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A roofing professional provides his candid thoughts
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Observations from the field during 1998
A roofing professional provides his candid thoughts
about various technical issues
by Dick Baxter
Editor's note: Following are the author's
opinions about technical roofing industry issues.
Views expressed are not necessarily those of NRCA.
During
the past few years, most technical roofing problems
involving litigation have been related to low-slope
membrane roof systems. However, in the steep-slope
arena, there have been problems associated with
wrinkling of organic felt underlay sheets (mostly
caused by using organic felts as temporary roof
systems); glass-fiber-reinforced asphalt composition
shingles; and no asbestos fiber-cement shingles.
About the time coated roofing felts gave hot-applied
built-up roof (BUR) membranes an almost unreasonable
black eye because of poor performance, no reinforced
PVC, CPE, CSPE and EPDM roof membrane systems were
introduced. Although many of these sheet membrane
roof systems have provided satisfactory and, in some
cases, exemplary service, their average service
lives have been relatively short. Problems arose
with some of these membrane systems because of
misuse and misapplication. Now, metal has become the
"hot" system. |
Metal roof systems
Many problems associated with metal roof assemblies
have occurred primarily because of poor design. For
example, some designers are content to specify only
that contractors follow manufacturers' installation
recommendations, rather than provide specifications
and details tailored to jobs. Submittal drawings
often do not match shop drawings for specific
projects, and manufacturer-recommended details often
come with caveats, such as, "XYZ Manufacturing Co.
shall be held harmless from any and all claims
arising from a lack of water tightness as a result
of following these recommended details."
Another problem is that some roof systems are
installed by unknowledgeable, unapproved applicators
(if, in fact, approved applicators are recognized by
metal roof system component suppliers). In addition,
most metal roof system warranties do not include
watertightness of finished roof assemblies—only the
panels' finish is included in many warranties, and
this could be voided if metal roof systems are
installed in coastal areas or exposed to animal
feces or acid environments.
Additionally, architectural metal roof system
specifications often are contradictory. For example,
many specifications stipulate Factory Mutual (FM)
and Underwriters Laboratories Inc. (UL)
classifications. These classifications may be
similar but contain irreconcilable differences for
roof system construction. In addition, some
specifications reference Sheet Metal and Air
Conditioning Contractors' National Association and
manufacturers' recommendations, which, in most
cases, are not similar. Many specifications include
designs that do not match roof assemblies'
configurations that were tested for wind-uplift and
fire properties. Following the roof assemblies'
designs may violate model or local building codes.
It is up to roofing contractors to determine
specification discrepancies and resolve them or take
exception to the conditions before starting roof
system installations.
Contractors should consider the following example:
Suppose design specifications stipulate that a
completed roof assembly have a UL wind
classification that meets UL 90 (90
pounds-per-square-foot [44-kg/m²] uplift). The roof
assembly to be installed must be constructed exactly
as the one tested by UL when the wind-uplift
classification was determined
A contractor should pay attention to the minimum
type and density requirements for foam insulation
over which the metal roof system is to be installed
and understand that commercially available
polyisocyanurate foam insulation rarely will
approach 2-pounds-per-cubic-foot (32-kg/m³)
density—not the 2.5-pounds-per-cubic-foot (40-kg/m³)
density of tested insulation. The contractor also
should note special requirements for attachment
devices.
In addition, the contractor should cross-reference
UL construction numbers listed in the UL Roofing
Materials and Systems Directory to the speci
roof section to determine whether the test and
specified construction essentially are the same. If
they aren't the same, the finished product will not
have the wind-uplift classification prescribed in
the specifications' performance criteria. The
contractor then may be in breach of contract for not
providing a roof system that meets the design
specifications' performance criteria (i.e., UL 90).
Such small discrepancies, as described in this
example, can lead to a building owner holding retain
age, filing a claim against the contractor's
performance bond or, worse, initiating a lawsuit for
noncompliance with specifications. In addition, the
problem can be expensive to correct because, in most
cases, removal and replacement of a metal roof
system is the only way the system can be brought
into compliance with the specifications' design
criteria.
If specifications call for UL and FM
classifications, the contractor should be aware that
the testing methods used by these organizations are
different, and UL-tested constructions may not be
similar to Factory Mutual Research Corp.-tested
constructions. In this instance, the designer or
owner must choose one testing agency's requirements,
preferably before commencement of roof system
construction.
It also is important that the roof system be tested,
listed and approved by the specified testing agency.
Some metal roof system suppliers have their systems
tested at independent testing laboratories or
perform the tests themselves. However, just because
a supplier says a metal roof system has passed the
UL 90 test does not mean the system was tested and
listed by UL. |
Metal warranties
Roofing professionals should understand that when
they purchase metal coil stock to roll metal roof
panels at job sites, they become, in essence,
manufacturers. This means they may be liable for the
terms of the specified manufacturer's warranty. The
supplier of the metal materials from which roof
system components are field-fabricated will not
issue a warranty for the roof system other than,
perhaps, for the coil stock or panel material's
paint or finish.
Roofing professionals and building owners also
should understand the implications of warranty
requirements included in bid documents. A roofing
professional either should select a warrantable
manufacturer's metal roof system to comply with the
contract documents or be willing to assume
responsibility for any long-term warranties required
by designers. Designers must understand that
conventional manufacturers' warranties may not be
available for field-fabricated metal roof systems.
Polyiso insulation
Problems associated with dimensional instability
(i.e., shrinkage) of polyisocyanurate foam roof
insulation remain and don't appear to be going away.
And the problems have not been made better with the
revision of ASTM C 1289, which allows manufactured
insulation boards to shrink or grow up to 4 percent
of their manufactured dimensions.
Although it appears most manufacturers have been
able to deal with new blowing agents, the
manufacturing results remain fairly unpredictable;
some batches appear to be better—some worse—than
others.
To help combat dimensional instability problems,
contractors should compensate for them. Installing
two roof insulation layers remains the best method
for addressing shrinkage and resultant open joints
between polyisocyanurate foam roof insulation
boards. The shrinkage still occurs, but the effects
are mitigated by offset or staggered insulation
board joints. When polyisocyanurate foam insulation
is covered with an adhered layer of perlite or wood
fiber roof insulation, the shrinkage effects appear
to be minimized by the stabilizing effect of the
cover boards.
TPO roof systems
Thermoplastic polyolefin (TPO) roof systems are a
hot topic of conversation in the industry. Those who
understand TPOs best believe the chemistry for this
roof system type is not yet completely acceptable.
Unlike PVC roof membranes, TPOs are not inherently
fire-resistant. Chemical formulations for non
fire-resistant TPO membranes generally provide
suitable weathering and handling properties for roof
system applications. The problems with the long-term
weathering properties of TPO membranes are
associated with the introduction of chemicals that
provide fire-resistant qualities.
Although TPOs offer more promise to the sheet
membrane roofing segment than any previous sheet
membrane roofing material, there will be some tough
lessons learned by those spearheading the movement.
Prudence dictates a conservative approach to TPOs.
Contractors should make sure all current application
recommendations are followed carefully. Then, if
problems arise with a completed membrane before
expected, the responsibility for the problem cannot
be blamed on application techniques or practices.
Coal-tar pitches membranes
A polymer-modified coal-tar pitch membrane roof
system was reintroduced during 1998. The latest
modified coal-tar membrane is flexible enough to be
installed with hot coal-tar pitch, a torch or
special coal-tar-pitch-based modified adhesive.
Although the waterproofing bitumen is
coal-tar-pitch-based, contractors should not expect
that a mineral-granule-surfaced, coal-tar-based
modified bitumen membrane will possess the same
cold-flow properties as thermoplastic coal-tar or
same resistance to ponded water as an
aggregate-surfaced, coal-tar pitch membrane.
A mineral-granule-surfaced, modified, coal-tar pitch
roof membrane requires positive drainage to minimize
breakdown of the fire-resistant top surface. Even
though the material is represented as being suitable
for use wherever coal-tar pitch BUR membranes are
used, it will not perform as expected under even
moderate pounding water conditions. With top surface
degradation (deregulation), crazing, etc.), roof
membrane deterioration—similar to that of an
SBS-modified asphalt roof membrane—can be expected.
Summing it up
Overall, it has been an interesting year. It is
important for the industry to continue development
efforts for cost-effective roof systems; it's just
as important not to get caught in the mire between
new products and time-tested, proven ones.
No roofing industry segment is much different from
any other. There are those who try to provide
tested, proven roof systems and materials and those
who don't care if a system or material works as long
as they collect their money.
It is, indeed, caveat emptor.
Dick Baxter is president of CRS Inc., Monroe, N.C |
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