In the complex world of building construction, it is the roof deck that provides the foundation of the roof system.
Recent statistics tell us that for commercial low slope roofs, roughly two-thirds of the market is reroofing of existing structures. Managers and owners of buildings need good information on the existing roof deck in order to maintain, re-cover, or replace their roof systems. Fortunately, there are excellent resources available, many as free downloads.
In many cases, only limited information is available on what’s currently in place. We know that building codes generally limit the structure to two roof systems, so if there are already two roof systems in place, at least one of them has to go. Codes also won’t allow us to re-cover over wet materials.
These issues strongly suggest that a roof consultant is needed to conduct a roof moisture survey by taking roof cuts down to the deck to determine what the deck and thermal insulation consist of, how many roof systems are in place, and the condition of the thermal insulation and roof deck. The consultant, in turn, needs a roofing contractor to patch the cuts, rendering them watertight. Wet materials may be discovered by infrared, nuclear, or capacitance surveys.
On a national basis, steel roof decks will be most common, so let’s begin with them.
Understand Steel Roof Decks
What are the gauge, yield strength, depth and span of the steel roof deck? This information should be part of a roof file that was started during the building’s original construction.
Need to bring yourself up to speed? Current ASTM standards for steel decks include:
- ASTM A1008 for painted steel grades C, D, and E
- ASTM A 653 Structural Grade SQ 33 or higher for galvanized (hot dip) steel deck
- Grades C and SQ33 refer to steel having a minimum yield strength of greater than 33 ksi. Yield strength refers to the maximum yield strength (load) that can be applied.
- Steel deck is designed using a safety factor of 1.65, resulting in a design stress of 20 ksi for allowable load carrying capacity for a 33 ksi yield strength. For steel with a minimum yield strength of 80 ksi, use 36 ksi as the maximum design stress limitation rather than the 1.65 safety factor. (The use of high-strength 80 ksi steel is necessary for some mechanically fastened single-ply roof systems to reduce the number of fasteners required.)
Troubleshooting for Existing Roofs
For existing buildings, higher stresses from installation of heavy HVAC equipment may cause unintended deflections. It may be necessary to bring in a structural engineer to verify that the roof deck and structural members meet current building codes.
The structural engineer is seeking answers to these questions:
- Does the deck show severe corrosion? Steel roof decks that were in contact with wet phenolic insulation will have major corrosion problems. If such corrosion is found at the roof cuts, the deck may be unsafe to even walk on and may require total deck replacement.
- How is the existing roof system attached to the structure? Generally, steel decks were attached by welds or mechanical fasteners to the purlins.
- Is roof slope adequate and does the roof drain freely? This is best determined by visual inspection after a rainstorm or by flood testing.
- Is some of the existing equipment obsolete? If so, can it be removed?
- Are existing expansion joints functional?