We have discussed what constitutes solid shelves within a rack structure but it can get a bit confusing when we consider shelf storage itself. NFPA 13 makes a distinction between shelf storage and rack storage. Shelf storage is defined in the standard in Section 3.9.2.6 as: Storage on structures up to and including 30 in (750 mm) deep and separated by aisles at least 30 in (750 mm) wide. The standard also acknowledges that it is possible to push shelves together and create Ba