Very good question. The reason we do all of our toppers in 5cm depth is that we believe that is deep enough to give a true feeling of latex or memory memory foam but is not too thick so as to lose support (and also keep the product affordable). We have looked at thicker overlays (up to 12cm) but we honestly do not believe the benefit is there. So long as the mattress underneath is flat and supportive (has not lost its shape) then the overlays at 5cm thickness should do their job beautifully. Hopefully this information is of help and please do not hesitate to call us on (03) 8585 6685 to discuss further.