May 2002 Interface • 3
Most metal roofs are installed on pre-engineered metal buildings
(PEMBs), supplied as a package from a metal building manufacturer.
PEMBs are installed for the most part by general
contractors. The National Roofing Contractors Association
(NRCA) surveys roofing contractor members and publishes data
on the quantity of various types of roofs installed (Figure 1).
Metal roofs account for 7.4% of all roof systems installed by
NRCA members. The NRCA survey data does not include metal
roofs installed by metal building contractors, either on new
metal building projects or retrofit roof projects. Data gathered
by the Metal Building Manufacturers Association (MBMA)
include new metal building and metal retrofit roof sales. When
the MBMA data are combined with the NRCA data (Figure 2),
the ranking of roof systems is adjusted slightly. The “Other”
category includes shingle and shake products used on nonresidential
History of Metal Buildings
PEMBs, based on the rigid frame design, came into existence
in their present form just prior to World War II. The rigid frame
design, the basis for PEMB construction, was refined during
WWII for use as barracks, hospitals, aircraft hangars, etc. In late
1945, manufacturers of PEMBs looked for ways to use metal
buildings to support the demands of the fast-growing, post-war
economy. In the past 56 years, metal building construction has
By Raymond K. Heisey Jr., PE, RRC
Figure 1 — NRCA Market Data (Year 2000). Figure 2 — Combined NRCA & MBMA Data (Year 2000).
New and Retrofit Roofing
4 • Interface May 2002
grown to become commonly used for warehouses, manufacturing
plants, distribution centers, schools, shopping centers, etc.
Metal building usage has grown to become almost two-thirds
of all new one-story and two-story non-residential construction
in the United States. Twenty-six gage (0.017″ nominal thickness)
coated steel, through-fastened R-panel type roofs have been the
predominant roof type installed on PEMBs over the past 56+
years. A through-fastened R-panel, with typical 36-inch-wide
coverage, has 1 to 1-1/2 inch tall trapezoidal shaped ribs, spaced
at 12 inches on center. Estimates range from 30 to 50 billion
square feet of existing through-fastened metal roofs in use on
buildings in the United States today. What does this mean to
the roof consultant? If one has not been on a problem throughfastened
R-panel metal roof in the past, there’s a good chance
he or she will be called upon to provide consulting in the
On metal buildings, the 26-gage (or in some cases, 24- or
even 28-gage) coated steel roof panels are typically installed
over some type of 16-gage or heavier, cold-formed “Z” or “C”
steel structural member. In some cases, the structural members
supporting the roof panels are bar joists, wood joists, or
a hybrid structural member. On buildings over 40 years old,
the structural members might be hot-rolled channels or
beams. The structural members are typically spaced at 5 foot
on center, but the structural spacing can vary from as little as
2 feet to more than 6 feet.
Screw-type fasteners are used to secure the panels to the
underlying structural members and to join (stitch) the panels
together at their sidelap and endlap joints. The fasteners used
to attach the panels to each other, at their lap joint, are commonly
referred to as “stitch screws.” Self-drilling screws are
the predominant fastener used today. Some self-tapping
screws requiring pre-drilling or pre-punching of the panels
and structural members are used. On early metal buildings,
the fastener of choice was the bolt and nut or the rivet. The
most common fastener material is carbon-steel. Carbon-steel
fasteners have zinc or cadmium plating to provide corrosion
resistance. In some cases, stainless steel or aluminum fasteners
are utilized. Carbon-steel fasteners provide a more consistent,
secure connection because they cannot easily be
over-driven (stripped out). Aluminum fasteners are typically
the rivet or clamping type. Fasteners are typically spaced
at 6 inches on center at the panel sidelaps and typically
12 inches on center for the panel-to-structural member
To weatherproof the fasteners, a gasketed washer is used
under the fastener head. Until the early ‘90s, the gasket
material was primarily Neoprene. After a few weather cycles,
the Neoprene gasket would start to dry and crack until the
fastener became a potential source of leaks. Today, more
durable ethylene polypropylene diene monomer (EPDM) is
available for the washer gasket material, although the less
expensive Neoprene material is still widely used.
Waterproofing at the panel sidelap and endlap joints is
accomplished with the use of field-applied tape or pumpable
butyl rubber sealants. Butyl sealants vary widely in quality. High
quality (high percentage of butyl rubber vs. chalk filler material)
sealants can last as long as 20+ years while lower-quality sealants
will last as little as a few years before losing elasticity and contributing
to leak problems.
Metal Roof Problems
When an existing metal roof has problems, the fix is typically
not simple and, in most cases, can be quite expensive. Problems
can run the gamut from poorly designed details, to incorrectly
installed materials, to use of inferior quality materials.
Figures 3 through 6 show typical metal roof problems:
• Figure 3 – Poor panel alignment leads to poor fit-up of the
• Figure 4 – Pipe penetration blocks water between
Figure 3 — Poor panel alignment leads to poor fit-up of the panel closure.
Figure 4 — Pipe penetration blocks water between panel ribs.
• Figure 5 – Curb penetration has exposed screws and
• Figure 6 – Poor roof transition design, detailing
Typical metal roof problems:
• Poor design and detailing used for trim, flashing, and
• Inadequate slope – poor drainage.
• Panels installed in an out-of-module alignment – poor
fit-up of panels and closures.
• Inadequate provision for thermal movement of panels
• Poor installation quality – bad workmanship.
• Poor material quality – inferior quality of materials used.
• Improper repairs and fixes used.
• Abuse and neglect of the roof by owners.
An example of out-of-module installation, as seen in
Figure 7, is the result of poor quality installation. Metal panels
have a defined width of coverage. For through-fastened
R-panels, this coverage is typically 36 inches. However, a
36-inch coverage panel can be made to cover more or less
than the 36-inch defined coverage. When this happens, the
installed panel is described as being “out-of-module.” Outof-
module panel installation causes problems due to poor
fit-up of panel lap joints, closures, and flashings. Long-term
problems can occur due to restricted panel movement. Roof
weathertightness is then dependent primarily on sealants
filling large gaps between panels, closures, and flashings.
Out-of-module installation and misalignment of panels
sometimes go hand-in-hand. Figure 8 shows a standing seam
roof that has been installed out-of-module and been misaligned
as evidenced by the changes in direction of the
Why aren’t metal roof problems easy to fix? The easiest
and least expensive solutions are applied to the surface of the
roof. Replacing problem fasteners and sealants, applying surface
roof coatings, and overlaying the existing roof with a
new roof are all relatively straightforward solutions. However,
to repair the existing roof, improperly installed panels,
flashings, penetrations, etc. must be disassembled and
reassembled to provide a long-term, weathertight solution.
When weighed against the potential for business interruption,
the “repair” of the existing roof becomes less attractive
than a roof overlay solution.
Figure 7 — An out-of-module installation is the result of poor quality installation.
Figure 5 — Curb penetration has exposed screws and ponded water.
Figure 6 — Poor roof transition design, detailing, and installation.
6 • Interface May 2002
Pros and cons of metal
In order to evaluate the suitability of
various metal roof solutions, existing conditions
must be investigated. The condition
of the existing roof must be evaluated for
the amount and severity of corrosion (on
both top and bottom panel surfaces). The
panel-to-structural member attachment
must also be evaluated for structural integrity.
Warning: Care should be exercised
when inspecting any roof, because
there are fall hazards at the edge of
the roof and even at the interior of
the roof. The interior roof hazard is
the potential to fall through a rusted,
deteriorated metal panel. This is
especially true of lighter, 28- or 26-
gage metal panels. Coatings can conceal
roof panel deterioration as well as make translucent
fiberglass panels difficult to detect.
The thickness and condition of any existing insulation should
be evaluated for the:
• Condition and location of the vapor retarder (insulation
facing – most likely vinyl).
• Presence of moisture in the insulation and/or on the
bottom side of the panel.
A dewpoint analysis based on the building usage should be
performed to determine the proper amount of insulation
required, combined with the location of the vapor retarder.
Important note: PEMB designs are optimized for the
original intended loads. PEMB designs do not provide
capacity for the added dead load of future roofing materials.
Any reroofing of a PEMB should include analysis of
the current system by a qualified structural engineer
familiar with PEMB design assumptions and construction.
Changes in the American Iron and Steel Institute (AISI)
Cold-formed Steel Design Manual (CFSDM), 1996 latest
edition, have reduced the capacity of cold-formed steel
purlins from the previous 1980 and 1986 editions. Therefore,
an existing purlin that worked using the 1980 or
1986 edition of the AISI CFSDM will typically not support
the same loads when designed with the 1996 edition
of the AISI
include very little,
roofs or metal
contains the following
to Chapter 15 –
that for a
Figure 8 — A standing seam roof that has been installed out-of-module and also has been misaligned
as evidenced by the changes in direction of the panel seam.
May 2002 Interface • 7
Figure 9 — Table A-15-A from the 1997 Uniform Building Code.
8 • Interface May 2002
metal roof, the only allowable retrofit is another metal roof. To
help explain the metal roof retrofit restriction, ask yourself, “Is a
26 gage through-fastened R-panel or 24 gage standing seam
panel an adequate supporting structural deck for the application
of conventional roofing materials?” Almost every conventional
roof weathertightness warranty contains an exclusion for “excessive
building or structure movement.” As we review the pros and
cons of the various metal roof overlay solutions, keep this consideration
Solutions for leaking or deteriorating metal roofs
come in many shapes, sizes, and costs. The pros and
cons of each solution must be evaluated against the
overall cost, expected service life, compatibility
with the PEMB, and suitability of the materials for
Available through-fastened R-panel metal roof
• Tear-off and replace the existing roof.
• Coat the existing roof with:
Ö elastomeric (acrylic- and urethane-based)
coatings for waterproofing.
Ö asphalt-based coatings containing aluminum
to inhibit ru s t .
• Overlay the existing roof with:
Ö sprayed polyurethane foam (SPUF).
Ö a single ply membrane roof.
Ö a plywood deck with various conventional
roof systems applied.
Ö a metal retrofit using a through-fastened
R-panel roof or standing seam roof.
Tear-off and Replacement
Tear-off and replacement of the existing roof
adds little if any additional weight. However, in
most cases, a through-fastened R-panel roof cannot
be replaced with a standing seam roof. The PEMB
design takes advantage of the through-fastened Rpanel
roof to help brace the top flange of the
purlins. Also, a through-fastened R-panel roof
might have been used as a wind diaphragm similar
to a structural roof deck. A standing seam roof will
not brace the top flange of the purlins and cannot
act as a wind diaphragm.
During tear-off and replacement, safety is a
major consideration while working over open
structural members. Also, the building contents
and/or operations are much more susceptible to
damage and/or interruption due to weather. Figures
10 and 11 show a through-fastened R-panel roof
(light-colored panel) being removed and a new
standing seam roof (dark-colored panel) being
installed from the right. The light material
between the old and new roof panels is the new
fiberglass blanket insulation.
Of course, a new through-fastened R-panel
roof could be used to replace the old through-fastened
R-panel roof without affecting the PEMB. In
order to evaluate a tear-off and replacement solution
using a standing seam roof, a structural engineer should
check the PEMB design, especially the cold-formed purlins. It is
always prudent to have any existing building checked by a structural
engineer prior to any type of roof work being done.
The weight of a coating adds minimal dead load weight to
the existing roof. The application of coatings can be done with
Figure 10 — A through-fastened R-panel roof (light-colored panel) being removed.
Figure 11 — A new standing seam roof (dark-colored panel) being installed from the right.
spray equipment, brushes, or rollers, giving the contractor flexibility.
Coatings come in a variety of colors. White coatings are
reflective, providing an energy benefit in summer (cooling load)
months. Most coatings have good flexibility and elongation
properties to accommodate movement and thermal stress. There
are many different coatings to address leaks and rusting of metal
panels. Acrylic-based elastomeric coatings are popular for
addressing leak problems and asphalt-based coatings containing
aluminum are popular as a rust inhibitor.
Important note: Any coating over integral skylight panels
or deteriorated panels creates a safety hazard for foot
traffic on the roof.
The success of a coating application is dependent upon the
condition and surface preparation of the existing panel to create
proper coating adhesion. If a coating loses adhesion, the resulting
delamination of the coating from the panel surface can allow
moisture to become entrapped, resulting in an accelerated deterioration
of the existing panel. To properly prepare the roof surface
for the application of the coating, any foreign material must
be removed. Foreign material can include surface-applied sealing
products, previous coatings, oxidation (steel, aluminum, zinc,
etc.), loose paint, etc. The applicator must be skilled not only in
the proper application methods for the coating, but also in the
permissible environmental conditions (temperature, humidity,
wind, etc.) necessary for successful application.
Figure 12 shows an attempted fix using asphalt-based products
that must be fully removed before application of an elastomeric
May 2002 Interface • 9
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Figure 12 — An attempted fix using asphalt-based products that must be fully
removed before application of an elastomeric type coating.
type coating. Figure 13 shows failure (bleed-through of
underlying material) of an elastomeric coating due to
improper surface preparation (asphalt products not
properly cleaned from the roof surface).
Most coatings can accommodate uniform panel
thermal movement. However, movement concentrated
at panel lap joints and flashings can exceed the coating’s
allowable elongation. Excessive vertical deflection
from live loads (foot traffic, equipment, etc.), snow
loads, or even horizontal deflection from wind loads
can exceed the allowable elongation of the coating
system. Coatings typically will not span holes or gaps
in the existing panel. In most cases, this can be
addressed by imbedding a reinforcing fabric in the
coating. The reinforcing fabric typically has little elongation,
reducing the allowable elongation of the
installed coating system.
Sprayed Polyurethane Foam
Sprayed polyurethane foam (SPUF) is a two-part
chemical compound that is mixed at the point of application
on the roof. SPUF provides a lightweight, thermally
efficient roof system that is easily applied with
spray equipment. Adhesion of SPUF is typically good
to many types of substrates. The foam can be sprayed to vertical
surfaces (parapet walls and curbs) and sloped surfaces as well as
applied horizontally. A protective coating must be applied to
shield the foam from the attack of ultraviolet light. Many protective
coating options are available for application onto the foam.
SPUF relies on the integrity and stiffness of the existing “roof
deck” to provide structural support.
The success of an SPUF application is dependent on the condition
of the existing panel (structural integrity) as well as proper,
allowable environmental conditions (temperature, humidity,
wind, etc.). Outside of the Southwest, much of the U.S. has few
available days with weather conditions that fall within the published
SPUF application guidelines. The applicator must be
extremely careful in windy conditions, as airborne polyurethane
foam can adhere to almost any surface.
Improper application, coating over foreign material,
and other factors can introduce voids into the
foam. These voids can become the vehicle through
which moisture enters and becomes entrapped,
resulting in accelerated deterioration of the supporting
panel. SPUF can accommodate little movement
(thermal or deflection) without damage. Isolating the
existing panel (roof support substrate) from temperature
fluctuations is key to avoiding damage from thermal
movement. Applying foam over integral skylight
panels creates a fall hazard. SPUF is susceptible to
attack from birds, animals, and plant growth.
Figures 14 and 15 show a through-fastened R-panel
roof with SPUF applied. Figure 14 shows an interior
gutter between two metal buildings. Figure 15 shows
the through-fastened R-panel rib pattern through the
Single Ply Overlay
Single-ply membrane systems are available in a
wide variety of materials (EPDM, TPO, PVC, etc.)
and in several configurations (ballasted, adhered, and
mechanically fastened). However, the application of a
single-ply membrane over an existing metal roof typically
utilizes a reinforced single ply membrane either
mechanically fastened to the underlying, supporting
Figure 13 — Failure (bleed-through of underlying material) of an elastomeric coating due
to improper surface preparation.
Figure 14 — An interior gutter between two metal buildings.
10 • Interface May 2002
structural members or to the existing panel. PEMB purlins typically
are spaced at 5′-0″ on center; however, on steeper pitches,
the spacing can be 5′-3″ (for a 4:12 slope) or more. It is recommended
that a registered professional engineer verify the design
and attachment of the single ply system to the PEMB. Using the
26-gage through-fastened R-panel roof as the only attachment
for a single ply membrane overlay is not recommended.
Single ply membrane systems are relatively easy for contractors
to install, and they require little or no specialized equipment.
However, on steep slopes, heat welding can become more
difficult to accomplish correctly. White membranes are reflective,
providing an energy benefit in summer months. Because the
single ply membrane sheet will be mechanically fastened at the
existing purlins, the sheet width will be 5 to 6 feet wide. While
un-reinforced single ply membranes can provide as much as 200
to 300 percent elongation, reinforced membranes provide 10
percent or less elongation. The elongation of the single ply
membrane must be capable of accommodating the expected
thermal and wind movement of the PEMB and the metal roof.
Figures 16 and 17 show a 26 gage through-fastened metal roof
with a two-layer insulation system and a single ply membrane
The application of a single ply membrane overlay requires
that a board insulation product (1 to 1-1/2″ thick, depending on
the roof panel rib height) be used to fill the area between the
ribs of the existing roof panel. A continuous layer of insulation
board is then applied over the panel ribs and secured to the
underlying panel using insulation plates and screws. These added
holes from the insulation fasteners provide additional moisture
entry points if the single ply membrane leaks. If polystyrene
insulation is used in the assembly, the fire rating should be investigated.
Edge details require the installation of wood blocking to
adequately secure the edge flashings and terminations. The
weight of a single ply overlay system, including 2.5 inches of
rigid insulation, will add approximately 1.25 pounds per square
foot of dead load to the roof.
A plywood or oriented strand board (OSB) deck, minimum
7/16-inch thick, is used as the base for many types of metal roof
retrofit systems. In many cases, this wood deck is used as the
base to install a modified bitumen roof system.
Nominal 1/2-inch thick wood deck adds approximately 2
pounds per square foot of dead load while nominal 3/4-inch
thick wood deck adds approximately 3 pounds per square foot of
dead load to the PEMB. The modified bitumen system base and
cap sheets weigh 1 pound per square foot each for a total added
weight (with a 1/2-inch nominal wood deck) of 4 pounds per
square foot. Metal buildings typically have little, if any, capacity
to carry this added load.
Wood decks are very stiff compared to a 26-gage metal
through-fastened R-panel roof or a 24-gage standing seam panel.
When added to a PEMB, a stiff, wood roof deck can concentrate
stresses in unintended areas of the metal building. Metal buildings
move differentially. When subjected to horizontal wind
loads, a metal building will move (deflect) less at the stiffer rigid
frame than at the more flexible z-purlins in between the rigid
frames. The opposite occurs when thermal loads are imposed.
Stiffer rigid frames move more under thermal loads than the
more flexible z-purlins that span between the rigid frames.
Manufacturers of modified bitumen and asphalt built-up roof
products recommend expansion joints (roof area separators)
every 150 to 200 feet. On a metal roof this would require wood
blocking to frame the roof expansion joint. These joints must be
worked around the existing through-fastened R-panel roof ribs
May 2002 Interface • 11
Figure 15 — The through-fastened R-panel rib pattern through the applied foam.
12 • Interface May 2002
located typically 12 inches on center. Modified bitumen systems
can be secured with asphalt (hot mopped), torching, or cold
adhesives. Hot asphalt and open torches introduce burn, fume,
and fire hazards, both to the installation process and the building
itself. Cold applied modified bitumen systems eliminate
most of the fire and fume hazards associated with torches and
Metal Roof Overlay
A metal roof overlay can utilize either a through-fastened
R-panel roof or standing seam roof as the new roof system. The
overlay can also incorporate a new structural member directly
over, and attached to, the existing structural member. This new
structural member can serve as the support for the new roof and
also create the depth necessary to add the desired amount of
new fiberglass blanket or rigid board insulation.
How do you determine whether
to use a through-fastened R-panel
metal roof system or a standing
seam metal roof overlay? The criteria
to evaluate are the:
1) Desired longevity and serviceability
of the new roof system.
2) Spacing of the underlying
3) Desire for added insulation.
4) Length of the panel run;
R-panels have limited expansion/
5) Slope (pitch) of the
Figure 18 — Existing “R” Panel with proprietary “Z” structural member and new “R” panel roof.
Figures 16 and 17 show a 26-gage through-fastened metal roof with a two-layer insulation system and a single ply membrane roof system.
May 2002 Interface • 13
1) Longevity and Serviceability
• Depending on the panel system, a through-fastened
R-panel metal roof can have a service life of up to 10
years. The panel finish can last well in excess of 10 years;
however, exposed fastener gaskets, panel lap joints, thermal
movement, penetrations, etc. can drastically reduce
the service life of a through-fastened R-panel metal roof.
• Depending on the panel system, standing seam metal
roofs can provide leak-free performance for 20 years
2) Existing Structural Spacing
• Through-fastened, 26-gage steel R-panel roofs can be
designed to span supports in excess of 5 feet; however,
the most common span is a nominal 5 feet.
• Due to wind uplift considerations, 24-gage steel standing
seam metal roofs are typically limited to a nominal 5-foot
span or less. Most standing seam systems have not been
rated in industry wind uplift tests such as UL 580 (Class
90 rating) or FMRC Standard 4471 (I-90 rating) at spans
over 5 feet.
3) Desired Insulation Thickness
Higher R-values (more insulation) cause all roof surfaces to
experience large temperature differentials and, in turn, more
• Through-fastened R-panel roofs also have insulation
thickness limitations as low as 3 inches of fiberglass blanket
in some cases. With through-fastened metal roofs,
insulation thickness is typically limited because of the
potential for indentations around the fastener heads
caused when the fasteners are tightened. These indentations
can hold water at low slopes and be a source of
• Standing seam metal roofs are designed to handle thermal
movement without damage much better than through fastened
4) Length of Panel Runs
• Steel, through-fastened R-panel metal roof systems have
limitations for continuous panel runs of less than 150 feet.
Some systems have limitations as low as a 100 foot continuous
run. This length limitation is due to the lack of
any provision to handle thermal movement inherent with
• Standing seam metal roofs, designed to accommodate
thermal movement, can typically be installed with continuous
runs from 200 feet up to 250 feet. This length is
determined both by allowable panel clip movement and
movement allowed by trim and flashings.
5) Existing Roof Slope
• Through-fastened R-panels should never be used at slopes
under 1/2 inch per foot and, depending on the roof system
details, not under 1 inch per foot. The greater the
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14 • Interface May 2002
slope, the faster the
water flows off the roof
panel and the less
chance for roof leaks.
• Standing seam metal
roofs eliminate up to 90
percent of the exposed
place the panel sidelap
joint well above the
water flow plane. The
typical standing seam
metal roof can be
installed on slopes as
low as 1/4 inch per foot.
The cost of a metal roof
overlay using a 26-gage
through-fastened R-panel roof
system will typically be about
15 percent less than a 24-gage
standing seam metal roof system
Standing Seam Retrofit
A standing seam metal roof overlay can be installed over an
existing through-fastened R-panel roof utilizing two methods.
Either a new structural support member can be installed as the
support for the new roof system or the new roof system can be
installed directly without a new structural support member. The
difference between the two roof overlay systems is the ability to
add insulation and the installed cost of each system.
Figure 19 shows 16-gage structural members in place directly
over the underlying structural members. The new top structural
member will be tied to the existing structural member with a
spacer of some type and fasteners. The new standing seam metal
roof and insulation will be installed over the new structural member.
Figure 19 also shows additional framing around the roof unit
for support of the new roof curb.
By using a different depth structural member (typically from
1 to 12 inches deep), the desired insulation thickness can be
accommodated. This type of retrofit system can be installed over
through-fastened R-panel roofs as well as
other types of existing metal roofs including
standing seam roofs. This retrofit system
can be easily installed over
out-of-module existing roofs. The weight
of the system is typically less than 2
pounds per square foot (psf). The 24-
gage metal roof adds 1.25 psf, the 16-
gage structural member adds 0.5 psf, and
the fiberglass blanket insulation adds less
than 0.25 psf.
The new standing seam metal roof
system is fully compatible with the existing
PEMB system. Another advantage to
this type of retrofit system is the ability
to change the pitch and direction of
slope of the new roof to eliminate valley
gutters or difficult transitions, to simply
increase the pitch if necessary, or to
change the appearance of the building.
Figure 20 shows a standing seam metal
retrofit system that utilizes a standing
seam roof clip attached directly to the
underlying structural member.
Extruded polystyrene blocks are used
to transfer dead, live, and snow loads to
Figure 19 — 16-gage structural members in place directly over the underlying structural members.
Figure 20 — This standing seam metal retrofit system utilizes a standing seam roof clip attached directly
to the underlying structural member.
the existing structural members. Since the new roof is only 1-1/2
inches above the existing roof, there is an insulation thickness
limitation of 3 inches. The 3-inch fiberglass blanket insulation
will provide little R-value; however, condensation control is the
This type of retrofit system can only be installed over
through-fastened R-panel roofs that have not been installed in a
severely out-of-module condition. The weight of the retrofit system
is typically less than 1.50 psf. The 24-gage standing seam
metal roof adds 1.25 psf and the fiberglass blanket insulation
adds less than 0.25 psf.
To compare initial installed prices, a metal roof overlay will
be more expensive than an inexpensive, single-layer coating. In
many cases, the price for the installed metal overlay will be comparable
with either a top-of-the-line, multi-layer coating system
over a properly prepared metal panel substrate or a single-ply
overlay properly designed and attached to the PEMB purlins.
Insulation, Vapor Retarder, and Dew Point
Most PEMB projects that are over 25 years old have a layer
of 1-1/2 inch fiberglass blanket insulation with a vinyl vapor
retarder. Typically, the fiberglass blanket insulation is compressed
and provides little if any real R-value. The old vinyl
vapor retarders become soiled, brittle, torn, and do little to prevent
vapor migration because of their 1.0 to 1.2-perm rating.
An important consideration for any reroof project is the
investigation and evaluation of the condition of the existing
insulation and vapor retarder. The existing insulation system plus
any added insulation must be evaluated against the project usage
and historical, local weather conditions. A dew point analysis is
the tool used to conduct the evaluation and determine the optimum
amount of required insulation.
On retrofit roof projects, insulation is typically added above
the existing metal R-panel roof. To address the correct vapor
retarder and dew point location, a good solution involves removing
the existing insulation from the underside of the existing
R-panel roof. This is accomplished by cutting through the vapor
retarder and insulation with a razor knife adjacent to the PEMB
secondary structural member. The existing R-panel roof not only
acts as the new vapor retarder, but also provides a like-new
reflective ceiling for the building interior. Based on the results
of the dew point analysis, the proper amount of unfaced fiberglass
blanket insulation is then added on top of the existing Rpanel
With the large number of existing metal through-fastened
R-panel roofs in service, a metal roof retrofit project is just
around the corner. When a roof consultant encounters his first or
next metal roof retrofit project, he should carefully consider the
suitability of the various retrofit roof solutions and choose the
system that meets the code requirements and provides the best
overall performance and value to the owner.
As in every retrofit application, a qualified structural engineer
must check the structural capacity of the existing building and
the connection of the new roof to the existing structural mem-
May 2002 Interface • 15
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When AA Flight 77, piloted by terrorists, hit the Pentagon on
September 11, 20,000 gallons of jet fuel set the building on fire. The structure’s
slate roof was nailed to wood sheathing, which was attached to a concrete deck with a slope of
23 degrees (5 in 12). This included a two-ply underlayment mopped to the deck during its installation in 1943.
Firefighters, at first unaware that the system included wood sheathing, poured more than 6 million gallons of water on
the roof, which did its originally-intended job very well: it shed most of that water. It was not until firefighters broke
through the slate that they realized it was not just insulation that was burning underneath and were then able to stop the fire
from spreading further. Over an acre of the Pentagon’s roof system – some 40,000 square feet – was damaged beyond repair.
But in a project launched by John Francis of Northern Virginia Roofing, and spearheaded wholeheartedly by National
Roofing Contractors Association Executive Vice President Bill Good, that organization is reroofing the damaged structure
with volunteer labor and materials.
Good notes that the total value of donations has been in excess of $500,000. Even the NRCA’s lone member in Ghana,
Africa, informed Good that the community there had taken up a collection to pay travel and housing expenses for a roofing
worker to travel to the Pentagon to provide labor.
bers or panels. The 26-gage, existing through-fastened R-panel is
most likely not adequate as a substrate for most proposed retrofit
systems. The condition of the existing through-fastened R-panel
can determine or strongly influence the retrofit solution chosen.
A dewpoint analysis can determine the optimum amount of
insulation to add with the retrofit roof. By selecting the right
retrofit solution, the new roof system can provide the owner
with many years of worry-free service. n
16 • Interface May 2002
ABOUT THE AUTHOR
Raymond K. Heisey Jr.,
PE, RRC, earned a degree in
civil/structural engineering from
Lehigh University in 1978. He has
worked at Butler Manufacturing Co.
for over 23 years and has a wide variety
of experiences, encompassing
plant engineering, metal building
design, computer system development,
roof product development, and
roof regional sales and sales management.
Ray has two U.S. patents and
numerous foreign patents on metal
roof system components. He received his RRC designation in
1993. Heisey has won six Butler sales awards in an eight-year
period. He presented a report on standing seam roof clip
design to the ASCE’s Structural Congress and has taught seminars
on various roof-related subjects.
RAYMOND K. HEISEY
JR., PE, RRC,
Sheet Metal Guidelines
Metal and Air
design data located in certain sections and the appendices
are designed to help users adapt the drawings to local climate
and project conditions. For information, visit
www.smacna.org and select “publications.”
Common roof styles, from SMACNA
Residential Sheet Metal Guidelines.