Steel Deck & Roof System Attachment

Perspectives
A Quarterly Information Source from Benchmark, Inc.
Volume 53 June 2005

Steel Deck & Roof System Attachment (In High Wind Uplift Exposures)

by Curtis Liscum, RRC

In recent years, a gradual change has taken place in the way the roofing industry addresses the attachment of roof systems to steel decks in areas of high wind uplift pressures. This article will attempt to explain those changes and how they affect the selection, design and installation of a roof system in an area of high wind uplift pressure.

Definitions

Let’s first define a couple of parameters used in the development of this article.

Attachment of a roof system

Attachment includes the mechanical attachment of an insulation or cover board under a built-up, modified bitumen or adhered single-ply roof system. Attachment would also include the mechanical fastening of a mechanically attached single-ply roof system.

Area of high wind uplift pressure

It is our experience that normal or frequently encountered wind uplift pressures are those with maximum design pressures equal to or less than 90 psf (pounds per square foot). In most of the inland United States, the wind uplift pressures do not exceed 90 psf for buildings less than 60 feet in height.

For the purpose of this article, high wind uplift pressures are those maximum design pressure values equal to or greater than 105 psf. Please note that these pressures are maximum design pressures and include a factor of safety. The roofing industry appears to have accepted a factor of safety (FS) of 2. This is primarily based on FM Global requiring a minimum FS of 2 in the design of insured roofing projects. Although a FS of 2 does not appear to be mandated by Building Codes, having a FS in the range of 2 does seem to be a prudent practice. Ultimately, the determination of a project’s factor of safety rests solely with the roof designer. If we use a FS of 2, the maximum wind load pressure for our roofs in areas of high wind uplift pressures would be 52.5 psf.

Steel Deck Types

Steel roof decking has been used for years and is the primary decking material in use today. Steel roof decking is classified by two ASTM International standards. ASTM A1008/A1008M-04b "Standard Specification Sheet for Steel, Sheet, Cold-Rolled, Carbon, Formability" includes carbon steel or painted steel decks and ASTM A653/A653M-04a "Standard Specification for Steel Sheet, Zinc-Coated (Galvanized) or Zinc-Iron Alloy-Coated (Galvannealed) by the Hot-Dip Process" includes galvanized steel decks. Steel decking is available in several profiles (A, B, F, N ect.) and gauges or thicknesses (16 to 22-gauge) with the most common thickness being 22 or 20-gauge. Decking can also be obtained with several minimum yield strengths (Fy) and measured in ksi or 1000 pounds per square inch. ASTM A1008 (painted steel) and ASTM A653 (galvanized steel) decking is generally available in Grade 33 - Fy = 33 ksi; Grade 40 - Fy = 40 ksi and Grade 80 - Fy = 80 kis. The most common grades of decking in the industry have a minimum yield strength of 33 or 80 ksi.

It is this difference in deck grades or minimum yield strength that is causing a lot of concern and confusion within the industry. The advantage of Fy = 80 ksi decking is that it has an increased yield strength and therefore can carry increased loads over wider support spacing. One might ask that if Fy = 80 ksi decks are good, why not manufacture Fy = 100 ksi decks? It seems that steel becomes very brittle when the minimum yield strength exceeds 100 ksi.

This brittleness decreases wind uplift resistance (increases tendency for tear through), and is prone to cracking and tearing. For the purpose of this article, I will refer to decking grade as either Grade 33 or Grade 80 to designate the minimum yield strengths, regardless if the decking is painted steel or galvanized decking.

Steel Deck Grade Identification

Some practical observations regarding the identification and differences between Grade 33 and Grade 80 steel decks: Twenty years ago, Grade 33 steel decking was the most common decking used and probably accounts for the majority of the steel decking in service. During my research for this article, I obtained conflicting opinions on the primary grade of decking being used today. Some industry experts believe that Grade 80 decks are the most common because Grade 80 steel is easier to manufacture than Grade 33. However, several deck suppliers indicated that they are selling much more Grade 33 decking than Grade 80.

Much to my disappointment, steel deck manufacturers do not grade mark or stamp decking. Anticipating that original deck specifications are missing or of questionable authenticity, the identification of deck grade can be difficult. Except for sending deck samples to a laboratory for expensive tensile testing, about the only way to begin to understand an existing deck grade is by performing fastener pull-out tests. Based on our experiences, and as confirmed by two fastener manufacturers, some measurable differences can be obtained in fastener pull-out values between Grade 33 and Grade 80 decks.

A #12 screw in a Grade 33, 22-gauge steel deck should have an average pull-out resistance in the range of 400 to 500 pounds. A Grade 80 deck should have pull-out resistance 75 to 100 pounds greater than Grade 33 decks of the same gauge.

A #15 screw in a Grade 33, 22-gauge steel deck should have an average pull-out resistance in the range of 500 to 700 pounds. A Grade 80 deck should have 100 to 150 pounds more pull-out resistance.

An Extra Heavy Duty #21 screw (extra heavy duty fasteners are used for 10 foot wide single- ply sheets) in a Grade 33, 22-gauge steel deck should have an average pull-out resistance around 750 pounds. A Grade 80 deck should have 200 to 225 pounds more resistance.

To complicate fastener pull-out testing, one needs to understand the historical selling of steel roof decking. Prior to the 1980s, steel decking was sold by weight. If a deck manufacturer manufactured a standard gauge deck a little thicker (and therefore a little heavier), he made a little more money in the sale. For example, the thickness of a standard 22-gauge deck is .028" to .0295". Older 22-gauge decks have been measured at .031" to .032" (14% increase in thickness). After 1980, steel decking was sold by area and thickness, or gauge. Any additional thickness over the minimum requirement would increase the manufacturing cost without being able to realize any increased sales price.

So what does this all mean in the practical determination of the grade of an existing steel roof deck?

The majority of steel roof decks under roofs being reroofed are most likely Grade 33.

Using #12 screws for fastener pull-out resistance tests may not be able to determine the differ- ence between Grade 33 or Grade 80 decks. In my opinion, the anticipated difference in pull-out resistance (75 to 100 pounds) is within the margin of error for performing pull-out resistance tests in an existing roof deck. A #15 or #21 screw is recommended for determining deck grade differences. In any case, fastener pull-out tests should be performed in accor- dance with a nationally approved standard such as ANSI/SPRI FX-1-2001 "Standard Field Test Procedure for Determining the Withdrawal Resistance of Roofing Fasteners."

Decks installed before 1980 may have increased pull-out resistance not because of steel grade, but because of increased thickness at the time of manufacture.

Steel hardens (the yield strength increases) with age. As long as there is no detrimental corro- sion, Grade 33 or Grade 80 decks should provide increased fastener pull-out resistance with age.

Wind Up-Lift Resistance

Roofing manufacturers obtain maximum design pressure ratings for specific roofing assemblies primarily through testing, although a few ratings are obtained by engineering analysis. The majority of ratings available today can be obtained from three primary sources; FM Global, Underwriters Laboratories (U.L.), and Miami-Dade County, Florida. Miami-Dade County does not always identify steel deck grade for each approval or Notice of Acceptance (NOA). Likewise, U.L. does not identify the specific grade in its product listings. However, FM Global does indicate deck grade in its assembly combination approvals.

FM Global

RoofNav, FM Global’s internet based ratings calculator and approval search database, was used to search for FM approvals. The following is a synopsis of my findings, when searching for approved listings with a wind uplift rating equal to or greater than 105.

Steel deck, min 80 ksi, 22 to 18-gauge, 1.5 inches deep, wide rib (>90 wind uplift rating) - 1,833 listed assemblies, 1,539 with single-ply covers and 849 with multi-ply (built-up or modified bitumen) covers.
Steel deck, 22 to 18-gauge, 1.5 inches deep, wide rib (>90 wind uplift rating) (33 ksi) - 1,441 listed assemblies, 413 with single-ply covers and 849 with multi-ply (built-up or modified bitumen) covers.
Many well known roofing manufacturers have a limited number of approved listings over Grade 33 decks and wind rating >/=105. However, those same manufacturers had numerous listings over Grade 80 decks.
In every listing I checked (and no, I did not look at all 3,274 of them), with either Grade 33 and Grade 80 decking, and wind uplift pressures greater than 90 psf, there is a requirement to verify and install required deck attachment fasteners into the structural steel and deck side lap fasteners.

Miami-Dade County

In Florida, the Miami-Dade County Building Code Compliance Office (BCCO) through the Miami-Dade County Product Control Approval System approves construction product and assemblies to be used in Miami-Dade County. The Product Control Approval System was established to "allow new and innovative ideas to be developed into useful, practical, lasting and safe products".

Building components, including roof systems and those that protect the envelope of the building from being breached, must be approved by the BCCO Product Control Division prior to their use in building construction or rehabilitation in High Velocity Hurricane Zones as defined by the Florida Building Code. Products require approval through the issuance of a Notice of Acceptance (NOA) from the Product Control Division.

Our search of the available roofing NOAs over steel decks and with maximum design pressure >/=105 psf revealed the following observations:

Very few approvals are available for roofing systems installed over steel decks with a maximum design pressure >/= 105 psf.

Most of the approvals do not identify deck grade. However, we did find that some of the approvals do identify the deck as a Grade 80 deck.

Because FM Global is an approved testing facility, to obtain NOAs, there is a direct correlation between those manufacturers who tested over Grade 80 decks at FM Global, and those who listed over Grade 80 decks in the NOA.

Underwriters Laboratories, Inc.

In reviewing the Underwriters Laboratories (U.L.) 2005 Roofing Materials & Systems Directory for roofing assemblies over steel decks with high wind uplift (>/=105 psf) resistance, numerous listings were available. The listings generally indicated steel deck gauge, but none listed steel grade. Most of the listings indicated that the steel deck gauge was 22 MSG (manufacturers’ standard guide). There was not a lot of correlation in wind uplift resistance between those systems tested over Grade 80 steel deck at FM Global or Miami-Dade and those tested over Grade 33 steel deck at U.L. I would surmise that the difference in wind uplift resistance might be as much a factor of test size and methodology than of a steel deck gauge and grade.

Conclusions

When specifying a roof system for a recover project over a steel deck, prudent practice should require that the following be considered:

Understand the wind uplift listing, approval or rating and the substrate that it was tested over.

Because listings, approvals or ratings are based on tested assemblies and not individual components, do not arbitrarily mix and match different components and expect that the system will meet the same wind uplift rating.

Unless you can verify or prove differently, most steel decking in reroofing situations should be assumed to have a minimum yield strength of Fy = 33 ksi.

If the listing, approval or rating is over a Grade 80 deck, verify and understand the requirements for attachment of the deck to the structure and side lap fastening of the deck sheets. In order for this rating to be applicable, this supplemental attachment must be accomplished as part of the reroofing project.

Some manufacturers would have us "engineer" and revise the fastener spacing for mechanically attached single-ply roofing membranes based on fastener pull-out testing. My concern with doing this is that we may succeed in adequately attaching the roof system to the deck, but is the steel decking adequately attached to the structure to resist the required wind uplift resistance?

The issue of deck grade is confusing, for specifiers, owners, contractors and manufacturers. According to building codes, every roof must meet an applicable wind uplift resistance.

Because reroofing over steel decks is and will continue to be a large share of the roofing market, we must all work together to understand just how these issues affect long-term attachment of a roof in areas of high wind uplift pressure.