Drainage design for extension

[MattPope]

Hi,

We're getting started on an extension and have run into potential trouble with the drainage design. The existing drainage seems to be very shallow - the inspection chamber that links to the sewer is just 600mm below finished floor level. This would not appear to be enough for the new drainage. I would imagine a new connection to the sewer via a new/deeper inspection chamber will be very costly....

We have a total run of 30m required for a BIG for the kitchen grey water which I think would not require the pipework to run under the new floor.

The main problem will be the three new soil stacks that will need drain below the new floor of the extension (up to 40m from the sewer). Given that we will have a solid floor construction (concrete slab) I can't see this working. How much lower than the finished floor height must these pipe runs be?

One option is to have the new drains running through the garage but this would involve a new inspection chamber in the garage which might not be ideal if we want to convert this in future.

Any good ideas much appreciated. Most of my research has involved drains that simply connect with the existing.

Thanks in advance.

Matt

 

[noseall]

Whilst you have access to drawings and are able to pop outside and have a look at your existing drains, we on the other hand are not so blessed as all we have is a keyboard, a monitor and an imagination.

Any chance of a drawing?

As you rightly said the drains need a minimum of cover, however this can be close to the surface internally as long as it does not interfere with the slab. For instance, the pipes can pass through at insulation level without being detrimental to the floor slab.

If the furthest stack is 40m from a 600mm deep drain then you are not going to achieve recommended fall (1:40) or even close. However you can have a successful system with half the amount of recommended fall so long as you are very precise.

A 40m run would ideally require 1m of fall but a well laid plastic system could still work with half that i.e. 500mm. However, the price to pay is there is no room for error. Also the runs would need to be dead straight without being overly 'busy' i.e. no junctions or branches etc.

 

[MattPope]

Thanks for your reply Noseall. I have put together a quick diagram of the existing drainage in green and what the architect has drawn up in blue.

Untitled

• MattPope
• 14 Aug 2012
• 1

I was originally not in favour of this as there is an inspection chamber in the middle of the garage floor for connection to one SVP (upstairs ensuite), two sub stacks (family bathroom and cloakroom) and the gulley from the kitchen at the back of the house. I guess I don't like the idea of the inspection chamber inside the building this limits future conversion of the garage. There will also be an inspection gulley close to the driveway entrance. The maximum distance on the proposed design is 30m

I would have thought it was a neater solution (but maybe not the most practical) to run all drainage to the rear of the house with a single inspection chamber outside. The run of the longest drain in this case would be 38m. If the existing inspection chamber along the side of the house could be used then better still.

The inspection chamber close to the sewer is 600mm below finished floor level, the other existing chamber is 300mm below.

Any good ideas and suggestions are much appreciated.

Many thanks.

Matt

 

[freddiemercurystwin]

Why does the blue run run all the way down the garden and then turn 90° instead of just running diagonally to the existing connection? What's the shortest run in metres that can be achieved by going as close as possible to the existing corner of the house and then diagonally?

Nothing technically wrong with a manhole in the garage even after a conversion, provided it has a sealed cover then no smells or risk of it backing up should occur, just more hassle to inspect if ever its required (ie its gonna be under the carpet).

And why hasn't your architect sorted this out?

 

[^woody^]

Can you identify what each blue dot is?

On the face of it, I'd take one straight run from the rear kitchen, with a RE in the back garden, connect the other drains to this on the basis of rodable stacks/access points and low usage, so no MH under the garage, put a chamber just outside the garage door and then run diagonally to the existing manhole at the front

 

[MattPope]

Thanks for the ideas Woody and FMT.

To answer your question about why the previous drawing had a drain run and 90deg angle, this was just how the architect drew it. The building drawing details are simply marked with a disclaimer that the drainage must be run in compliance with Part H of the building regs - bit of a cop out perhaps.

Based on your suggestions, I wonder if Woody's suggestion is this is the way to go? The total run from the rear of the extension to the sewer connection is 28m - 14m front to back and just under 14m diagonally.

Untitled

• MattPope
• 15 Aug 2012
• 1

If the inspection chamber level is 600mm below the finished floor level then a 28m run and various gradients requires the following drops:

1:40 - 700mm
1:60 - 470mm
1:80 - 350mm

Is this really possible given the concrete slab at the rear of the house?

To answer the question about the blue dots, from front to back there are 4 connections:

SVP for ensuite bathroom upstairs
sub stack for cloakroom downstairs
sub stack for main bathroom upstairs
BIG for kitchen

Once again thanks for your help with this.

Matt

 

[theoldun]

Take finished floor level datum as 1000. Invert level at top of run 300 below FFL = 700. Invert level at IC before main sewer 600 below FFL = 400. Gives a fall of 300 over 28m.
Over 28m you need minimum fall of 350 at 1in 80. Therefore theoretically, drainage will not go in on gravity, how ever if we were doing it we would go in at 1 in 95. If bottom of trench is hand trimmed and pipe is laid very accurately then with three WCs on the line you should have no blockages, repeat should not have, but not guaranteed. The existing 600mm deep inspection chamber will need new channel junction and not a three quarter section bend as you will need the additional 50mm in the IC.
If you do not fancy fall of 1 in 95, then you will require single dwelling 1178 litre pumping station. Not cheap, and can be troublesome.
Regards oldun.

 

[^woody^]

And one more thing, you need to be clear about the levels - and its not clear what you are measuring to either

You need to be working to invert levels, which is the level of the half channel in the bottom of the chambers, and you need to work out you actual ground levels to see if the pipes can be laid below ground

The crucial thing, may well be that first gully at the rear kitchen. I would consider altering this to an sealed internal connection for the sink, which may get you a bit more height. But your garage floor is probably lower than the other floors in any case, so it may not matter

 

[theoldun]

Looking at your drawing again there is one other possible solution and if you are prepared to give us all the information that we require, then we will tell you whether it is possible.
Really you should have some one knowable about drainage and levels come out and throw a few levels about and design a scheme that is workable.
This is one of the problems in employing half a*sed architects.
Regards oldun

 

[MattPope]

Looking at your drawing again there is one other possible solution and if you are prepared to give us all the information that we require, then we will tell you whether it is possible.
Really you should have some one knowable about drainage and levels come out and throw a few levels about and design a scheme that is workable.
This is one of the problems in employing half a*sed architects.
Regards oldun

Thanks for your ideas on this. Do you think the architect should design the drainage run? Is that normally what happens? The building drawings are fairly loose on the drainage details.

The levels were provided by a builder I had round to give a quote for doing the general blockwork. It was a bit rough and ready and we agreed it would be a more specialised task.

I guess my question now is what the best approach should be. Get a builder in who specialises in drainage? I'm happy to do a lot of the groundwork labouring to keep costs down. How long would this job take?

Thanks.

Matt

 

[^woody^]

Drainage is part of the design. Its no good your Architect putting in toilets/bathrooms/sinks etc if they can't be connected to the drainage system!

Some Architects will try and pass this onto the builder, but the Architect should be sorting this out so that the builder can give you a proper accurate quote with no surprises when the drainage layout wont work or needs altering

A competent builder can sort this out, and you wont need a specialist.

Sometimes, drainage can be sorted out after the building has started or even at a later stage, but in this case it seems like it should be sorted out before hand, so that you know what works or what changes are needed.

The inspector will need to approve any altered design on site, and you may need a pragmatic inspector and not the moody one or the one just out of college

 

[Nige F]

Does the sink have to go under the window ? chefs prepare food. Porters do the washing up if the sink - or at least the dishwasher was opposite the window it could go through the wall to stack in garage . The gulley outside the window could go round the outside left - unless that`s next door . Don`t like that long flat run from the kitchen gulley with no real volume of water behind it . Also the modern WC`s have silly little amounts of water per flush ( 6 litre max). It could all be a problem .

 

[MattPope]

I'm feeling depressed with all the bad news.... but many thanks anyway!

The sink by the window has been confirmed as essential....

I guess by the time I get the levels measured accurately I might find that we have a bit more depth to work with and with an accurate measure the run I might have a couple of meters less but it won't be far off what I have suggested to date.

Going back to the original idea of the inspection chamber in the garage floor for the three soil pipes, would it be feasible to run the sink waste to the existing inspection chamber at the rear of the house? This IC is 300mm below FFL but could go around the perimeter of the house rather than cut across the slab.

Thanks once again.

Matt

 

[Nige F]

That idea is feasable - and a good one - If you use a gulley with rodding access and have an outside tap above it , you`ll be able to flush the pipe if necessary . The problems I have encountered with sink/dishwasher/washing machines/waste disposal ( in sink) is due to the fine debris and fat making a sludge . So with this drain run the worst case scenario is you MAY have to rod it with a brush on the rods from the existing manhole . Also Build Control might let you use a long radius bend @ the corner of the house , instead of a manhole Any work is outside the house , also the double seal internal manhole in the garage could be shallower because it now takes just the WC`s - giving more opportunity to get a better fall

 

[maltaron]

I think you will find that the 40mm sink waste cannot exceed 3metres long, so you would have to use a 110mm from there to the manhole.