by Lawrence Evensen
The combination
of drawings and specifications encompasses the require-
ments for a project. Although far more like a technical
paper than a novel, these combined documents can in some
ways be seen as the concise narrative detailing the design
and construction of a project. The writer's goal is to
create comprehensive documents telling the building's
'story'. Unfortunately, some drawings and specifications
leave out enough details that they read more like a Sherlock
Holmes mystery, but without the 'gotcha' finale to tie
up the loose ends. This situation can be particularly
true when it comes to roof penetrations.
In this case, the mystery is what component will provide
a watertight assembly for each of the various penetrations
shown on the roof plan. Without penetration details and
complete specifications, the story is incomplete and the
roofing installer is left to assume unfair responsibilities
or attempt to interpret the design team's intention.
An understanding of roof flashings and counterflashings,
and how they perform their waterproofing tasks, is a
necessity. This article concentrates on roof penetrations
and the methods of installing those aforementioned products
and systems to aid in comprehensive roof plans. Following
some of the practices suggested in this article should
result in a smoother-running construction project.
The specifying pitfall
The building envelope designer must be extremely diligent
when writing the specifications for roofing details because
the result of poor planning is confusion and a higher
risk of litigation. A roof membrane is a watertight,
non-permeable cover unless damaged, penetrated, or bypassed.
In a perfect world, roofing contractors would not have
to deal with complication typically arising during the
construction process and there would never be any leaks.
A problem is created for the design professional when other
trades (e.g. electric, refrigeration, structural
steel, and HVAC) install their products and break the watertight
cover. Clearly written designs directed at each of the
trades will place waterproofing responsibility in the hands
of the individuals with the expertise and knowledge required
for creating failsafe, waterproof, penetration solutions.
When the chain of responsibility for waterproofing a roof
penetration is not properly delineated, conflicts can be
created since most sub-trade contractors are concerned
only with the equipment or structure they have contracted
to produce.
It is imperative subcontractors take responsibility
for the roof penetrations they create and start considering
how their work affects the overall building envelope.
Properly written specifications should put the responsibility
of appropriate waterproofing design into the contracts
of each of the parties. If specifications and details
are clearly defined in advance, the roofing contractor
is not put into the impossible position of creating details
for penetrations that are proprietary to the various
sub-trades. Closing this communication gap goes a long
way toward preventing water from bypassing the monolithic
roof cover, and keeps the lawyers at bay.
Flashing basics
Over the centuries, roof construction has evolved
to include two waterproofing techniques - flashings
and counterflashings. These materials create a rise in
the roof's membrane high enough to keep the elements
from entering the waterproofing membrane, with a cover
over the rise. The roof rise feature, or flashing, facilitates
water runoff as long as weather conditions are not
extreme enough to overflow the rise. The cover of the
flashing, or counterflashing, is designed to allow water
to shed over or around the flashing opening. there are
many classes of roof flashings and counterflashings,
including those products specifically designed for:
- bases;
- chimneys;
- copings;
- eaves and fascias;
- valleys and
- roof penetrations
For each class of flashing, the law of gravity and the
rules of physics for the water's flow are the same. The
roof flashing is constructed to rise higher than the
uppermost expected water level from a weather event,
and is counterflashed to cap its opening, allowing gravity
to direct water away.
Potential problems with penetration
Flashings for roof penetrations, projections, and equipment
stands are designed with the flashing/counterflashing
methodology. Pipes, conduits, vents, and support legs
use a sleeve or 'jack' flashing to create the rise in
the roof's level.
As a general rule of thumb, roofing product manufacturers
follow the guidelines of the National Roofing Contractors
Association's (NRCA's) Roofing and Waterproofing
Manual. In the third edition of its "Handbook
of Approved Roofing Practices," Sections 1 & 2
recommend using a metal flashing or inserting roof jacks
into the membrane for projections through the roof's
membrane no lower than 203 mm (8 in.), and not higher
than 356 mm (14 in.) above the finished roof level on
low slope roof applications.
Steep slope roof applications can have a rise as minimal
as 63.5 to 76 mm (2.5 to 3 in.) since the grade virtually
guarantees a water event will not be high enough to overflow
the flashing even in extreme weather conditions.
Many penetrations through a roof covering can be waterproofed
using the respective manufacturer's standard details.
Round penetrations - such as plumbing vents, electrical
conduits, HVAC chiller lines, domestic water lines, natural
gas, and other pipes - can be matched to a pipe flashing
jack with the proper outside diameter. Nearly all designers
call out a flashing method at these locations.
It is the responsibility of the general contractor to
ensure the appropriate installer provides properly sized
flashing for each of the roof penetrations on a project.
However, when the roof penetration is not a standard
round geometry or is not detailed on the drawings, or
when sub-trades do not provide the necessary flashing
as part of their work, the roofing professional is forced
to create a waterproofing detail on the fly. This is
complicated by the great variety of structures and mechanical
devices used on a roof.
For example, many structures have equipment located
on the roof; for safety or aesthetic reasons, this equipment
is often hidden behind a screen. The screens are built
out of very solid material (e.g. structural
steel) to ensure their capability to withstand high wind
loads. It is not unusual for equipment screens to employ
several hundred roof penetrations made from square, angle
iron, or H-beam steel. The use of non-round steel support
structures at these locations makes standard details
difficult to write, creating a problem for the roofing
contractor that can be especially complex to solve.
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Help from above for
St. Luke's
To better understand the need for clarity in waterproofing
specifications, it can be useful to see the lessons learned
from a previous project. St. Luke's Outpatient Center is a
new medical building located in St. Louis, Missouri. In late
2007, Ryan Freeman, the job superintendent of McCarthy Building
Companies, discovered the roof details provided by the roofing
material's manufacturer were inadequate. He was faced with
120 steel support posts (102 or 152mm [4 or 6 in. tall] that
were part of the equipment screen. If improperly waterproofed,
the leaks would have been numerous.
The flashing detail included a site-fabricated flashing
to be created by the roofing crew out of the white, single-ply
roof product. The plan was to make the flashing, seal the
top edge against the steel post, and draw it tight using
a stainless steel band clamp. The problem with this technique
is no matter how much torque is applied to the clamp, there
will be loose gaps on the flat part of the square post.
Square pegs do not fit in round holes.
In search of a better solution, Freeman
contacted Ron Carter of RNC Enterprises, a local technical
advisor for innovative construction products. Carter suggested
installing storm collars over the field-fabricated flashing
to provide the waterproofing each square post required.
(NRCA approves storm collars for this design purpose.)
The steel support posts had already been installed so any
counterflashing would need to be a retrofit design.
Choosing the appropriate product was the next challenge.
Any storm collar used for these posts had to fit certain
design criteria. The storm collar needed to be:
- a retrofit design (as the equipment screen posts had
already been established);
- installed using the existing roofing crew labor;
- made of material compatible with the metal posts, without
worry of corrosion to the post or structure;
- free of sharp edges;
- able to spring back into place in the event of disturbance
by workers or pedestrians walking on the roof; and
- aesthetically in line with the building's white roof
covering.
Additionally, the storm collars had to have a life expectancy
of at least 20 years - the intended duration of the roof.
Carter used retrofit storm collars made of ethylene propylene
diene monomer (EPDM) rubber as the counterflashing. By
ordering these products with square cut-outs sized exactly
to match the metal posts, the storm collars were suitable
for the field-fabricated roof detail. There were neither
corrosion compatibility issues nor sharp edges; further,
if the rubber is impacted by pedestrian traffic, it bends
and springs back automatically. Installed using a simple
nut driver, the 'off-the-shelf' collars can open up to
wrap around myriad geometric shapes, while creating the
rise in the roof level needed.
A/C specifications are not always cool
By using the same roof rise and cover technique, almost all
other odd or difficult roof penetrations can have waterproofing
specifications written for them. Another example illustrating
how a common problem can be solved is the luxury apartments
built by Trammell Crow Residential on the corner of Walnut
Street and Colorado Boulevard in Pasadena, California.
This project includes 265 luxury dwellings that are cooled
using individual split central air-conditioning systems.
A split system is made up of two copper
refrigerant tubes connected to an indoor coil and an outdoor
condenser heat pump unit. (The smaller of the lines is
called a liquid line and the larger, a suction line.) The
lines are filled with a chemical refrigerant with a boiling
point low enough it evaporates at relatively low temperatures
and takes heat and moisture out of the air as it passes
through a coil installed inside each apartment dwelling.
The refrigerant travels in a closed loop between the coil
and the roof top condenser.
Additionally, a low-voltage wire inside
a watertight conduit provides an electrical connection
from the coil to the outdoor condenser. The two tubes and
conduit combination create a tightly grouped roof penetration
called a 'line set'.
The 265 heat pump condenser units on the
Trammel Crowe project were mounted on platforms located
on the flat areas of the multi-story building's roofs.
Line sets are a prime source of water intrusion because
it is extremely difficult to seal between the groups of
tubes. As a further complication, the large suction line
tube is always insulated with soft foam as an energy conservation
measure. Water that gets inside the insulation follows
the tube like a highway through the walls of the building,
ultimately creating leaks in apartments. The Walnut Street
project had first floor apartments five stories below the
roof decks, so a leak at ground floor could create hard
to identify long-term problems.
EPDM storm collars again offered a solution
for this roof penetration problem. In this case, the rise
in the roof level was created using a standard 37 to 50-mm
(1.5 to 2-in.) diameter lead pipe flashing jack and covering
the rise with an EPDM molded storm collar. The factory-made
storm collars are pre-engineered to accept three closely
grouped tubes and include two 20-mm (7/8-in.) and one 10-mm
(3/8-in.) nipple holes. By passing the three independent
components of the line sets through nipples molded into
the rubber collar, each is separated and secured by a stainless
steel hose clamp. An added benefit for using rubber is
the assembly has no bi-metal contact and zero corrosion
potential.
A plan was put in place to have each line
set enter the building through lead pipe flashing each
with 200-mm (8-in.) high risers installed into the roof
system following the manufacturer's standard construction
details The roof was a five-ply, built-up, smooth surface
roof installed over a protective layer of rosin sheet.
By using the roofing manufacturer's installation details,
the flashings were installed in a watertight manner and
the result was a specification that allowed issuance of
the 20-year manufacturer's warranty.
After the roof jack flashing had been
completely installed, the rubber storm collars were placed
down onto the copper tubes by way of the pre-molded nipples.
The storm collars were located over the already installed
lead flashings; the stainless steel hose clamps on each
of the three nipples permanently secured the storm collar
as covers over each flashing. Line-set storm collars create
the perfect counterflashing for any type of pipe jack by
yielding 'umbrellas' that keep the water on the roof.
Conclusion
The National Roofing Contractors Association (NRCA) recommends
metal flashing or roof jack insertion into the membrane
for projections between 203 and 356 mm (8 and 14 in.) above
the finished roof level. These flashing jacks can be fabricated
from many classes of materials including lead, steel, aluminum,
and even single-ply materials. (The material used is often
determined through manufacturer preference, design/construction
professional experience, compatibility, aesthetics, and
cost.) All that is required is a storm collar that attaches
to the penetration, or group of penetrations, which acts
as the counterflashing atop the flashing jack.
The installation of the retrofit storm
collars to solve St. Luke's waterproofing problem is a
great example of how an innovative product can improve
roof penetration details. By adding these storm collars
to the roof plan, each penetration becomes manageable,
taking the mystery out of inadequately written details.
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