How to deal with old adhesive when re-tiling?

[yz606]

Hi everyone,

Could you help me with some advice? I’m renovating my home and have removed the old tiles from my living and hallway areas (around 80 square meters). There’s old adhesive left on the floor, and my builder suggested screeding over it, which would raise the floor level by 15mm. Since we have low ceilings, I’d prefer to remove the adhesive rather than add extra height. The builder then mentioned that we could tile directly onto the concrete slab, after removing old adhesive, without any additional surface preparation, even if there are some dents and potholes, as the adhesive would help with leveling.

Here are my questions:

1. I’ve read that tiling directly on a slab can lead to issues like efflorescence, cracking, and moisture damage. Are these concerns valid, or am I overthinking this?
2. I was considering using a thin 3-5mm layer of Dunlop Ardit self-leveling compound and primer after removing the adhesive, as our slab is relatively even. However, my builder thinks a 10mm layer would be better, as they believe a thinner layer could crack under the tiles. Is a 3-5mm self-leveling layer really too weak, or could it hold up fine for years ?

3. If a thinner layer isn’t advisable, should I follow the builder’s suggestion to screed over the adhesive and live with the 15mm height increase, even if it creates a noticeable gap between the tiles and timber floors?

Thanks a lot for any suggestions and advice!

Would love to hear your thoughts and any advice you might have. Thank you!

[MitchellMc]

Welcome to the Bunnings Workshop community @yz606. It's sensational to have you join us, and many thanks for your question about old adhesive.

When it comes to tiling directly onto a concrete slab, your concerns about efflorescence, cracking, and moisture damage are valid, especially if the slab has moisture issues or isn't properly sealed. However, if the slab is in good condition—dry and structurally sound—you can tile directly onto it, but it’s important that the surface is clean and free of contaminants like old adhesive. If the slab has any moisture, it can lead to efflorescence (a white powdery deposit) or weaken the adhesive bond over time. As long as the slab is well-prepared and you use a high-quality, moisture-resistant adhesive, tiling directly onto the slab can work, though it’s important to be cautious about potential moisture problems.

Regarding the self-levelling compound, a 3-5mm layer is generally sufficient if the slab is relatively even, but your builder's suggestion of 10mm might be based on the idea that a thinner layer could crack over time. Typically, a 3-5mm layer is enough for smoothing out minor imperfections and can hold up well for years if the slab is stable. If you're concerned about the durability of a thinner layer, it might be worth considering the builder's advice to go for a thicker layer, but it’s important to assess if the extra height will cause any issues with floor transitions, such as the gap between the tiles and timber floors. If the 15mm screed option is problematic in terms of height, you may need to decide if the additional floor height is manageable or if you prefer to stick with a thinner, more level approach.

Ultimately, it’s about balancing the condition of your slab, the need for durability, and the potential floor height issues. Make sure to use a quality self-levelling compound and adhesive, and consult your builder to ensure you're choosing the best option for your space.

Please let me know if you have any questions.

Mitchell