Let's Talk Waste Pipes

Firstly an apology. This is post-dated - I am writing on the 25th, but pretending it is the 14th. Slipping back into old habits.....

Getting the waste water out of the en-suite bathroom is going to be difficult. It was not really allowed for at the design phase, hence becomes problematic during implementation. The problem is that we do not want unsightly pipes running across the outside of our nice new Cotswold stone gable, so they have to be routed inside. This is a problem because the bathroom is quite large, and consequently the items that need draining are quite a long way apart.

The plan is to position the toilet so the main soil pipe can go straight through the wall into a new soil stack that joins up with a drain put in at the very start of this project by the groundworks contractor. The waste water from the other items will be joined into a single outlet pipe that goes out through the wall and joins onto the stack :

The problem is, and this is where we get back to the design, that no allowance has been made for this in the joist arrangement under the floor. The joists run from top to bottom of the diagram, so there is no problem with the waste pipes from the bath, basin or bidet, which will sit neatly between them. However, the main pipe, from the shower out to the soil stack needs to traverse through 7 joists. There is no way that this can be achieved within the letter of the building regulations. These state :

1. The maximum diameter of a hole in a joist is 25% of the joist depth
2. Holes must be drilled on the centre line
3. Holes cannot be drilled in the last 25% of the length of the joist

A hole for an 1 1/2" waste just about meets the first of these. The second cannot possibly be met as the pipe needs a drain slope minimum of 18mm per metre. The third is also impossible, as the pipe needs to pop out of the wall in the right place to join the soil stack. This should really have been addressed by the structural engineer at design time.

There is also, of course, the practicalities of how you actually get a pipe into the holes in 7 consecutive joists. The only way I can see is that it is pushed through from outside, drilling one joist at a time. The option of making it up from numerous short lengths joined with straight connectors is definitely not attractive !

I have chickened out of doing this myself, and the builder is booked to come tomorrow to put the soil stack in, connect up the toilet, and run the main waste pipe through the joists. To be fair, it is only partly chicken. There is also the small matter of having appropriate tools to cut a 6" diameter hole through a stone and block wall over a foot thick, and also the time it would take to get it done.

To help him on his way I have bought a trap for the shower, and today I cut a hole in the floor to install it. This is an adjustable depth P-trap, and I have it set up to show exactly where the pipe from outside needs to aim for. You can see it in the left of this picture, taken from below. Note also, the two central heating pipes to the bathroom radiator, both of which need to be avoided.