How to fix shower drain cutout in slab flexing?

Question

Hi everyone

Currently in the final steps of a bathroom reno.

The old shower base was a thick solid porcelain unit (1996 house) and when removed it’s covered a 300mm deep cutout in the slab for the drain.

The new base is a Mondella 900x900 Torbex unit. The plumber who installed the base didn’t back fill the hole as he said it wasn’t necessary. There’s now a hollow sound around the drain and it also has a bit of flex.

Is this going to cause issues?

The tiles are not yet grouted and the shower screen is not installed. Should it be ripped up and the hole filled?

Thanks in advance

Nailbag · Answer

Hi @michaeli

I agree with the plumber, I don't think filling the gap is required. I wouldn't be concerned about any hollow sound only the flex in the floor. If it's just around the lip of the opening, then again I think thats fine. Whats important is that the shower base is supported as per any specific guidelines.

Nailbag

Dave-1 · Answer

Good Afternoon @michaeli

Having a talk with the plumber is the first step I would try.

The second step is to find a supplier of the same type of base unit you have and see if it also flex's and sounds hollow. I am thinking of @EricL's words as its a different type of construction "so it could feel different" (my words lol) Being able to see a bathroom setup up with the model of base you have could go a long way to allying your fears. It would also be better to find out now in case the base has to be lifted. I have not had a lot of experience installing showers or baths but have definitely stomped around the showroom ones (without shoes) and know that flex you refer to.

Dave

EricL · Answer

Hello @michaeli

Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about your shower base.

I would recommend contacting your plumber and asking them to return and inspect the installation, particularly as the work should still be covered under warranty. Explain your concerns about the shower tray flexing and the unpleasant noise it makes when you step on the unsupported section. It would be much better to have this assessed and rectified now, before the shower is fully installed, as addressing the issue later could involve considerably more work and expense.

Please also keep in mind that any alterations involving the movement or reconfiguration of the pipework would be considered regulated plumbing work and would need to be carried out by a licensed plumber.

In regard to the shower base itself, there is an important difference between the two types of construction. A solid porcelain shower base is generally very heavy and rigid, allowing it to support a person's weight with very little, if any, noticeable flex. Torbex, on the other hand, is a high-strength, long glass-fibre composite material manufactured using Sheet Moulding Compound (SMC). While designed to be strong and durable, it is not the same construction as a solid porcelain tray. This makes it particularly important that the Torbex base is installed and supported in accordance with the manufacturer's installation requirements.

Let me call on our experienced members @Dave-1, @Nailbag and @DIY4Knuckleheds for their recommendations.

If you need further assistance, please let us know.

Eric