tier 4 datacenter requirement "translation"

Hello, It may be somehow offtopic, but maybe someone can help understand the meaning of a requirement for Tier 4 DC. We have a project to build a Tier 4 datacenter. One of the requirements specified in TIA942 and related to roof of the building sounds like this: "double redundant with concrete deck (no mechanically attached systems)" For my understanding (being not a native English speaker) what exactly does this mean ? 1. no mechanically attached to the roof systems (gear, equipment) like air-conditioning, generators etc. or 2. the roof (double) is made from components that are not mechanically attached with bolts and such. Thanks in advance. -- Valeriu Vraciu RoEduNet