Toilet 100mm soil pipe under concrete floor

travisonline

I would be grateful if anyone could share their views on whether the below suggestion is doable and makes sense. What I want to do is to increase the size of the second bedroom (which is very small at the moment and can only accommodate a child) and make the bathroom smaller as a result. To make it possible (i.e. having a smaller bathroom) I will need to relocate the toilet from the bathroom to the cloak room under the stairs (the cloak room is 100cm wide and 210cm high). The most difficult part of this will be laying the pipe in the concrete channel (the floor in the bathroom and the corridor is solid concrete) with a gradient (let’s say 20mm per meter). The length of the WC soil pipe (the pipe is shown in green on the new plan) will be approximately 3.5-4meters. The lowest point where such pipe will need to laid before being connected to the soil stack is approximately 180mm (i.e. the pipe itself is 100mm plus 80mm to have the required gradient). I showed this to the building control in Camden and they think it is doable but they asked me to check whether there are any foundations in the concrete which I can accidentally penetrate when cutting the channel in which case I cannot lay out the pipe as shown on the plan.

I have a few questions in this respect:

1. What’s the best way of checking if the concrete floor can be cut to make a channel with the depth of 100mm-200mm? Are there any plans somewhere showing what was made in my flat? My flat is in a Victorian building building built circa 1890-1900. The building control suggested making a few holes in the concrete and invite the structural engineer (£200 minimum a visit) to check this. I googled and found that some firms provide services of brining ground penetrating radars which scan the floor and provide pretty accurate information what is underneath without any cuts in the concrete – is it reasonable to request such scan or I am overcomplicating things?
2. Does the pipe have to be straight or it can have some bends (minimum bends)?
3. What’s the best way to cut the concrete in this case to avoid too much damage to the floor – using concrete diamond saw?
4. Once I laid the pipe in the channel – do I need to sleeve it in a particular way?
5. I hope the length of the pipe (max 4m) is not an issue as the building control standard limits the length to 6m – is this correct?
6. Any nuances where the pipe near the new toilet should be located. Should it start at the floor level (i.e. being laid in the concrete so its top should be at the same level as the floor)?

Many thanks!

anotherbaldrick

All that and you have not said if the flat is ground floor or upper level and if ground floor is there any basement below ?

Stooby2

Well as he mentions the risk to foundations and the possible use of ground penetrating radar after knocking some holes in the floor, I think it's safe to assume it's a ground floor flat with no basement.

anotherbaldrick

Never assume, If its a ground floor flat why not just trench out to connect to the external drain ?

travisonline

Sorry, should have mentioned that - it's a ground floor flat without any basement. Do I need to worry about "foundations" if I am going to cut the channel in concrete up to 20cm? The floor is wooden in my flat apart from the corridor and kitchen and I will need to cut the channel in both kitchen and corridor to lay the pipe there.

keystone

1:50 isn't enough fall. It needs to be 1:40.

travisonline

Keystone, noted I suspected that 1:50 fall is a bare minimum so I will go will go with 1:40 (i.e. 2.5 cm fall for every meter). This means that on a 4 meters pipe the fall shall be 10cm. Therefore that the the depth of the channel must be at least 25cm at its lowest point (100mm the pipe itself, 100mm fall and 50 mm leeway for sleeving the pipe etc. As I understand, I need to connect the pipe to the external drain rather than to the vertical soil stack - is this correct?

keystone

Actually 1:40 is the minimum. If you can connect to your existing stack above ground do it but I can't quite visualise how you would given the description of your project.

The moment you go underground with the connection to conform with the rules you need to invite LABC along to inspect the connection before you fill the hole in. If its your private drain underground thats the limit of your expsoure to the outside world. If its a lateral drain then your water company will, at the very least, want to inspect the connection as well for an enormous fee. Thames Water, for example, is currently £ 485!

Cheers

anotherbaldrick

I would suggest digging a trial pit outside the external wall . this will tell you the level of the U/s of the foundation or footing . Your best bet would be to cut a trench to the wall and foundation and tunnel under to run a new branch to the foul drain. The floor slab will be about 225-250 mm thick and trying to cut a 200mm chase in it is pointless, the bottom will just break out.

travisonline

Sorry when you say the concrete breaks out you mean I cannot do it because of the foundation issues? I am not sure how you can put such big pipe under concrete from outside. Is it doable for a 4 meter pipe?

I_have_spoken

makes sense.

Nope. Surely you can design the shrunken bathroom to accommodate the toilet pan?

Anyhow, if you did move the WC to under the stairs, you'd also need a basin. Why not use a Saniflo rather than trying to cut a huge trench in the concrete slab?