Ground floor shower waste must penetrate DPM

[1justin]

I'll be building a ground floor shower room. It won't be a wet room, but the tray will be directly on the floor. (Concrete, early 1970's, no insulation)
The 90mm wide shower trap has pipe invert at about 70mm. Allowing 18mm/M fall, I'll need to dig down to ~ 110mm below ex floor at the point where the waste exits the wall to join the stack. (That's still above external ground level).
The screed will be only ~50 max (not yet dug any up). Assuming for now that the DPM is below the screed (not yet found that out), I'll need to penetrate the DPM and cut into concrete for the waste pipe. (I'd be fairly sure there will be no steel in the site concrete).

What's going to be the best way to repair the DPM and to seal it round the pipe? I'm thinking I'll need to remove a wider strip of the screed in order to expose more polythene have something to tape to. I have a concept that I'd create a smoothed trough with some new adhesive mortar (so as not to puncture new polythene) and line it with a DPM taped on. Only I'm not keen of relying on sticky tape. Are there better solutions?

The job is under Building control, so I'm sure the inspector will like a nice solution as well!

The shower can't be lifted higher (elderly person).

 

[tomfe]

How about you get a younger person to help lift the shower?

I would not bother to repair the dpm, just don't concrete over the pipe.

 

[1justin]

LOL. - But on your second point, I get that air won't bridge a DPC, only something needs to fill it in.

 

[Doggit]

You're going about this the wrong way, albeit maybe the cheapest. There are trays that just have an outlet pipe at the side of the tray, and that connects to a pump that activates as water flows though the outlet. I'll try and find a few more details for you.

If you continue the current route, then you'd just apply a liquid dpm to cover the trench

 

[1justin]

I'm aware of pumps, but trying to avoid them. Also no space at all.
A liquid DPM wouldn't physically connect to the polythene though would it, or is overlap and pressure regarded as adequate?

 

[Doggit]

The pumps are normally fitted in another room, so space shouldn't be an issue, but it's your choice. You'd paint the liquid dmp under the membrane, fold it down into the wet bitumen, and then paint over the join before it dries; just make sure the membrane isn't too dusty.

 

[1justin]

No- space is really really tight ! Think I'll go with a liquid then. Thanks.

 

[lostinthelight]

You're going about this the wrong way, albeit maybe the cheapest. There are trays that just have an outlet pipe at the side of the tray, and that connects to a pump that activates as water flows though the outlet. I'll try and find a few more details for you.

If you continue the current route, then you'd just apply a liquid dpm to cover the trench

This could be a solution to a situation I can see coming in the near future.
litl

 

[Doggit]

Yup, it's going that way for most of us. Some are just luckier in having a wooden floor rather than a concrete one.

 

[noseall]

Some are just luckier in having a wooden floor rather than a concrete one.

I would never refer to having a wooden GF as lucky.

 

[1justin]

So my first excavation at the trap. Solid concrete, hard going (no screed). ~12 cM, then sub-base hardcore below. No DPC at all. The (all falling off) internal Marley tile appears to be the only form of DPC in this case.

 

[Nige F]

I would never refer to having a wooden GF as lucky.

Indeed - concrete doesn't get woodworm

 

[oldbutnotdead]

Hmmm. Entirely your call but if space is tight presumably you're dealing with a small room? Might be worth thinking about hooking the whole floor out while you're making a mess and redoing it with insulation, DPM etc etc? And cast a trench in the concrete for trap and waste pipe? If you're lucky (as I have mostly been in the House of Pain) the clowns who put the concrete in were too tight on cement in the mix so it all fell apart with lovepats from the sledgehammer and wrecking bar. If you're not lucky then a 9" angle grinder with a diamond disc works very well