TechnologyRoofing Screws
Figure 1: Metal Roof Made to Look Like a Tile RoofA

Figure 2: Example of a Standing
Seam RoofFigure 3: Installation of Exposed
Fastener Metal Roof Systemlthough metal has been a material option
for roofs for hundreds of years, because ofchallenges posed by high cost, lousy appearance,and poor longevity, it has only recently becomea feasible mainstream choice. In fact, today in
the United States metal roofing represents oneof the fastest growing areas in the residential
roofing segment. It is now possible to choose
from a selection of metal roofing solutions thateither provide large, continuous panels or metal
alternatives to traditional roofing materials suchas slate, tiles, shakes, and shingles in attractive
designs and at affordable prices. Although someof these solutions are more expensive in the shortterm than their traditional roofing options, theirsuperior durability and life expectancy makes
them a smarter long-term investment. Of course,
like any roofing material they must be securely
fixed to the roofing substructure to protect
the interior spaces below. Since metal roofing
solutions exhibit their own unique qualities andchallenges, many of these solutions require special
or different fasteners from nails and staples usedwith traditional roofing solutions. As metal roofing
has grown then, so has the need for innovation andengineering in the fasteners for this market space.

Metal Roofing
Metal roofs can be classified into three distinct categories: Exposed Fastener Panel Type, Standing Seam Type, and Specialty Panel Type.

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Specialty Roofing Panels: These include both small and large metal panels that have been fabricated to reproduce the look ofceramic tile, slate, shakes, and shingles (See Figure 1). Just like the items they are made to mimic, the panels are fastened at their top
and the attaching fasteners subsequently hidden by the next panel above. These types of systems normally do not require specialty
fasteners and often use the same or similar fasteners to the ones utilized by the traditional products they replace.

•Standing Seam Systems: Standing Seam roofs are connected to the roof substrate by hidden clips. These clips are screwed to thesubstrate, often with special low profile head screws, but without regard to special sealing because the screws are covered with theroof panel and thus not vulnerable to water intrusion and exposure (See Figure 2). Panels are interlocked together at the seams.
Metal roofs undergo considerable expansion and contraction from changing heat and cold conditions. Because a Standing Seam
Roof essentially “floats” over the roof substrate there is no limitation in the length of the roof panel, and they often are seamless anduninterrupted. Standing seam roofs cost more to install but have a cleaner look and, more importantly, last longer.

•Exposed Fastener Systems: Exposed Fastener System roof panels are installed directly into the roof understructure andare the most economical of all the metal roofing types (See Figure 3). The fasteners must be able to pierce the metal roofing
panel and self-tap into the supporting understructure. For this reason, there are different versions for wood and metal connections.
Since the fasteners are exposed on the surface of the roof, they must also have the capability to seal the penetration from water
intrusion. The screws used in this method normally possess an external hex flange or washer head with a bonded EPDM or Neoprene
washer. The heads and washers are usually painted to match the color of the roofing panel. When properly installed the washer iscompressed to form a tight seal around the roof penetration. Since the panels are constrained by the fasteners holding them down
and the metal panels undergo significant expansion and contraction with temperature changes, roof panels are usually limited to nolonger than about twenty-five feet (about eight meters) in length. Therefore, they may be overlapped over long spans.
Fastener World no.204/2024 惠達雜誌TechnologyRoofing Materials
Today there are a variety of material choices. When properly installed someof these materials are known to last a hundred years or more.
• Steel: Steel is the strongest and least expensive metal roofing material. It is
usually galvanized or coated with an Aluminum-Zinc alloy called Galvalumeand then painted. Steel roofing panels will generally last 30 to 50 years.
• Aluminum: Aluminum is lighter, softer, and more expensive than steel but
will withstand the elements better than steel, especially in areas of salt air oracid rain. Aluminum roofing panels are almost always painted and will last
30 to 50 years.

• Copper: Copper is the longest lasting, mostexpensive, and perhaps most attractive of the metalroofing material choices. Copper forms a natural
patina over time and copper roofs have been known
to last up to 200 years.
• Terre: Terre used to refer to a roof made of tin
with a lead coating. Today it is stainless steel that is
sandwiched between layers of tin. It does not holdpaint but can last more than one hundred years.
• Zinc: Zinc is often alloyed with Titanium for added
strength and corrosion. It will last 80 to 100 years.

Challenges
Although all three metal roof types exhibit
application challenges, the exposed fastenersystem exhibits the greatest challenge related to
the fasteners. Thus, purchasers of these systems
must wrestle with the question of whether theadded cost for premium longer lasting solutions isjustified while installers must be very careful andintentional about how they install the fasteners thatare specified. To perform properly, some of thefastener challenges that must be overcome withexposed fastener systems are:••262

Condition of the Seal: The screw must beable to cleanly penetrate the metal panel
and generate an effective and long-lasting
seal. Screws that are installed off-angle,
under tightened, or over tightened may upon
installation or over time begin to leak. Thus,
a successful roof seal is not only a function of
how well the installer seats the screw but often
also a function of the screw design, quality ofmanufacture, and type of sealing element.
Sealing Material: Sealing washers comein two varieties and two different materials,
bonded and non-bonded washers and EPDM
(Ethylene Propylene Diene Monomer) and
Neoprene materials. A bonded washer is one
where the rubber gasketing material is bonded(adhered) to the bottom of a slightly beveled
metal washer (See Figure 4). The ID and OD
of the rubber gasket are slightly smaller thanthose of the metal washer it is attached to.
This provides an overhang at the outer edge
protecting the gasket from UV rays and asmall amount of interference from the ID toretain the washer to the screw and to guarantee
a tight seal around the screw itself. During
installation, the bevel in the metal washer
flattens providing an even stress distribution
across the surface of the rubber washer. This
prevents the screw head from embedding into
the washer and squeezing out at its periphery.
This is particularly advantageous because
when overtightened the rubber gasketing
material will want to creep away from theensuing stress concentration which contributes
to loss of clamp load and sealing capability.
Fastener World no.204/2024 惠達雜誌

Figure 4: Exampleof Bonded Dealing
WasherFigure 5: Example ofCorrosion to the RoofPanels Around theFastener

Figure 6: Example ofCorrosion to the Headof the Fastener and theMetal Roofing Panel

Bonded washers can also be larger in diameter because the metal washer is able
to distribute pressure over its entire area. This provides a greater sealing areaand makes it harder for water to penetrate the roof panel. Non-bonded washers
are limited in diameter by the size of the bearing diameter of the hex flange
or washer head or, when utilized, a head cap. Both EPDM and Neoprene aregood choices for roof sealing, although EPDM is considered superior
since it has better heat, light, and ozone resistance than Neoprene.
•Embrittlement of the Sealing Material: One of the challenges of using
Neoprene and EPDM sealing material is vulnerability to environmental
exposure. These materials are susceptible to temperature changes and get stiffer
in the cold and creep in the heat. Prolonged exposure to such temperature cycles
eventually weakens the gasketing material. Worse than the temperature cycling,
however, is the exposure to UV radiation. These materials are embrittled by UV
rays so that the unprotected periphery that is exposed ultimately breaks down
and potentially opens pathways for water intrusion.

•Damage to Sealing Material: Burrs and debris that are created when thescrews pierce the roofing panel can tear the gasketing material creating
pathways for water intrusion. It is important, therefore, that the screws bedesigned with sharp and efficient piercing points that either prevent or
minimize the creation of burrs.

•Corrosion and Head Appearance: Exposed screws must be durable so thatthey do not corrode while in service. Screws that begin to rust will, in thebest case, be unsightly against the uncorroded roof panels but, in the worst
case, begin to weep and leave streaks or stains of rust from each fastener.
Additionally, care must be taken in proper fastener material selection
to prevent negative galvanic interactions with some of the roof panel
material types. Galvanic corrosion can result not only in corrosion tothe fastener, but depending on the metal pairing, to the roof panel.
Once again, this could result in unsightly corrosion but also damage to theroof panel around the screw, opening leak pathways (See Figures 5 and 6).
Head appearance is also important and usually screws are painted to matchthe color of the roofing panels. Not all paint is the same, however, with liquid
applied paint being far less durable than powder coated paint. Paint
type is important because paint methods that provide poorer adhesion may bevulnerable to damage during installation which can lead to premature corrosion.

TechnologyScrews••

For Woo d Appl ic at ion s: Sc r ews for wood
applications may be made of either hardened carbon
steel or stainless steel. Naturally, the stainless steelvariety has superior corrosion resistance to thosemade of carbon steel. However, the austenitic gradesof stainless steel make poor choices for self-drilling or
self-piercing points. Therefore, many of these roof
screws may be made of the stronger but poorer
corrosion resistant 410 martensitic stainlesssteel. Screws for wood to metal applications usually
have either a sharp Type #17 point or a small #1 drill
point. In either case, the point provides sufficientpiercing capability to make it through the thin roofing
panel but not enough strength to pilot holes in thicker
metal trusses and purlins. These screws are usually
Hi-Lo or spaced high-thread styles of threads for easy
tapping into wood. Head styles are shared between
wood and metal applications with the most common
head form being hex flange or washer head varieties.
For Met a l Appl ic at ion s: Sc r ews for m et a lapplications are usually either hardened carbon steel
or bi-metallic. Since stainless steel does not perform
well in drilling through thicker metal, screws entirely
made of stainless steel are seldom used in theseapplications. Instead drill point screws made ofcarbon steel are most common. There is a drill screw
variant, however, that is bi-metallic with the upper
body and head made of stainless steel and the drill
point and thread forming zone made of hardenedcarbon steel. These are produced by friction welding
the two materials together. The lower half is hardenedcarbon steel and houses the drill point and thread
forming threads while the upper half is stainless steel
and houses the upper threads and head. Screws formetal applications usually come with either a #3 or a
#5 drill point. The #3 point is capable of self-drilling
thicknesses up to 0.210” and a #5 capable of 0.500”
thick material. Like screws for wood applications theheads are usually either a hex flange or hex washer
head design.

Figure 7: Example of Roof Mounting Solar Fastener System

Sealing Washers•Non-Bonded: A Non-bonded sealing washer is simply a washerof EPDM or Neoprene. It can be slipped onto the screw andretained by making the ID of the washer a little smaller than theOD of the screw. These washers perform similarly to an O-ring
except that the cross-sectional geometry of these washers is notnarrow and round like an O-ring but flat and rectangular like aflat washer. These washers rely on the underhead bearing surfaceof the screw to distribute load and compress them. With the headof the screw providing the only protection from exposure, they
are more prone to potential embrittlement from UV rays.

•Bonded Washers: A bonded washer adheres the EPDM or
Neoprene washer to the underside of a flat or slightly beveled
metal washer. The metal washer is slightly larger in OD and ID,
so that the smaller rubber washer ID retains it to the screw andthe slightly larger OD of the metal washer protects the rubber
washer underneath from most UV exposure as well as providing
more evenly distributed compression. Naturally, bonded washers
are more expensive than the non-bonded variety. However,neither is completely effective in protecting the rubber washer
from exposure to the elements and, most importantly, from UVradiation.

•Capped Heads: Although technically not a washer, there aresome premium roofing screws that have added a zinc-aluminumalloy or stainless-steel cap to the head. These oversize caps serve
not only to improve the corrosion resistance of the hardenedcarbon steel screw below, but they fit over the rubber washer
completely encapsulating it from the surrounding environment.
In this way it is pretty well protected from the elements and far
less prone to embrittlement.

Solar Fasteners
Increasingly solar panel installations are being made on residential
and commercial buildings these days. Although the screws associatedwith mounting these solar voltaic panels are completely independentof the screws used to fix the roofing panels, it is worth a mention here.
Like roofing panels, roof mounted solar panels must be firmly fixed toprevent them from being damaged or torn off in a storm or by gusting
winds. Therefore, the panels are fixed to a framework of mounting rails.
These rails must be rigidly fixed to the structural underbody of the roof.
The screw systems that typically make up this mounting hardware are a
sophisticated combination of screw, sealing washer, bracket, and mounting
nut (See Figure 7). Like the roofing screws described above, the screws inthese systems are also designed with self-tapping threads and self-piercing
or self-drilling points to effectively work in wood or steel. Althoughusually much larger in diameter than the screws for fixing roofing panels,
these screws often are bi-metallic to provide hardened carbon steel on theportion of the fastener going into the roof substructure and stainless steel
for the portion extending above the surface of the roof. Like the exposed
fastener screws, these fasteners have a sealing washer to seal the roof
penetration from water intrusion.

Wrap-up
Exposed metal roofing screws, although simple in concept, must address multiple challenges. Like many similar fastening situations, there
are multiple choices for a user to make, and various levels of performance that can be achieved. The user, therefore, must educate themselveson whether it is wiser in the long run to purchase a more premium fastener for the initial installation and avoid potentially costly rework laterin the life of the roof or use less costly fasteners to ease the cost of roof installation. In any event though, there are many choices and those
producing, purchasing, and installing these fasteners should know a little bit about them to be able to make the most informed choices on how
to proceed.
Copyright owned by Fastener World / Article by Laurence Claus惠達雜誌 Fastener World no.204/2024

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