Unfortunately if the mattress has sagged i don't think the overlay will be able to save your mattress. You will need to get something that is new and flat. You do not have to spend a lot but get a medium or firm bed (do not get the pillow top). You can then use any overly over the top to soften to your requirements. Please call if we can be of further assistance. 03 8585 6685.Warm regards, Brett Parnham.