A Few Simple Rules: What To Do To Ensure A Built-Up Roof Will Perform As Expected

May 15, 2006

Built-up Roofing (BUR) has
been around a long, long
time, and as most are aware,
it has an excellent record of
providing long-term performance
for property owners.
This long-term performance is predicated
on a few simple rules that most roof consultants,
specifiers, and designers know,
but on occasion, may overlook. While these
rules are applicable to built-up roofing, several
of them apply to most low-slope systems.
This article revisits these rules and
why they should be heeded.
While ASTM D312 Type III and Type IV
represent approximately 95% of the hot
asphalt sold for use in the roofing industry
today and are acceptable for use in most
geographic areas of the country, this rule
must still be stated. ASTM D312 is the standard
specification for asphalt used in roofing,
and asphalt used in a built-up roof
should meet this standard. The exception to
this rule is when a modified, mopping-grade
asphalt is specified and used. These newer
asphalts may meet
the intent of the
standard but not
the “letter of the
law.” The roof consultant
or specifier
should require that
the published performance
characteristics
of modified,
mopping-grade asphalts
be met when
ASTM D312 is not
applicable.
When using
D312 asphalt, the
correct type must
be used for both the
climate and the
slope of the application;
asphalt is a
viscoelastic material,
and an asphalt that is too “soft” can cold
flow on a fairly small slope on a hot summer
day.
Because Type II asphalt is not readily
available in most markets, and many roofing
contractors use the same Type III or
Type IV mopping-grade asphalt for built-up
roofs as for SBS modified bitumen installations,
this is a rule that is not often broken.
It’s simple: the saying, “if a little is good,
a lot must be better” does not apply to the
construction of a built-up roof. In general,
specifications and manufacturers’ recommendations
state that a built-up roof is
constructed with approximately 25 lbs. of
OC T O B E R 2006 I N T E R FA C E • 2 1
Figure 1. Cap sheet installed on steeper slope with the wrong type
of asphalt.
USE THE RIGHT ASPHALT
USE THE RIGHT AMOUNT OF ASPHALT
asphalt per square per
ply sheet with a tolerance
of +/- some
amount, e.g., 15%.
This quantity of asphalt
equates to a
cross-section about
the thickness of a
dime for each ply installed.
Why is breaking
this rule a problem?
Too little asphalt,
and the roof does not
have enough of the
component in the finished
membrane that
keeps water out. Too
much asphalt, and the
roof now has a plane
where slippage of the
plies and asphalt can
occur; in addition,
heavy interply mopping
weights reduce
the stabilizing properties
of the ply sheets.
Getting the right
amount of asphalt was
made much easier
with the introduction
of “equiviscous temperature”
(EVT) and the determination by
researchers that there is an optimum viscosity
for asphalt. When installed at this
optimum viscosity, the resulting amount of
asphalt will be approximately 25 lbs per
square. EVT also takes into consideration
how the asphalt is being installed (either by
hand mopping or by mechanical spreader),
specifying an apparent viscosity of 125 centipoise
for hand mopping and 75 centipoise
for mechanical spreader applications.
Adhering to the EVT specific for the asphalt
being installed helps ensure uniform, consistent
application of the asphalt in the
right amount because asphalt that is too
“cold” results in heavy moppings and
asphalt that is too hot causes light interply
mopping weights.
Built-up roofing systems have sufficient
strength to resist normal expansion and
contraction forces that are exerted on a
roof; however, they typically have a low ability
to accommodate excessive building or
substrate movement. Rephrased, if the roof
must be used to “hold the walls” together or
if the use of “loose-laid insulation” has a
benefit, then a traditional 3- or 4-ply builtup
roofing system is not a good choice.
A built-up roof typically provides high
tensile strength with low elongation. Guidelines
about where expansion joints should
be installed in the roofing system need to be
followed and should not be ignored by the
designer.
These guidelines include installing
expansion joints where the deck changes
direction, approximately every 200 feet
(although many consider that this dimension
can be expanded for membranes with
high elongation properties), where there is a
change in deck material, anywhere there is
a structural expansion joint, etc.
In addition, insulation or decking below
a built-up roof must be properly anchored.
In fact, Griffin and Fricklas claim that
“unanchored insulation boards is by far the
most common cause of BUR membrane
splitting.”1
It seems elementary, but roofs should
not pond water. Certainly there are exceptions
to this, such as limiting precipitation
run-off to assist antiquated stormwater systems
that cannot handle water from deluging
rains; but in general, ponding water
should be avoided on low-slope roofing
applications. A common definition of ponding
is water that remains on the roof 48
hours after precipitation has ceased. This
definition takes into account that there may
be some small areas of standing water
immediately following a rainfall, but the
water should have drained or evaporated
after a couple of days of drying.
Ponding water causes a myriad of problems
that most property owners want to
avoid: acting as a source for vegetation
growth; self-perpetuating deck deflection (a
small amount of water adds a concentrated
load to the deck, causing deflection, which
in turn increases the area ponding water,
22 • I N T E R FA C E OC T O B E R 2006
Figure 2. Ponding water reduces the life expectancy of a roofing system.
DESIGN FOR FORCES THAT ACT
ON THE ROOF
ROOFS SHOULD NOT HOLD WATER
which increases the load, etc.); allowing a
much larger amount of water to enter the
roofing system (and, ultimately, the interior
of the building in the event of a small defect
in the roofing system); and accentuating
and accelerating the effects of ultraviolet
exposure on asphaltic-based materials.
To avoid ponding water, designers
should provide a minimum of 1/4 inch per
foot slope in the structure. This is often the
best method for the property owner,
because when slope is properly constructed
into the structure, one doesn’t have to
worry about it in future re-roofing programs.
Where the structure does not provide
adequate slope, the use of tapered
insulation or a tapered fill can provide
slope. The installation of additional drains
and the use of crickets and saddles can also
either eliminate ponding water or minimize
its extent.
While today’s fiberglass built-up roofing
felts are much more forgiving than their predecessor
organic felts, all materials must be
kept dry before and during installation to
prevent blistering in the roof system. Proper
storage is key. Do not overstock the roof. Use
breathable tarps to cover material on the roof
(plastic sheets used as covers can result in
condensation, causing the stored materials
to become wet). Store material on pallets to
minimize the possibility of material sitting in
water. Store rolls on end to prevent crushing,
and keep materials stored on the roof neat
and organized to prevent physical damage.
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OC T O B E R 2006 I N T E R FA C E • 2 5
Figure 3. Improperly stored materials should not be used in the construction of a
built-up roof.
KEEP MATERIALS DRY
In addition to proper storage of materials,
roofing materials should not be
installed during inclement weather or when
precipitation is predicted. Care should be
taken to make sure the substrate is clean
and dry before roofing and if the job
requires roofing at night or in the early
morning, make sure dew is not forming on
materials or substrate as the work progresses.
Each of these steps can help prevent
problems with the finished roofing system.
In straight mathematical terms, two
plus two does equal four, but in built-up
roofing, the math works this way: two plus
four equals four. Many roof consultants and
product manufacturers clearly state that
there should be no phasing of a built-up
roof. This is a clean and simple rule to
understand; if the roof being constructed is
a four-ply, built-up roof, then only as much
insulation should be installed as can be
covered the same day with all four of the
plies in the built-up roofing membrane.
Phased construction of a built-up roof
greatly increases the potential for blistering
of the membrane and does not allow for the
total number of plies to be installed in a
shingled fashion. Phased application contains
perils, such as roofing over a very
small amount of overnight precipitation or
dew that, even with the best of intentions,
can cause harm.
In reality, there are times when a roofing
contractor may get ahead of the ply
sheet installation and have installed more
insulation than can be covered in the same
day; or, weather may change quickly, both
of which necessitate getting the building
protected through the installation of two
plies in hot asphalt. When this occurs, both
the Asphalt Roofing Manufacturers Association
(ARMA) and the National Roofing
Contractors Association (NRCA)2 agree that
the continuation of the membrane on the
next work day should start with making
sure the roof is dry and clean, and then
with the installation of the required total
number of plies over this temporary roof;
ergo, 2 + 4 = 4.
This rule is controversial simply
because in practice, gravel stops have been
stripped-in with two plies of felt in many
applications across the country for many
more years than modified bitumens have
been available – and they’ve been available
for over 30 years!
The reality is that splitting of these felts
at the joints in a gravel stop will probably
occur because the 2-ply application cannot
accommodate the movement in the edge
metal and the two plies themselves have a
high coefficient of expansion and contraction.
Because of this, many tend to treat
gravel stop strip-ins as a maintenance item,
whereas this rule suggests that this maintenance
can be avoided altogether or minimized
by simply switching to a better alternative.
26 • I N T E R FA C E OC T O B E R 2006
TWO PLUS TWO DOES NOT
EQUAL FOUR
USE MODIFIED BITUMEN FLASHING
MATERIALS TO STRIP-IN METAL
Modified bitumen
membranes
are compatible
with built-up
roofing systems
and typically provide
superior performance
when
used as a flashing
material in the
construction of a
built-up roof.
Even on gravel
stops, when properly
installed,
modified bitumen
membranes can
accommodate the
metal and provide
a maintenancefree
solution for
the life of the built-up roof. In fact, modified
bitumen membranes provide the basis for
our final rule – Rule #8.
In general, the growth and acceptance of
modified bitumen membranes has had the
added benefit of creating flashing details
that have been shown to provide great performance
over a long period of time. These
flashing details are now recognized as often
providing the best performance available for
the completion of a built-up roofing system.
Another benefit of using modified bitumen
flashing membranes is that the construction
of these details is often the same or
similar, whether the finished membrane is a
built-up system or a modified bitumen system
– so contractors don’t have to worry
about training their crews on another set of
criteria, and confusion on how a particular
detail should be flashed is minimized or
eliminated.
Test your knowledge of roofing with the
following questions, developed by Donald E.
Bush Sr., RRC, FRCI, PE, chairman of the RRC
Examination Development Subcommittee.
1. Simplified heat transfer
calculations normally used for
building design require
knowledge of what four
indexes of heat transmission?
2. To qualify as thermal
insulation, what must the
maximum k-value be of the
material being used?
3. Radiation accounts for the
extremes in roof-surface
temperatures above and below
atmospheric temperatures.
How much can an insulated
roof’s surface temperature
differ from the atmospheric
temperatures during clear,
sunny days and clear,
cloudless nights?
4. Solar cooling load is
dissipated via what four basic
mechanisms?
5. To qualify as a cool roof, what
are the requirements of the
standard set by the Heat
Island Group of the
Environmental Energy
Technology Division of the
Lawrence Berkeley National
Laboratories?
Answers on page 28
Figure 5. Flashing details are critical to the long-term performance of any roofing
system.
OC T O B E R 2006 I N T E R FA C E • 2 7
Figure 4. Modified bitumens perform well as metal flashing strip-in
material.
USE MODIFIED BITUMEN MEMBRANES
AS A FLASHING MEMBRANE
Final Comments About These Simple Rules
There are more options available to the
roofing professional today than at any other
time in history. These eight simple rules
represent basic items that should not be
ignored in the design and construction of a
built-up roof. Certainly there are other
guidelines and requirements that must be
met to realize a good roofing system; in fact,
for each type of roofing technology, every
professional has his or her own set of rules,
such as “do not install product that has an
expired shelf-life.” However, regardless of
how complicated this industry becomes, if
we remind ourselves of the simple rules that
we should know and understand, then
specifying, installing, and enjoying a roof
that performs will be much easier.
References
1 C. W. Griffin and R. L. Fricklas,
Manual of Low-Slope Roof Systems,
4th Edition, McGraw Hill, New York,
NY 2006
2 ARMA/NRCA BUR Quality Manual
Helene Hardy Pierce, FRCI, is executive director of contractor
services at GAF Materials Corporation. She was named a
Fellow of RCI in 2005 and is on the board of the RCI
Foundation.
Helene Hardy Pierce, FRCI
Answers to questions on page 27:
1. a. Thermal conductivity – k = heat
(BTU) transferred per hour
through a 1-inch thick, 1-foot2
area of homogeneous material per
˚F temperature difference from
surface to surface.
b. Conductance – C =
is the corresponding unit for a
material given thickness
c. Thermal resistance – R = 1/C
indicates a material’s resistance
to conductive heat flow. The unit
for R is ˚F • h • ft2/BTU.
d. Overall coefficient of
transmission U is a unit like k
and C, measured in BTU
transmitted per hour (BTU/h)
through 1 ft2 of construction per
˚F from air on one side to air on
the other,
All component materials that
make up the roof assembly,
including the inside and outside
air films, must be considered in
the total resistance computation.
2. k-value of 0.5 or less.
3. a. Clear sunny days – +75˚F
b. Clear cloudless nights – 10˚F or
more
4. 1. Surface reflectance
2. Surface emittance
3. Conductance
4. Convection
5. 1. Minimum solar reflectance = 0.70
and minimum thermal emittance
= 0.75 or
2. Minimum emittance less than
0.75, minimum reflectance =
0.955 – 0.34 emittance
Reference: Chapter 5 — Manual of Low
Slope Roof Systems
thickness
k
ΣR
(U = 1 ) .
28 • I N T E R FA C E OC T O B E R 2006
An updated and revised version of the ANSI/SPRI “Standard Test Procedure for
Determining the Withdrawal Resistance of Roofing Fasteners, ANSI/SPRI FX-1-
2006,” has been officially canvassed and reaffirmed as an official national standard
in accordance with protocol established by the American National Standards
Institute (ANSI).
SPRI, the association representing sheet membrane and component suppliers
to the commercial roofing industry, developed this standard for measuring the
pullout resistance of roofing fasteners in field conditions. It was first published in
1996, revised in 2001, and now, in accordance with ANSI requirements calling for
the re-canvassing of standards every five years, FX-1 has been reviewed and reissued
in a more user-friendly version.
“While it can be used on both new and older decks,” SPRI Technical Director
David Roodvoets adds, “this standardized test is particularly useful in a reroofing
situation.” The test’s significance is enhanced by the fact that its findings are
accepted by all industry segments, including fastener suppliers, membrane manufacturers,
and Factory Mutual Engineering and Research Corp. In cases where fastener
spacing or amounts are questioned, this test method can be used to validate
actual field requirements. The FX-1 Standard has become a key component in verifying
the suitability of steel deck with 10-ft-wide mechanically attached systems,
particularly when complying with Factory Mutual and the tensile strength of the
deck is unknown.
All SPRI standards can be downloaded free of charge at the SPRI website
www.spri.org.
UPDATED VERSION OF ANSI/SPRI
FASTENER STANDARD FX-1 NOW AVAILABLE