Implications of ASCE 7-16 on Re-Covering or Replacement of Existing Roofs

INTRODUCTION
Since the initial edition of ASCE 7,
Minimum Design Loads and Associated
Criteria for Buildings and Other Structures,
in 1988, the design component and cladding
(C&C) wind pressures on a roof have varied
little for a given location. While the method
of determining wind speeds has changed
from the “fastest mile” to the “three-second
gust,” and the mean return interval has
increased, the allowable strength design
(ASD) pressures are remarkably consistent.
The diagram for the pressure zones—Zone
1 (interior), Zone 2 (edge), and Zone 3 (corner)—
has not changed since that initial
ASCE 7-88.
However, the new edition, ASCE 7-16,
has implemented significant changes to
the diagram for the pressure zones, which
makes the design of reroofing a challenge,
because the existing roof structure may not
be able to resist the changes in wind uplift
pressures within building code requirements.
A PROTOTYPE BUILDING
As an example, consider an office building
located in Kansas City, MO.
The building has plan dimensions of
100 by 100 ft., with a mean roof height of
30 ft., without parapets. The roof is “flat,”
with minimal pitch for drainage provided
by tapered insulation. For wind pressure
design purposes, the Risk Category is II and
the Exposure is C.
For ASCE 7-10 and older editions, the
roof zoning is shown in Figure 1, and the
ASD uplift pressures are shown in Table
1. Users of the
ASCE 7 standard
should be aware
that prior to the
2010 edition,
the wind velocities
used and the
pressures calculated
are ASD
pressures, and
would be multiplied by 1.6 to be converted
to ultimate pressures. Starting with ASCE
7-10, the wind velocities used are higher,
ultimate wind velocities. The calculated
pressure is therefore an ultimate pressure,
and would be multiplied by 0.6 to convert
the pressure to an ASD pressure. Structural
engineers would use an ASD pressure when
using allowable strength design, and an
ultimate pressure when using the load and
resistance factor design (LRFD) method of
design.
As can be seen in Table 1, while the
design wind velocity went from “fastest
mile” in ASCE 7-88, to the “3-second gust”
in ASCE 7-02, to a longer return interval in
ASCE 7-10, the final design pressure did
not change by more than 7% in any zone.
Now consider the zone diagram (Figure
2) and ASD uplift pressures (Table 2) for
this same building using the ASCE 7-16
standard.
The implications of this change become
apparent when the ASCE 7-16 pressures
are overlaid upon the ASCE 7-10 pressures
(Figure 3). You can see that large sections
of the roof must now resist substantially
higher uplift pressures.
The result clearly shows that the roof
1 0 • RC I I n t e r f a c e J u l y 2 0 1 8
ASD Pressure (psf)
ASCE 7-10 ASCE 7-02 ASCE 7-88
Zone 1 -20.0 -20.4 -21.1
Zone 2 -33.5 -34.2 -35.4
Zone 3 -50.4 -51.5 -54.2
V (mph) 115 90 78
Figure 1 – C&C roof zones – ASCE 7-10 and previous. Table 1 – ASD C&C wind uplift pressures, from 1988 to 2010.
Photo credit: iStock.com/cash14
J u l y 2 0 1 8 RC I I n t e r f a c e • 1 1
structure that was designed for ASCE 7-10 or older wind pressures can be
substantially overloaded when analyzed using ASCE 7-16 wind pressures.
While it is quite possible to design the roofing membrane for the ASCE 7-16
wind pressures, there may be a problem if the permitting agency (the authority
having jurisdiction) requires the existing steel roof deck or other areas to
be able to resist these same higher pressures.
Figure 2 – C&C roof zones – ASCE 7-16.
Figure 3 – ASCE 7-16
pressures over ASCE
7-10 pressures.
ASD Pressure (psf)
ASCE 7-16
Zone 1 P -20.6
Zone 1 -35.8
Zone 2 -47.2
Zone 3 -64.4
V (mph) 122
Table 2 – ASD C&C
wind uplift pressures –
ASCE 7-16.
1 2 • RC I I n t e r f a c e J u l y 2 0 1 8
One instance where this structural retrofit
may come into play is when Section
503.12 of the 2018 International Existing
Building Code (IEBC) is enforced. When
removal of roofing materials from over 50%
of the roof area of a building occurs in an
area where the ultimate wind speed is over
115 mph, the roof deck and connections
must be able to resist at least 75% of the current
wind loads. The 115-mph ultimate wind
velocity encompasses the entire states of
Hawaii and Florida, as well as the Atlantic and
Gulf coasts for a Risk Category II (ordinary
risk) structure. For a Risk Category III or IV
structure (which includes essential facilities
such as hospitals, fire and police stations,
and structures designated as storm shelters),
the entire U.S. falls into this condition.
EFFECT ON THE STEEL ROOF
DECK ANCHORAGE
Let us focus specifically on steel roof
decks. When IEBC Section 503.12 is applicable,
the first thing to consider is if the
attachment of the steel roof deck is adequate
for the increased uplift pressures.
For instance, there are regions of the
roof that were in the previous Zone 1 that
are now in the new Zone 2. This would
increase the uplift pressure from 20.0 psf
(ASCE 7-10) to 47.2 psf (ASCE 7-16). A
22-gauge, 33- or 40-ksi roof deck, welded
to the supports using ½-in. visible diameter
arc spot welds (“puddle welds”) in a 36/3
pattern, would have an allowable uplift
capacity of 32 psf (RDDM, 2012). While a
36/3 pattern is not commonly used today,
it can be found in some older structures.
Seventy-five percent of 47.2 psf is 35.4 psf,
which exceeds the 32-psf available capacity
of this connection pattern. This would be
adequate for the original design by ASCE
7-10 or older, but not adequate for ASCE
7-16, and would require additional deck
anchoring. This may require removal of all
existing roofing materials in order to add
additional fasteners.
Furthermore, concentrated line loading
for uplift of mechanically attached membranes
can severely overload the steel roof
deck and its attachment, if the system was
designed considering a uniformly distributed
uplift load. This is addressed in an article
originally published in Structure (Fisher and
Sputo, 2017), which is reprinted in this
issue of RCI Interface (see page 23).
RECOMMENDATIONS
When re-covering or replacing an existing
roof, consideration must be paid to
increased uplift pressures that can be
required by code or the permitting agency.
Additionally, the effect of concentrated line
loads resulting from mechanically attached
membranes also needs to be considered.
Solutions do exist—some more costly than
others. The services of both a roof consultant
and a licensed structural engineer
who are familiar with the design of both
the proposed roofing system and the structural
system involved should be engaged to
determine options that will comply with the
codes in effect.
REFERENCES
ASCE 7-10: Minimum Design Loads
for Buildings and Other Structures.
American Society of Civil Engineers,
Reston VA, 2010.
ASCE 7-16: Minimum Design Loads and
Associated Criteria for Buildings and
Other Structures. American Society
of Civil Engineers, Reston VA, 2016.
J.M. Fisher and T. Sputo. “Are Your Roof
Members Overstressed?” Structure.
March 2017.
Roof Deck Design Manual, 1st Edition.
Steel Deck Institute, Glenshaw PA,
2012.
Thomas Sputo, PhD,
PE, SE, FASCE, is
the technical director
of the Steel Deck
Institute, a trade
organization of steel
roof and floor deck
manufacturers.
Additionally, he is
a consulting structural
engineer with
the Gainesville, FL,
firm of Sputo and
Lammert Engineering, LLC, and an emeritus
senior lecturer in the Department of Civil
and Coastal Engineering at the University of
Florida. He holds a PE license in twelve states
plus an SE license in Illinois.
Thomas Sputo, PhD,
PE, SE, FASCE
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