Philip Ashdown
Hi Andy Usually on the back of floor tiles you would have a raised ridge normally in the form of small squares, you need to apply a what is call a scratch coat of adhesisive using the flat edge of your trowel ensuring that you push your adhesive into & fill all the small squares before fixing onto you're notched application of adhesive applied to the floor. Also, when tiling onto undertile heating, at least 14 days should be allowed before turning on your undertile heating, so to allow the adhesive to completely cure Phil PA Tiling
