Advice on replacing solvent weld shower waste pipe into concrete floor

[dave100]

Hello there!

I hope your weekend has started well.

TL/DR: Should I replace 90-degree bend with a smaller type to increase the fall from the shower tray?

I am replacing a bathtub with a 1500 X 800 Sonas shower tray which I will place on 4-inch blocks, probably with some 18mm marine ply on top. The shower will be a thermostatic one with two outlets that can be used at the same time, fed with a 2-bar universal ST pump.

The problem is the waste pipe for the old bath is solvent welded so I can't undo the fittings (I am new to this). Also, I am concerned about getting enough of a fall from the shower tray to the waste pipe, although the distance is only about 3 feet.

Could I cut away the existing 90-degree waste pipe bend at floor level (concrete floor so no access below, it's an apartment building) and replace it with a 90-degree bend like the one shown in this image? Would this drastically reduce the flow rate as the lower end of the 90-degree bend must be smaller than the current one?

Here are some more photos that give more info.

Above: The current waste pipe leading to the hole in the concrete floor. The fall is impacted by the type of 90-degree bend used for the old bath.

Above: The existing solvent welded pipe. It connects to the 4-inch waste stack below the concrete level.

Above: My vernier calipers tells me the existing pipe is 43mm, but that's probably a standard 40mm pipe.

I will be fitting a 1500mm X 800mm slate shower tray with a fast-flow waste.

Above: This shows how the pipes connect below the concrete level. There is another waste pipe too (was for the basin waste) but it protrudes onto the floor too much so I plan to blank it off and connect the basin waste into the shower waste, probably.

This seems like a small thing but if I get it wrong my fancy new shower won't work properly. My 2-bar ST Monsoon pump will be pumping water into a regular shower head and rain shower head at the same time so there could be a lot of water flowing. Would reducing the size of the lower end of that 90-degree bend reduce the flow capacity enough to be a problem? How would you tackle this?

As always, your help is greatly appreciated!

Dave

 

[ShaunCorbs]

I would cut the concrete from around the pipe carefully

Around 2” below the the existing fitting

Then cut the pipe just below the existing elbow socket and then install a new elbow onto the pipe etc this will give you more fall

 

[dave100]

Hi Shaun. Thank you for this idea, I hadn't considered that.

It's the top floor of an apartment building, built 25 years ago, I think the concrete floors are about 9 inches thick.

So you suggest leaving the existing elbow in place, cutting out the concrete 2-inches below the floor level, then cutting off the existing elbow right at its lower edge, which would give me a floor-level pipe opening with a newly made hole around it big enough to get a new elbow on at a lower level.

Did I get that right Shaun?

 

[ShaunCorbs]

Yep spot on

 

[dave100]

Hi again,

I started digging into the concrete using a 4-inch grinder. It looks like the bit of pipe below the 90-degree elbow almost immediately connects to another 90-degree elbow, based on what is visible here:

The distance between these two elbows, where you can see the 40mm pipe is only 8mm.

If you look at the 5th image from my original post you see that there is a bend in the pipe buried in the concrete, that bend is very close (about 30mm) from where I am in the concrete now so it's getting risky, I don't want to break that pipe. The top of the 40mm pipe that enters the 4-inch stack is 100mm from the top of the black soil pipe and my concrete digging has taken me down 70mm from the top of the black soil pipe so I have very little left.

I bought the following 40mm fittings but not sure how I can use any of them now.

Here is how those new fittings compare to the old one.

The single-collar one:

So I am unsure how to proceed.

Is it possible to connect two pieces of plastic pipe by melting them with a heatgun? Maybe not, as I am working on the top floor of a 6-storey building so there are 5 other bathrooms below me and therefore there will probably be methane gas in the pipe so maybe applying heat there might not be a good idea.

What is the minimum acceptable overlap between a pipe and an elbow? Could the 8mm be enough if I use solvent weld glue to connect them? To improve the fall could I cut off half of the wider part of the elbow (the collar) and place that over the 8mm available pipe?

Is there any way to undo solvent welded pipes?

Any advice would be appreciated!

Dave

 

[ShaunCorbs]

8mm won’t be enough

Two options raise your shower up to make the existing drainage work or

Cut the existing waste just below the top elbow socket and then get a hacksaw blade internally of the pipe and cut a vertical cut just the pipe as for depth of cut just so it’s enough to cut through the pipe but not into the socket

And then carefully use a heat gun to warm the inside of the pipe up so the glue softens and you can peel / remove just the pipe leaving you a socket

 

[ShaunCorbs]

Or you could try this

 

[Ben-gee]

Another method is to use a very sharp chisel to pare away the pipe that remains in the socket. Once you have removed a section fully, cutting a slot through the pipe. Same as above, warm the remainder and it should peel away.

However in your case. Could you not excavate further, down to where the pipe enters the boss on the 4”, and remove everything and start again? Or this could be the fallback position if you get in trouble with the above.

 

[dave100]

Thank you Shaun, thank you Ben.

Okay, I'm going to try to cut out that pipe from inside the lower 90-degree elbow as you both suggested.

I've got an oscillating multi-tool and I'm about to buy one of those small rotary power tools, I think they are known as Dremel tools, it has a tiny rotating disc which I think will help cut the inside of the pipe. I'll also try heating up a 35mm hole say as seen in Shaun's video to see if that helps.

I presume heat is the best way to loosen things up here?

Is there any danger of me blowing up the apartment building by bringing a heatgun (I have a cheap Black % Decker one from B&Q) into contact with pipes that may contain methane gas from the 5 bathrooms below me? That would be bad.

Thanks,
Dave

 

[ShaunCorbs]

You will be fine as long as your not smoking while your doing it 🙂

 

[dave100]

Shaun, Ben, these suggestions worked great! Problem solved. Thank you both very much indeed!!! 😀

Here I will post some photos of what I did as that may be helpful for others who need to do something similar.

Here are the steps I took.

1> Cut off the old elbow with a Multi-Tool. It helps to leave 5-10mm sticking up as it's easier to grab with pliers later.

2> I bought this hacksaw at Woodies, it allowed me to score the inside of the piece that I intend to remove.

3> Making the cut, I didn't go all the way through the piece that I removed as that would damage the elbow that I need to stay in place and be used again.

4> I used an old hole saw and heated it using a heatgun.

5> Applying the heated hole-saw bit to the inside of the pipe that I need to remove. Hold it with pliers obviously.
It's worth spending some time doing multiple rounds of heating and melting, I did this about 8 times, 30 seconds heating, 30 seconds melting, and moved the hot piece around the pipe as I went.

6> Now use your pliers to bend and twist the inner pipe. If you feel it's going to break it's best to release it and catch it further down.

I almost got it out in one piece and then used a file to knock off a bits that didn't come out with the pliers.

7> Done! I was able to insert a new elbow with a male lower end (spigot) and it works well! 🙂

Thanks again, guys!

I have one follow on question, the particular elbow that I bought has a very long spigot. Can I cut that in order to get a lower elbow and therefore a better fall on my shower pipe?

Cheers,
Dave

 

[ShaunCorbs]

Yes you can cut the spigot end down to tbh tight if you need to