looked at a job today customer is having piped underfloor heating the substrate is joists with the pipework running inbetween them the problem i have with it is the builder is planning to overboard the floor in caberboard p5 is this suitable to fix a membrane too then tile over it ?

Can you afford another 8mm or so on the height of the floor? If you can, I would stick and screw a 6mm cement board over it first. Here's a thread from a while back with a similar scenario Ditra or ply?

yes bri i can afford 8mm for hardie i take it its not suitable to go straight onto the p5 this area is 46m sq i wil need to discuss the costs with the customer thanks for your reply

if your interested in using 6mm cement tile boards,i have got 50 of them for sale for £380 or will split at £8 each,hope this helps.