are a no go or just avoided if possible? 

[BenGillyHills]

19 minutes ago, -rick- said:

Can you do something like this (red line):

 

Thank you, this looks good. would the shower then need rodding point? Also there would be a triple socket/Y fitting at the junction where the two pipes meet.This would be a tricky fit as I believe the angle is 45° the angle would be much less than this. Also, is kind of junction this ok to have under the slab? I like the idea to simplify it and remove a bend.

[BenGillyHills]

30 minutes ago, -rick- said:

Can you do something like this (red line):

 

This is very similar to what @Oz07 suggested. I will amend model to work as shown with the shower running into the WC/basin run resulting in one exit point one junction under the slab. Looking back, @Oz07 also suggested a rodding point for the shower which can go 45 up to ground level outside the nearest wall. In hindsight there will probably need to be same sort of external rodding point for the basin/wc run too as the current one on the diagram is above a 90° bend.

[Oz07]

Rather than having your wc coming into the top of the run like that put a y junction in on flat below the slab then bring a 90 up thru the floor. The poo will slide down the pipe better then rather than falling into the top and the water running away from the solids. It will prob work ok your way but I'd rather my way. You have a rodding access upstream so the y under the floor could be accessed if ever blocked. 

[BenGillyHills]

14 minutes ago, Oz07 said:

Rather than having your wc coming into the top of the run like that put a y junction in on flat below the slab then bring a 90 up thru the floor. The poo will slide down the pipe better then rather than falling into the top and the water running away from the solids. It will prob work ok your way but I'd rather my way. You have a rodding access upstream so the y under the floor could be accessed if ever blocked. 

That’s really helpful, thank you. Do you think that the preferable design for the rest of the layout is to bring the shower in to the WC/basin run with another Y underground OR the two separate runs as shown in the most recent diagram?

[Oz07]

Maybe just one run and eliminate the bend before the main run exits the house. It'll be easier to do because you can align with basin instead of wc

[BenGillyHills]

7 minutes ago, Oz07 said:

Maybe just one run and eliminate the bend before the main run exits the house. It'll be easier to do because you can align with basin instead of wc

Yes that makes sense. I’ll re-do the model and post back. All advice so far much appreciated. 

[BenGillyHills]

@Oz07I’m feeling pretty happy with this so far.
 

Is the extra 45 coming off the Y on the WC ok?

 

Where do I need to add rodding access?

[saveasteading]

Excuse that I haven't read all the above. But I don't like most of what I see.

Get drains outside of the building by a short route. Avoid bends and junctions under the floor.

Why can't you take them out of the right  side?

 

Is the bco OK with footings 420mm deep?

Is there to be stone under the slab?

 

2 layers of dpm should be unnecessary.

[BenGillyHills]

1 hour ago, saveasteading said:

Excuse that I haven't read all the above. But I don't like most of what I see.

Get drains outside of the building by a short route. Avoid bends and junctions under the floor.

Why can't you take them out of the right  side?

 

Is the bco OK with footings 420mm deep?

Is there to be stone under the slab?

 

2 layers of dpm should be unnecessary.

@saveasteading I appreciate all input. I’m really trying to work to best practices but have some limitations. I can’t take the drains out by the shortest route as there is not enough space around the building on the right side.
 

Also, the foundations are stepped down where my diagram shows the pipe exit.
 

So with 3 courses of trench blocks in this area it gives plenty of room for the pipe to pass through with a lintel.
 

This is not the case with the footings at the top and right of diagram which have just one course of trench blocks then dolly blocks as per detail.

The rest of the foundations are as per the detail (except concrete is deeper than 220mm in reality).
 

Footing depth is 450mm including mortar between courses as per detail. It’s on slatey rock and BCO was happy with depth.
 

The reason for stepping down the trenches in one area was because there was some made-up ground to get through. 
 

Regarding the ground floor detail with x2 DPM’s. This was led by the BCO. I originally had one DPM as well as hard-core and blinding sand but BCO suggested this as a preferred alternative. The secondary DPM is mainly there to help the slab go off consistently I believe. In all honesty though if he’s happy, I’m happy. The building is not going anywhere, it’s a single story cabin.

 

My main focus is to refine the layout of the foul drainage under the slab to be compliant.

 

I think it’s close (I may be wrong), but given the limitations, I am really keen to have other peoples ideas and if anyone has a better way of doing it, I’m keen to hear it.

 

You can see in the picture attached that there is not much room outside the bathroom end of the building. There will also be a retaining wall there reducing space further.
 

You can also see where the stepped foundations allow more room for pipe exit. This is also in direct line of sight with the existing drainage which we will be connecting into.

Edited 3 hours ago by BenGillyHills