Maintenance and care Wear problems occur with most mattresses and can include sagging, mildew, and staining. These are prevented by proper support, rotation and flipping, keeping it dry, and using a mattress pad or protector. Some symptoms of a broken or worn-out mattress include springs which can be felt poking through the upholstery layer, visible permanent sagging or deformity, lumpiness, and excessive squeaking. Mattresses require a solid foundation which does not itself sag – a sagging foundation, such as by weak slats on a wide bed, will in turn cause the mattress to sag. Consistently sleeping in the same place and body position causes excessive wear, and thus rotating or flipping mattresses is used to reduce this: double-sided mattresses can be alternately flipped width-wise (about the long axis) and length-wise (about the shorter axis), or alternately flipped and rotated; while single-sided mattresses are only rotated, which is simpler but less effective. Flipping/rotation schedules vary between materials and manufacturers, but typically recommended is monthly for the first six months and every two or three months thereafter. Foundations should also be rotated, if possible, though less frequently – rotating box springs twice a year is recommended. While sagging is undesirable, some level of indentation (about ) is natural if natural materials are used in a comfort layer. Excessive wear on mattresses can occur when folding and bending takes place, placing heavy objects in one spot, or excess force on the handles, will also cause more rapid deterioration. Care should particularly be taken during transport or storage. Mattresses require ventilation to remain dry and prevent mildew, and thus should not be placed directly on the floor or on a solid surface – slats or a box spring provide space for airflow, while solid wood or plywood (as in cheap bunkie boards) does not. Additional ventilation is recommended for natural materials, in which case leaving the mattress "naked" after stripping sheets (for example while laundering) is recommended. If a mattress is allowed to become damp, for example by wet cleaning, mildew may develop inside the upholstery; cleaning with a vacuum cleaner or mild surface cleanser and a slightly damp cloth avoids this. Mattresses absorb fluids and stains readily, notably from nightly sweating (which results in a yellow stain), seminal (or Cowper) stains which are darker, menstrual fluids which are dark red, and other bodily fluids in addition to accidental spills. These visibly stain the