A sagging mattress is the #1 reason people replace beds before they need to. The good news: in roughly half of cases we see, the mattress is fine — the foundation is the problem.
CAUSE #1: INADEQUATE FOUNDATION. A queen mattress needs a center support leg touching the floor. King and Cal King need three legs. Slatted bed frames must have slats no more than 3 inches apart. Most warranty claims are denied because the customer used a bed frame that voided the warranty. Check yours first.
CAUSE #2: BODY IMPRESSIONS. All foam compresses over time. Manufacturer warranties cover impressions deeper than 1.5 inches (Tempur-Pedic), 1.5" (Saatva), 1" (Helix), 0.75" (Purple). Measure with a yardstick laid across the mattress with no bedding — the gap underneath in the sleep zone is your impression depth.
CAUSE #3: ROTATION NEGLECT. Rotate any non-flippable mattress 180° every 3 months for the first year, then every 6 months. This evenly distributes wear. Flippable mattresses (Saatva HD, Brentwood Cedar) should be flipped twice a year.
CAUSE #4: COIL FATIGUE. Older innerspring mattresses (8+ years) lose coil tension. You'll hear new squeaks and feel "soft spots" where coils have weakened. This is not repairable.
CAUSE #5: WATER OR HUMIDITY DAMAGE. A mattress in a humid basement or one that suffered a spill can develop sag from foam degradation. Look for yellowing or mildew odor as confirmation.
TEMPORARY FIXES. (1) Plywood under the mattress — distributes weight and stops slat sag, adds 1–2 years of life. (2) Mattress topper (3" of latex or memory foam) — restores comfort but doesn't fix structural issues. (3) Foam pads in the sag zone — short-term only.
WHEN TO REPLACE. If sag exceeds your warranty threshold, file a claim — reputable brands honor them. If under threshold but you wake with back pain that resolves at hotels, your mattress is functionally done at year 6–8 even if visually fine.
PREVENTING THE NEXT ONE. Buy from brands with 10+ year warranties (Saatva, Avocado, Tempur-Pedic). Avoid mattresses without explicit foam density specs (lower than 4 lb/ft³ memory foam fails fast). Use a proper foundation. Rotate on schedule.