Skipping demolition sounds like the kind of renovation shortcut everyone should have mentioned earlier. Sometimes it is sensible. Sometimes it simply gives the old problem a second outfit.
The old floor becomes part of the new installation
MAPEI's overlay system guidance requires clean, well-bonded existing tile. That example explains the principle, not a universal Canadian product specification. Have the installer identify a system suited to your substrate and use.
Follow the added thickness around the room
Check the doorway, toilet connection, vanity, and transitions before approving the buildup. In a shower, the existing waterproofing and drainage need their own assessment. A bonding primer cannot establish that concealed layers are sound. Price removal as a comparison when the existing assembly is uncertain.
Before the next step
- Identify why the existing tile or finish is being replaced.
- Check the complete assembly and proposed product compatibility.
- Draw the finished height at fixtures and the doorway.
The useful question is whether the old installation deserves to stay underneath the new one. Saving demolition is a benefit only when it leaves a dependable result.