18 or 25mm plywood subfloor - Tile Advice

[mattkelpie]

Some advice would be greatly appreciated! So I'm putting in wet underfloor heating and limestone tiles in kitchen/living. The underfloor heating pipes will be laid inside 20mm routed cement coated xps board which can be tiled directly on top off. My question is, would an 18mm plywood subfloor (directly on joists) be adequate, or should I go for a 25mm subfloor? The joists are at 400mm centres and will be strengthened with noggins. Thanks in advance!

 

[Dave]

25mm and nog all joints that are without support

 

[mattkelpie]

25mm and nog all joints that are without support

Thanks for that Dave. Much appreciated