BUT!
At the junction of the two substrates vertical movement can likely occur. Even the smallest amount can cause tiles, (especially those of natural material), to crack at this point. CI+ matting cannot absorb this movement. Even over boarding both surfaces would not be guaranteed to remove this risk.
The only safe answer to this situation is to fix a movement joint exactly at the junction point of the two substrates and carry it through the tiled surface as recommended by BS 5385. Some clients find the idea of a movement joint at this point less than ideal. We can offer help to reduce impact by providing a colour co ordinated movement joint to match either the tiled surface or grout lines. (Minimum order 1 x 2.5mtr length). If this is unacceptable, the client must be advised, (preferably in writing), that no responsibility will be accepted if subsequent cracking occurs.
LARGER AREAS
The above guidelines would also apply if the tiled area covers any other substrate movement joints. At these points CI matting should be split directly at the joint and a surface movement joint positioned directly in line above the subfloor / CI matting split. If no subfloor joints exist, then intermediate joints should be fitted at no more than 10mtr x 10mtr field size.
SUMMARY
CI Matting absorbs horizontal movement very effectively. It cannot absorb the vertical movement that occurs where separate sections of subfloor abut.