The Hidden Reason Your Bathtub Might Be Cracking – A Plumber’s Confession

Ordinarily, I wouldn't be blogging about installing a mere bathtub. But there's something I noticed a couple of years ago - even currently, while replacing damaged bathtubs that I simply have to bring to the attention of any homeowner contemplating a bathtub installation.

My observation has led me to conclude that many a plumber is no longer adhering to the nitty‑gritty of the trade when it comes to how bathtubs ought to be installed. Let me briefly explain what I mean.

In a short space of time, I was asked to replace two bathtubs – each revealing the same troubling flaw.

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Case 1: Kempton Park – Moreblessing

The first request involved a bathtub that had developed a crack at the bottom, resulting in water leaking into the apartment below. Incidentally, the unit above had had a similar issue some time ago, and that bathtub had also needed replacing. As we commenced the installation for my client, Moreblessing, we did not realise that a common thread would emerge.

One fine weekend, we set about installing the new bathtub. We proceeded to knock down the bathtub wall, ripped the damaged tub out, and only then – when the old tub was completely removed – did we discover the cause. Apparently, when this apartment complex was erected, the plumbers had set the bathtubs on a concrete base. The tub was sitting on a big, unyielding block of concrete.

Slab of concrete clearly visible
where the tub was sitting,
Carlswald, Midrand
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Case 2: Carlswald, Midrand – Akhona

Shortly after, I received a call from Carlswald, Midrand, from a gentleman named Akhona. His bathtub also needed replacing. I met him, discussed the matter, and we settled on a suitable replacement – a standard 1,700mm x 700mm acrylic tub with an integrated overflow and waste.

Just a few days after Moreblessing's installation, another bathtub was being ripped out. Again, once the demolition was complete, lo and behold, we were confronted with a now‑familiar sight: a bathtub sitting on a specially poured, unyielding concrete bed.

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Case 3: Fairleads, Benoni – William

Then came the third case - a few days ago, Fairleads, Benoni. My client, William, had a bathtub that he intended we replace with a Quadrant Shower Cubicle. We arrived, knocked down the wall, ripped out the old tub, and there it was again – a concrete base and sure as expected, cracks had already started to appear at the bottom of the tub. Same problem. Same predictable outcome. A cracked tub waiting to happen.

Three different clients. Three different suburbs. One identical mistake.

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Why Concrete Is the Wrong Choice

Now, consider this: a modern bathtub – typically made of fibreglass (GRP – glass‑reinforced plastic) or acrylic – has to support upwards of 60 to 80 litres of water plus your body weight when you bathe. That is easily over 150kg of dynamic load. When that tub is placed directly onto a rigid concrete slab, any minor settlement of the building, any thermal expansion, or even the natural flexing of the tub when you step in, creates localised pressure points. The concrete does not give. The fibreglass eventually does – by cracking.

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The Right Way: A Sand Base

The thing is, bathtubs should not be placed on a concrete base. Rather, a base of sand – either riversand or plaster sand – should be employed.

Here is the correct procedure, as taught by good plumbing practice and modern building standards:

1. Prepare the subfloor – Ensure the area is clean, dry, and level.

2. Create a sand bed – A layer of damp plaster sand or riversand, about 50mm to 100mm deep, spread evenly across the footprint where the tub will sit.

3. Shape the sand – Form the sand to gently slope toward the waste outlet, allowing any spilled water or condensation to drain away.

4. Lower the tub – Carefully place the bathtub onto the sand bed. The sand will mould itself perfectly to the contours of the tub's underside, providing continuous, even support.

5. Check for level – Adjust by pressing the tub into the sand or adding a little more sand where needed.

6. Secure the tub – Fasten the tub to the wall battens or brackets as per the manufacturer's instructions, leaving the sand bed to do the supporting work.

7. Test – Fill the tub with water and check for any movement or settling. The sand will compress slightly and then hold firm.

The sand acts as a flexible yet supportive medium. It distributes the load evenly, absorbs minor movements, and prevents the stress points that crack fibreglass. It is also breathable and helps prevent moisture entrapment, reducing the risk of mould or rot.

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Why Do Plumbers Still Use Concrete?

The answer is simple: laziness or lack of updated training. Pouring a concrete bed is faster in the moment – you mix it, dump it, level it roughly, and press the tub in before it sets. But that speed comes at a cost. Concrete is rigid, heavy, and unforgiving. It does not allow for future adjustments, and it sets up a bathtub for failure.

Modern bathtub manufacturers almost universally recommend a sand bed or mortar bed (a slightly damp, soft mortar that remains pliable). The South African National Standard (SANS 10252) for plumbing installations also implies that bathtubs should be supported evenly across their base, which concrete does not truly achieve.

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A Final Word to Homeowners

If you are planning to replace a bathtub – or building a new bathroom – ask your plumber one question: "Will you be installing the tub on a sand bed or a concrete base?"

If they answer "concrete", politely find another plumber. Your bathtub will thank you. And so will the downstairs neighbour.

For Moreblessing, Akhona, we installed their new bathtubs correctly – on a properly prepared sand base and, William's Quadrant Shower Cubicle is in the process of installation. But for the first two clients, there will be: No cracks. No leaks. No callbacks. 

A little sand costs nothing. A cracked bathtub costs everything. 

(Photo Album).

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For further details on my plumbing and bathroom installation services, please reach out.

📞 Call, SMS, or WhatsApp: 063 888 5573

📧 Email: info.aptdesigns@gmail.com

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