Tiling exterior concrete floor - which backer board to use?

[halfabubble]

Installing tiles 400x600 over an exposed exterior cement screed - it has 20mm fall over 2300mm length away from house (5750mm width) but there might be little pits and bumps of a couple mm or so. Tiling directly on the screed seems risky to say the least outside with temperature fluctuations and weather etc.
The tile surface needs to be more or less flush with a deck which only gives me ~11mm to work with for a board+adhesive. Which 6mm backer board should I be using?
Holding strength under soaking conditions is obviously important (and frost resistance etc). Though the tiles and grout will be impervious (porc and epoxy), some water will still get through.
Marmox is least expensive truly waterproof option - but the foam core is less strong (impact, flexural wise)? than cement board such as NMP.
Will coating the NMP with their primer (SBR) which they say make it waterproof be good enough to use on exterior floor?
What about STS's 6mm construction board (with same sealer) to provide more strength since only working with 6mm depth?
Or am I worrying too much about strength and not enough about waterproofing?

 

[Dave]

Installing tiles 400x600 over an exposed exterior cement screed - it has 20mm fall over 2300mm length away from house (5750mm width) but there might be little pits and bumps of a couple mm or so. Tiling directly on the screed seems risky to say the least outside with temperature fluctuations and weather etc.
The tile surface needs to be more or less flush with a deck which only gives me ~11mm to work with for a board+adhesive. Which 6mm backer board should I be using?
Holding strength under soaking conditions is obviously important (and frost resistance etc). Though the tiles and grout will be impervious (porc and epoxy), some water will still get through.
Marmox is least expensive truly waterproof option - but the foam core is less strong (impact, flexural wise)? than cement board such as NMP.
Will coating the NMP with their primer (SBR) which they say make it waterproof be good enough to use on exterior floor?
What about STS's 6mm construction board (with same sealer) to provide more strength since only working with 6mm depth?
Or am I worrying too much about strength and not enough about waterproofing?

Hi , your over thinking it imo , are there any cracks in this slab your wanting to tile.?

 

[halfabubble]

Hi , your over thinking it imo , are there any cracks in this slab your wanting to tile.?

Thanks for replying. My dad said the same. No cracks, though it must be rather weak because the top is dusty/sandy.

 

[Dave]

Thanks for replying. My dad said the same. No cracks, though it must be rather weak because the top is dusty/sandy.

Try a test stick. Prime the screed and once dry stick a tile down. Leave to cure and see how hard it is to get up. If the screed surface is weak the tile will pop up easily. If so then maybe the screed mix is too weak to adhere to.

 

[canadianfloor]

http://https:/canadianfloorcoatings.com/benefits-ofepoxy-floor-coatings-for-vancouver breweries/