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?
All About Attachment
Roof decks need to be adequately attached to underlying structural framing members to prevent wind uplift. With some designs, the attachment of the roof deck plays an important role in the roof deck “diaphragm” to resist horizontal loads and provide lateral bracing for the building’s structure.
Steel roof decks are typically attached to the underlying roof structure using welds or mechanical fasteners. SDI and FM Global provide two generally recognized but somewhat differing guidelines applicable to the attachment of steel roof decks. The most stringent fastening recommendations of SDI’s Design Manual for Composite Decks, Form Decks, and Roof Decks
or Diaphragm Design Manual
should be used.
FM Guidelines can be found In FM’s Loss Prevention Data Sheet 1- 29, Roof Deck Securement and Above-Deck Roof Components (dated 9/10), and reference data sheets 1-28R and 1-29R. If your building is insured by FM Global, they should be part of the decision process for reroofing or roof replacement.
Steel roof decks generally are considered to be nailable, or in the case of steel roof decks specifically, mechanically attachable. That is, steel roof decks generally require the installation of rigid board insulation prior to the insulation of the roof membrane.
Built-up, polymer-modified bitumen, and single-ply membranes are always installed on top of a rigid board substrate that’s either mechanically attached to the steel roof deck or loose-laid, depending on the roof membrane system.
NRCA does not recommend the use of low-rise foam or liquid-applied adhesive as the primary means of attachment of rigid board insulation to steel roof decks. If you’re using low-rise foam or liquid-applied adhesive to adhere rigid board insulation to steel roof decks, NRCA recommends that you obtain agreement from the adhesive, rigid board insulation, and membrane manufacturers regarding the appropriate adhesive type and its application rate.
Also keep in mind that the best class required by model building codes for a fire-resistant building is a B, not an A.
Looking to learn more? Chck out these resources:
- ANSI/Steel Deck Institute: RD-2010 Standard for Steel Roof Decks
- Steel Deck Institute Publication No. 31, Design Manual for Composite Decks Form Decks and Roof Decks
- NRCA Roofing Manual, CD-2011
- FM Global datasheets
- Approval Standard for Profiled Steel Panels for Use as Decking in Class 1 Insulated Roof Construction Class Number 4451, June 2012
- SEI/ASCE Minimum Design Loads for Buildings and Other Structures (7-10)
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