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Gap between floor and wall tiles.
There is a gap of say 5mm to 8mm give or take high between where the floor tiles meet the wall tiles in the bathroom.
Gently push the tiles together so the spacers are tight in the joint.
Never fill this gap with grout because tile grout is not flexible.
Gap between floor tiles and wall tiles.
The large format 18 inch tiles should have inch space in between.
Fill gaps larger than 1 8 inch.
Place them between the tiles are you lay each one and grout straight over the top.
The tile council of north america recommends at least a quarter inch gap between the tile floor and the walls.
If your gap is smaller than 1 8 inch in width use regular caulk.
I m guessing its the thickness of the dot and dab then plaster board then tile adhesive then tile.
These can be left in place and will not be seen when you have completed your tiling.
If you have tile on the wall adjoining the tub select a caulk that matches the color of the grout.
For smaller tiles is advisable to use small spacers.
It shows how to seal a gap in the corner where the tile floor.
Instead fill it with an elastic material capable of sustaining the movement of the floor and the movement of the wall.
Gently spread the tiles and place two spacers between them one at the top edge of the seam and one at the bottom edge.
Caulking is the best choice for this application.
It might loosen them and might rot the floor.
The d i y video how to seal a big gap between tile floor and tile wall in a bathroom or shower.