Type F Roof Deck

What is Type F Roof Deck?

Type F intermediate rib roof deck is rarely used for new construction projects, but is often needed for repair and re-roof projects. Although Type F improved upon the strength-to-weight ratio of Type A Narrow Rib roof deck, it is still less efficient than Type B Wide Rib roof deck. As with all steel products, the lighter the section, the more economical it will be. And since F deck cannot carry as much load at longer spans as B deck, the steel supports below the metal deck must be spaced closer together. Therefore, unlike Type B, Type F’s smaller support spacing results in an increase in the number of supports needed, and in higher costs for the building owner.

To help illustrate the large disparity in capacities, consider the Factory Mutual maximum deck spans. For 22 GA 1.5 F Deck, the maximum span is only 4’-11”, but the maximum span increases to 6’-0” for 22GA 1.5 B Deck. Type A roof deck can only span 4’-10”. The Maximum Construction spans published by the Steel Deck Institute for two or more spans show a similar pattern. These maximums are 5’-3”, and 6’-11” for Type F, and Type B, respectively, but only 3’-9” for Type A deck.

When Type A and Type F decks were developed in the 1950s and 1960s, the strength of the metal deck was only one factor that the engineering community was concerned with, and it was secondary. The primary objective was to maximize the width of the top flange to provide the maximum amount of support for the roofing insulation that would be installed on top of it. Over time roofing materials improved and steel deck manufacturers were able to reduce the width of the top flange, increasing the efficiency of the section from Type A to Type F deck, and finally from Type F to Type B deck. In general terms, Type A deck was used in buildings that were constructed from the late 1940s to the late 1950s. Subsequently, Type F deck was used from the late 1950s through the late 1960s. And now, Type B deck is the preferred decking choice, dominating the market for roof decks in new construction since it was developed in the early 1970s.

The reason that Type F roof deck has a lower load capacity than Type B may not be immediately apparent, but consider the fact that at mid-span, the top flange of the metal deck is in compression. When thin metals like 22 gauge or 20 gauge (.0295” and .0358” thick, respectively) are compressed, it is much more difficult to predict how and when they will buckle when compared to thicker sections of 0.5 inches or more. However, it is easily demonstrated that bending thin metals strengthens it significantly. While the moment of inertia of the bends can be reliably calculated, the moment of inertia of the thin, flat compression flanges is more complex. The material closest to the bends will resist buckling better than the flat material further away from the bends. Since Type F roof deck has a larger top flange (4.25” for F deck versus 3.5” for B deck), a large part of the top flange is not considered effective in resisting the compressive forces that result from bending.

Like Type A deck, replacing Type F roof deck in a repair or re-roof application may present some challenges compared to replacing Type B metal roof deck. Since the design methodology used by the SDI has changed over time and become more conservative in the case of construction clear spans, the supports may be spaced further apart than Type F deck can span at the same gauge – according to current span tables. For example, a roof that was constructed in the 1960’s may have supports at 6’-0” on center with 22 gauge Type F roof deck. If new Type F deck is to be installed on this roof, it will need to be 20 gauge, since the maximum span for 22 gauge is only 5’-3” (or 4’-11” to meet FM requirements).

When using F deck to cover skylights, note that 20 gauge deck or heavier will probably be required since 22 gauge will only span 4’-3” (see maximum un-shored spans for one (1) span). As shown in the Maximum Construction Span table on this page, the maximum span for one span is 8 inches shorter than the maximum allowed when two or more spans are covered by each deck sheet.