drainage extention question

[5teves]

I want to extend a drainage pipe from the back of kitchen (plastic manhole) to the outbuilding at the rear of my house but not sure if the fall will be ok.

The run would need to be 7 metres and I have around 250mm before hitting ground level. I have a feeling this won't be enough of a fall, is there anyway I can achieve this?

I'm looking to put in a toilet sink in the rear workshop

 

[oldbutnotdead]

The minimum fall for 110mm plastic is 18mm per metre (max is 90)- so over 7 metres you'll be up to 125mm below ground level (minimum) assuming your 250 is to the top of the existing pipe. So there's enough fall BUT you need to protect the pipe, ideally cover with 50mm paving slabs all along the run (as well as pea shingle bedding etc etc, its all in the Approved Documents- Google Part H and look in Section 1 Sanitary Pipework). And since you're covering the pipe all the way, go for the maximum fall you can get (about 30mm/metre)- it'll reduce the likelihood of solids not flushing properly, getting stuck, drying out and causing grief.

You'll need a rodding point (or a manhole) at the workshop end and ideally put a stack pipe in as well (makes terminating the loo easier). Given your limited fall, be wary of plastic manholes- the cheap ones have a rise of about 70mm on the angle incomers so try and avoid using them.

I suspect that this is all notifiable work as well.

 

[5teves]

Thankyou so much. Your reply is very informative.
What would you say are the best manholes to use then?

Also, this is purely to benefit of of own understanding.
In a situation where the run is to long to meet the required gradient can anything be done?

 

[oldbutnotdead]

I'd still use the cheap plastic manholes, 'avoid them' meant avoid using the side branches, just use the main in and out- it is the side runs that are higher, having manhole access for any future unblocking is a lot easier than a rodding point. If you really really needed an angle manhole then you could do what I did years ago (not sure how legit it was/is but it is still working now)- took a Y branch, cut the top off it (left the sockets intact but cut the pipework down so the actual Y bit had 180 ish degrees of pipe rather than 360) and set it in a load of concrete, concrete formed up the sides of the pipework to above the sockets, brick surround and cast iron manhole lid on top. Could post a pic if anyone is desperate but it is cold and dark outside )

Ultimate run length- if you're on mains drainage then the main sewer is likely to be at least a metre below ground level (in the street out the front of your house). A metre drop at 18mm/metre run allows you 55 metres run from the main sewer to your outbuilding as a maximum- if your garden is that long then you can probably afford the connection charge from your friendly local utility company .........

In my current circs the existing drainage is all salt glaze set to a much steeper gradient than required for plastic- this may apply to you as well. So if there wasn't enough fall to your manhole behind your house you could look to linking at the side or front of your house (and putting another manhole in). Getting a CCTV survey here is real handy- you don't just get pretty pics of the state of the drain, you also get info on depth and direction of travel. Depending on your layout, you might be able to run the pipe above ground for some of the distance (it would have to be encased and protected but it is an option, typically that would get you another 300mm which translates to an extra 17 metres or so pipe run). After that I think you'll be getting into some serious technical solutions (pumps, macerators)- well above my pay grade. If you are in difficulties with run length then a chat with your BCO might be quite rewarding...... mine up here are getting quite paternalistic about my refurb efforts and have come up with some very very useful suggestions, hints and options along the way.

If you're on a septic tank then get a bigger one (if you need to- they are sized for the load put on them, an extra loo doesn't necessarily increase the load if the number of people using the loos hasn't changed significantly) and bury it a bit deeper.