Pull the steel measuring tape so it extends a few inches from its housing. Place the end of the tape so it touches the bottom of the sagging portion of your mattress alongside the string. Hold the housing upright at a right angle to the mattress. Read the measurement on your tape where it intersects the string. This tells you how much your mattress is sagging.
I read on another site that the problem with king mattresses sagging is the split box – which causes the middle to develop a hump. I am wondering about getting a metal platform instead in order to prevent this. Do you think this is a good idea? We are switching from a queen to a king for the first time and looking at a hybrid coil/foam mattress.
Inspect both the slats and box springs of in your bed regularly. The aim is to ensure that the slats and springs are in good position. Ensure the springs offer relative support and are still springy. Inspect for signs of wrapping on the slats. Keeping the spring box and slats strong is the key to preventing mattress sagging.