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Having had helpful advice on tile cutters I'm back with a query on how thick I should make the adhesive.
I'm using 300 by 600 porcelan tiles on a base of composition. This is a dark red material trowelled (I would guess) about 20mm thick on top of concrete. The floor is the basement of a 1900 stone built Edinburgh house, I think the composition dates from when the house was converted into flats in the 50s. Tile adhesive sticks well as I discovered when removing quarry tiles with a hammer and bolster. This process has left the composition with a few furrows and scars and I discover that it isnt as flat as it might be. This suggests to me that I should use a trowel with 10mm rather than 6 mm notches, and first fill the obvious low points with adhesive. Ideally I should level the floor with a self levelling compound but I am resistant because the existing shower and a fitment sit directly on the composition floor and I would have arrange some sort of dam to stop the compound flowing under them. Any suggestions please?

id use a half inch trowel on that size anyway, but thats just me!

First and foremost notch sizes are corresponding to tile sizes and so a 6mm notch would be no good for 300x600 tiles anyway. They also correspond to adhesive....i.e you cant use a 10mm notch with readymixed tubbed adhesive.

A 6mm trowel is ok for mosaics. It is also ok for small tiles with a perfect wall.

You need at least 10mm notch for those tiles and as marc1106 advised, I would also use a 13mm notch trowel with round notches.

The 13mm will give you more leeway for the floor base which as you say is a bit ooerrrr matron. Fill any furrows with rapidset let it go off and then away you go.

Thanks for the advice guys. That is a very helpful setting out of the rationale for notch sizes, RO. I have been looking at a few tiling websites as well as this forum and I have never seen much help on notch size - but it now looks clear: bigger the tile, bigger the notch!

yes, basically bigger the tile the bigger the notch but also how good the background to be tiled is, makes a difference also. The size of the notch also dictates whether you can use tubbed adhesive or you have to use powdered.

In my opinion once you use a notch bigger than 8mm on walls that would be the max size for tubbed adhesive.