Water ingress around a waste pipe

[DavidB]

Hi

I've posted previously two questions about my shower room renovation and the damp and rotten sole plate and posts that I am patching. As I explore more of the sole plate my latest discovery is that when I removed the old sink and pulled off the (damp) plaster board I actually found water droplets on the wall. The sink itself was on a thick piece of tiled plywood ( which was dry) and then attached to the wall. So I don't think that water came from the sink. But the dampness does seem to be !ocalised around (slightly below and to the left of) the waste water pipe. We have had a heavy downpour on Friday and raining saturday.

This area used to be a doorway in the past, and someone has built a new plinth wall around the door posts and put a new post in the middle. All three of which are rotten at their bases. I went round the other side and the exterior doesn't show any major problems - just the faintest of cracks around the waste pipe.

I think I have two options:

1. Re-route the waste pipe internally so that it joins onto the shower waste which exits from a similar hole about 3 feet away - this means I will have less external pipework and I can seal up the hole. But if that proves too awkward with the pipe run and stud wall for the shower enclosure; then
2. Alternatively I may reseal the pipe. I'd probably take the old one out put a new metric one in that matched the sink fittings etc.

So my question is - how should you install a waste pipe through a lime rendered wall (which has an outer skin of cement and paint)? I would have thought that the pipe would expand with heat and the wall will always resist that so you'd always get a break in the seal? My thoughts were:

A. Make the hole slightly larger and fit a slightly bigger pipe ( just a small section) like a sleeve for the waste pipe to go through. Then backfill around the sleeve with lime render and silicon seal or expanding foam between the two plastic pipes.
B. Some kind of flashing I.e. lead - not too sure how I would do that? Perhaps cut out a section of the wall surface and lay lead over the pipe and then render over the top of that.
C. Re-render around the pipe exit making a similar structure (but much smaller ) ) that you find over a window to shed water off the wall.
D. Just use silcon sealant around the pipe as it exits the wall.

What do you suggest? Thanks.

 

[Zebra]

Well, you're not absolutely sure of the cause. It could be condensation trapped in the wall behind the non-breathable tiles, it could be that moisture is condensing on the pipe because the pipe is cold.
I think option D is probably the best solution. It's the easiest anyway: silicone around the pipe - on the outside of course - then you can continue to monitor the situation while you carry on with the works.

 

[DavidB]

Thanks Zebra

You are right - I am not sure, but suspect a leak rather condensation trapped, because the tiled area was very small (4 small tiles) and the dampness is about 1 foot below the tiles and looks very localised. And the sink has hardly been used for the last two weeks. There is another waste pipe (for the shower) on the same wall that is a couple of feet lower and goes through the plinth rather than render - this is dry. So I think my preference is to just re-route the pipework. But as I am messing around with patching the sole plates over the next week or so - i could try your suggestion just to reassure myself that is the source and before I cover it up again. Thanks for the advice.

 

[Zebra]

When pipes are run through walls of course the hole should always be drilled with a slight fall to the outside. Even more so when it's a waste pipe. Perhaps this was overlooked.

 

[MatthewC]

When pipes are run through walls of course the hole should always be drilled with a slight fall to the outside. Even more so when it's a waste pipe. Perhaps this was overlooked.

I fitted three plastic waste water pipes and the cast iron soil pipe, all with a slight fall, and applied lime mortar around the exit end. There has not been a problem in almpst 5 years since.

Mind you, I didn't drill the holes - the issue was stopping bits of the wall from collapsing of their own accord!

 

[DavidB]

Thanks, guess what - yep you got it - mine has a slight rise (not fall)! Perhaps when the sink was connected it pulled it level, but the pipe is firm ( I was able to rest my level on it). On the exterior the pipe has the same finish as the wall I.e. thin textured cement coat with paint. I suspect that holds the water and allows it to track down the pipe into the wall. Thanks - I now have a simple plan of action and which doesnt need tricky lead flashing, sleeves or rendered run-offs !

 

[MatthewC]

It's amazing, isn't it, how someone doing something just a little wrong can have such a big effect? Or put another way, if you do things right to start with, don't you avoid such a lot of little annoying issues? That's where I have found this forum so useful!

 

[Tony.]

Sleeve the pipe with a pipe the next size up, it's what we do in commercial buildings.

It will have the advantage of catching it any condensation and running it to the exterior.