Unique king size beds can be wobbly for many years, so they do not support mattresses and spring box as well. Antique beds use wooden ribs that rest on a ledge on each side of the side rails. The wooden ribs deteriorate over time and may shatter or break, causing the mattress or box springs to sag in that area. You can make new bedside slats with common tools around the house to make your bed frame stronger.
Place the tip on a measuring tape inside the unique king size beds frame near the end face against a side rail. Stretch the tape measure to the opposite side rail and write down the measurement. Measure the width near the foot of the bed in the same way. If the measurement is the same as the first one, the frame is square and all require the same width of boards. If the measurement varies by 1/2-inch or more, you will need additional measurements.
Take additional measurements of the width of a unique king size beds frame in four equal distances down the length of the bed. Measure a queen bed frame in five places and a king bed frame in six equal distances. Number your measurements on a piece of paper with a slat near a headboard, then slap two, three and so on with a measurement for each one. Subtract 1/2 inch from each measurement. Cut a 1 inch through 4 inch board with one saw for each bed slat in this size and number each on the table with a marker.