What trowel size do I need to get a solid 5mm bed

18 May 2012
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United Kingdom
The aquapanel / plasterboard walls of my new shower room are close to vertical. So, to make the bedding easier, my idea has been to place small 10 x 10mm pieces of 5mm thick plastic cut from glazing packers close to each corner, which would ensure that the tiles would be vertical to the underlying wall.

The reason for 5mm is that my 75mm border marble inserts are 10mm in depth whereas the bulk of the tiles are 8.5mm. Given that the adhesive manufacturer specifies a bed minimum of 3mm, 5mm seemed the right depth to ensure the smooth finish needed to accommodate the angle trims around the window area.

I've experimented with a square notched 8mm trowel and with a half round one having notches 10mm deep by 18mm across at 10mm spaces, but, when applying the tile with the 5mm spacers in place, I'm not getting the expected 5mm solid bed needed for the wet area. The only way so far is by also buttering the back of the 250 x 500mm tiles.

Is there another trowel size that will give me a 5mm solid bed?

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