Hi all Just looked at a job where builder wants to put down 22m chipboard over joists then Ditra. I said nooooo. The heating system is wet on a polystyrene board with aluminium trays screwed to joists. I have mentioned giffa flooring. The joists are 400 centres so could probably use the 18mm. Any advice on this would be helpful as the builder will do whats best for a change.