Mattress fit
The guardrail only protects the sleeper if it stands taller than the mattress top. That single sentence decides which mattress belongs on an elevated deck.
Paediatric guidance we cite on the safety page asks for the rail to stand at least 5 inches above the mattress top, counting any pad you add later. Work backwards from that: measure your frame’s rail height above the deck, subtract five, and you have the maximum mattress-plus-topper height for the upper bunk or loft. For most frames in this range the practical answer is a 6-to-8-inch mattress up top.
Lower decks and platforms
Down low the constraint disappears and comfort takes over. Platform frames in this catalogue carry slatted steel decks, so any mattress type works and no box spring is involved. Corner frames with bookcase surrounds sit low by design; a thicker mattress there costs you nothing but the look.
Snug matters as much as thin
A mattress that slides in the frame opens a gap at the rail, and gaps are exactly what the federal bunk standard exists to close. Standard US sizes fit these frames as specified in our size guide; if your mattress is an odd import size, check its true width against the frame before it goes up top.