Does grout need to be waterproof in a bathroom?

Just wondering, as I can't seem to find anything definitive. even after hours of searching.

Our bathroom was recently redone and we have some concerns. It has all the waterproofing sealant behind the tiles on the floors and walls up to 1800mm and all that. Then we have the tiles, but the installer used cement based grout, which I've learned from AS/NZS 4858 does not classify as waterproof, even if grout sealers are used. Epoxy grout would be waterproof, but that wasn't used on the shower floor or wall tiles.

So basically, is this okay? The waterproofing sealant behind will stop water getting through the rest of the house, but to my mind, this will result in water getting under the tiles, which just seems... not great.

The installer is coming back to correct other issues, but seems to brush away our concerns about the type of grout used. If he's right that's okay, but if not I'd like to be able to present something concrete so we don't have water issues at a later date.

Mike

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Accepted answers

EricL

Hello @MikeB

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

You are correct, cement-based grout is not waterproof, but in a properly waterproofed bathroom, this is generally not a structural issue. Here's why:

Primary Waterproofing Layer: The most important part of a bathroom’s water protection is the waterproofing membrane applied behind the tiles. This membrane is designed to prevent water from reaching the structural elements of the house. As long as it was installed correctly and extends into the appropriate areas (such as junctions, corners, and shower areas), it should effectively prevent water damage.
Water Absorption in Grout: While cement-based grout does absorb some moisture, in a well-constructed bathroom, this water should eventually evaporate rather than accumulate under the tiles. Grout sealers can reduce moisture penetration but do not make the grout fully waterproof.
Epoxy Grout vs. Cement Grout: Epoxy grout is indeed waterproof and more resistant to staining and bacteria growth, which is why it’s preferred in high-moisture areas like commercial kitchens or hospitals. However, in standard residential showers, cement-based grout is still widely used as long as proper waterproofing is in place underneath.
Possible Issues to Watch For: While water reaching under tiles is not ideal, in a well-installed bathroom, this moisture should not cause damage as long as:

The waterproofing membrane is intact and correctly applied.
There is proper drainage in the shower (including a well-functioning fall towards the drain).
Adequate ventilation helps moisture evaporate over time.

What You Can Ask the Installer:

If you’re still concerned, you can request the installer to:

Confirm the type and extent of waterproofing used (it should meet AS 3740:2021 standards).
Ensure all junctions, corners, and penetrations were properly sealed (these are weak points if not done correctly).
Check the drainage slope to ensure water is efficiently directed towards the drain.

If there were installation shortcuts (e.g., no proper membrane or incorrect application), then water could build up under tiles and cause problems over time—including loose tiles, mould, or efflorescence. If your waterproofing was done correctly, though, the cement grout should not pose a major risk. But for added protection, I still recommend sealing the grout for that extra layer of defence against moisture.

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

If you need further assistance, please let us know.

Eric

All comments

EricL

Hello @MikeB

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

You are correct, cement-based grout is not waterproof, but in a properly waterproofed bathroom, this is generally not a structural issue. Here's why:

Primary Waterproofing Layer: The most important part of a bathroom’s water protection is the waterproofing membrane applied behind the tiles. This membrane is designed to prevent water from reaching the structural elements of the house. As long as it was installed correctly and extends into the appropriate areas (such as junctions, corners, and shower areas), it should effectively prevent water damage.
Water Absorption in Grout: While cement-based grout does absorb some moisture, in a well-constructed bathroom, this water should eventually evaporate rather than accumulate under the tiles. Grout sealers can reduce moisture penetration but do not make the grout fully waterproof.
Epoxy Grout vs. Cement Grout: Epoxy grout is indeed waterproof and more resistant to staining and bacteria growth, which is why it’s preferred in high-moisture areas like commercial kitchens or hospitals. However, in standard residential showers, cement-based grout is still widely used as long as proper waterproofing is in place underneath.
Possible Issues to Watch For: While water reaching under tiles is not ideal, in a well-installed bathroom, this moisture should not cause damage as long as:

The waterproofing membrane is intact and correctly applied.
There is proper drainage in the shower (including a well-functioning fall towards the drain).
Adequate ventilation helps moisture evaporate over time.

What You Can Ask the Installer:

If you’re still concerned, you can request the installer to:

Confirm the type and extent of waterproofing used (it should meet AS 3740:2021 standards).
Ensure all junctions, corners, and penetrations were properly sealed (these are weak points if not done correctly).
Check the drainage slope to ensure water is efficiently directed towards the drain.

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

If you need further assistance, please let us know.

Eric

Nailbag

Hi @MikeB

I can only speak in Victoria, but as long as the base waterproofing has been applied to standards, then there its no need or requirement for epoxy grout. Essentially, once the bathroom floors and walls are completed, the room is waterproofed. Then normal tiling is laid with conventional adhesives, grouts and mould resistant silicones.

Epoxy grout, though would add an additional level of waterproofing, it's typically only used in situations where the waterproofing for whatever reason has failed. Then any silicone is removed, grout to a level of height required is also removed, then epoxy grout is used. After that a clear waterproof coating over all applicable areas is applied, then the silicone.

Nailbag

Dave-1

Evening @MikeB

and Woooo @EricL , I had maybe half that knowledge 🙂 Just learned several new things. Id be stepping through @EricL's list as they certainly ease my concern and my missing gaps in knowledge 🙂

Is their a warranty period on the bathroom/sealer? Id keep an eye on the bathroom about 6 months before the warranty runs out, mostly just peace of mind for anything with a warranty more then things fail at warranty ends.

Dave

MikeB

Hi Eric,

Thank you so much for this well thought out and very prompt reply. You're definitely giving Bunnings a good name.

We will confirm all the points with the builder.

Again, many thanks, to Nailbag and Dave-1 as well.

Mike

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