Size of Mattress and Foundation The length and width of the upper and lower bunk mattress and foundation are as follows : Size Length Width Thickness Twin 747-75" 37 max You must use only this size mattress. Ensure thickness of upper and lower mattress and foundation combined does not exceed 9 4” and that the mattress is at least 5” below upper edge of guardrails. This Bunk Bed uses a slat kit for mattress foundation. iN SAFETY WARNINGS /N\ (1) Follow the information on the warnings appearing on the upper b