A mattress warranty is not a durability promise. It is a defect promise, and it comes with conditions the buyer usually never reads until the mattress has already sagged. Four conditions account for the overwhelming majority of rejected claims, and every one of them is decided before the mattress ever develops a problem.
1. THE BASE UNDERNEATH. This is the big one. Brands exclude any indentation that results from an improper or unsupportive foundation, which means the base can disqualify a genuine sag. The published slat limits differ by brand: Purple requires slats no more than 3.5" apart, Tempur-Pedic requires cross slats at least 3" wide and no more than 4" apart, Saatva requires solid surface or slats under 4" apart, Brooklyn Bedding voids coverage above 5" and Helix above 5.5". Queen and larger almost always need a centre support with a leg to the floor — Saatva voids the warranty on a standard frame that is not 5- or 6-legged with vertical centre support, and Purple requires 5–6 legs plus a centre support.
2. STAINS AND LIQUID DAMAGE. Helix excludes defects caused by burns, cuts, tears, liquid damage or stains. Tempur-Pedic excludes physical abuse, burns, cuts and liquid damage. Saatva's warranty does not apply if the mattress has been stained, burned or otherwise abused. A waterproof protector fitted on night one is the cheapest way to keep a warranty alive.
3. THE LAW TAG. Purple states that the law tag sewn into the head or foot of the bed must be intact for a claim to be processed. It is the tag that says do not remove under penalty of law — that instruction is directed at retailers, not you, but removing it still costs you the claim.
4. WHO OWNS IT. Every warranty in our comparison that publishes a transferability rule says the same thing: original purchaser only, non-transferable. Buy a mattress second-hand, or give one to a family member, and the warranty does not travel with it. Purple goes further and terminates coverage if the original purchaser resells or donates the mattress.
TWO MORE THAT CATCH PEOPLE OUT. Purple voids the warranty if the mattress is used uncovered, because UV light degrades the cushioning. Casper limits coverage to mattresses bought from Casper or an authorised retailer, so a marketplace bargain from an unknown seller can leave you with no manufacturer cover at all.
WHAT IS NEVER COVERED ANYWHERE. Comfort. Not liking the feel is what the sleep trial is for, and once the trial ends that door closes. Normal softening, a change in perceived firmness, and impressions shallower than the published threshold are explicitly excluded rather than accidentally omitted.