Lightweight Insulating Concrete
As a Choice Re roofing System
By Hubert T. Dudley, Ray Wetherholt, Zen Szewczyk, and Herb Cannon
ightweight insulating concrete has been used as a roofing
cases, a minimum of one inch. This design innovation resulted in
substrate for more than 60 years. It is a combination of
a lightweight insulating concrete system that is viableeven
cement, air, water, and an air-entrained aggregate. The aggregate advantageousfor use in reroofing applications.
provides improved surface characteristics, including a smoother Lightweight insulating concrete systems are placed using
finish, and a reduction in cracking. When aggregate is not mixing and pumping equipment. An average of 8,000 square feet
included, the lightweight insulating concrete is referred to as cel of lightweight can be installed in one day on reroofing projects.
lular concrete. Both types--aggregate lightweight insulating A higher coverage rate averaging 15,000 square feet per day can
concrete and cellular concrete--provide similar solutions to be achieved on new construction projects.
common reroofing problems. On a typical reroofing application, a lightweight insulating
The typical density range for lightweight insulating concrete concrete system can be installed directly over the existing roof.
is 22-38 pcf. (Lightweight insulating concrete should not be con Following a structural review and repair of the existing roof and
fused with lightweight structural concrete, which has a minimum flashing to a watertight condition, the installation of a light
density of 50 pcf.) Table 1 shows the typical range of physical weight insulating concrete system begins with placement of a 1/8
properties for materials used in insulating concrete systems. inch minimum slurry coat of lightweight insulating concrete.
The slurry corrects any substrate
irregularities or low spots, and
bonds the polystyrene to the
substrate. Type I expanded
polystyrene board, designed with
a pattern of holes, is placed into the
slurry in a stair-step configuration
to create the needed slope-to-drain
contour and achieve the required
insulation values. A top fill of
insulating concrete encapsulates
the EPS board, bonding it to
the substrate. The pour is
screeded to a smooth, planar,
monolithic surface suitable for
Lightweight insulating con
crete systems are used effectively
to solve common reroofing and
recover problems in applications
where bottom side venting is
Table 1 not available, including:
Originally, lightweight insulating concrete systems were Providing positive in-place slope-to-drain
installed with a varying thickness of concrete to provide slope. Correcting existing surface irregularities
In the early 1970s, a constant thickness of polystyrene was Avoiding disruptive tear-off noise and debris
incorporated into the system to provide increased insulation. In Avoiding asphalt fumes that accompany hot-mopped,
the late 1970s, the system concept evolved to include stair- tapered board systems
stepped thicknesses of polystyrene. This created a slope-to-drain
Creating potential initial cost savings due to labor savings,
contour that effectively removes water from the roof's surface,
and long-term life cycle cost benefits
reduces weight, and increases insulation values. Additionally,
maximizing the use of polystyrene allows a significant reduction
Encapsulating/covering roofing membranes that cannot
in the thickness of lightweight insulating concrete to, in some be disturbed
4 Interface July 2001
deck with a gravel-surfaced, hot asphalt built-up roof that
included four plies of 15# asbestos felt. Located in a park-like
setting surrounded by tall fir trees, the roof was difficult to access
from the sides. The original surface was virtually flat, and the
drains were located in a geometry that did not allow a standard
tapered system to be economically feasible.
Like most hospitals, Eastside required that the noise and
fume smell be minimized. Slope had to be created for drainage,
and the hospital wanted a system that would provide increased
insulation value, if possible. With lightweight insulating con
crete, we could meet all of these objectives.
The old roof was removed down to the existing insulating
concrete and allowed to air dry for a few hours. An upside-down
Photo 1--Lightweight Insulating Concrete system crosssection cap sheet was nailed down using Zono-tite fasteners and was
To illustrate the viability of lightweight insulating concrete sys covered with a heavyweight SBS-modified base sheet torch
tems as a problem-solving tool in such applications, the follow adhered to the upside-down cap sheet. This provided a tempo
ing case studies of LWIC systems on reroofing and recover rary roof and allowed for removal of an entire roof area before
projects are provided. the lightweight insulating concrete subcontractor performed his
portion of the work.
Ray Wetherholt Roof specification and slopes were laid out initially on the
Wetherholt and Associates design drawings for bidding purposes. Once the contractor was
Let me start by explaining that, initially, I was not a believer selected, the slopes were reviewed and modified as needed to
in lightweight insulating concrete, but began using it when I accommodate doorways and embedded reglet heights.
became involved in several projects. In those projects, it was the Overflow scupper openings were too low and had to be mod
best option for the architect and construction team to get slope ified to allow for placement of the flanged, stainless steel sleeve.
to drain and avoid relocation of drains and complicated tapered The lightweight insulating concrete subcontractor completed
system details. the 20,000 square foot job in a few days. The system was
Lightweight insulating concrete can be another tool in the allowed to cure for three to seven days, and an inverted cap
toolbox of the professional roofing consultant. sheet and new SBS-modified two-ply system were installed over
The three projects I would like to review are located in the the lightweight insulating concrete.
Seattle, Washington, metropolitan area. Each was unique and The lightweight insulating concrete contractor was able to
had different requirements. The product used on all three pro set up his tractor trailer rig in the parking lot, batch on the
jects was NVS ("Non-vented System," marketed by W.R. Grace) ground, and pump the material six stories to the roof. The pro
lightweight insulating concrete. ject required extensive coordination among the hospital staff,
facility managers, and the contractor. The project was completed
eight years ago, and the roof is still performing well.
Eastside Hospital with GravelSurface, Hot Asphalt Roof
The existing roof of the Eastside Hospital was lightweight
insulating concrete over a cast-in-place concrete
Photo 2--Eastside Hospital
July 2001 Interface 5
An inverted cap sheet was attached
over the lightweight insulating concrete
using NVS fasteners, and a torch-grade,
heavyweight SBS-modified bitumen
membrane was applied. The roof has
been performing successfully since the
project's completion in 1997.
Medical Clinic with Old Hot Asphalt
In 1991, the roof of the medical
clinic needed slope for drainage, and
the geometry worked for a tapered
insulation system over the structural
concrete roof deck. The roof was orig
inally designed as a plaza deck area
but was never used as such. The bid
documents specified a foam adhesive
to secure the insulation with an alter
nate for use of hot asphalt. The roof
system specified was polyisocyanurate
board over the concrete deck, 3/4-inch
perlite, a mopped fiberglass base
sheet, and a 2-ply heavyweight SBS-
modified bitumen system.
The owner and the low bid con-
Photo 3--Lutheran Bible Institute
Flat Roof Church with Coal Tar Pitch Roof
The existing roof on this church was flat, grav
el-surfaced, and had complicated drainage geome
try. Several "consultants" (including church
members and roofers) examined the roof and pro
posed adding drains, recovering the existing roof,
and spraying foam and a coating. Contractors were
worried that coal tar pitch would be considered
hazardous waste and associated disposal costs
would be incurred.
The addition of drains through the structural
concrete deck was not economically feasible
because the underside of the roof deck is exposed
to the church interior. The Seattle weather, togeth
er with the dearth of qualified SPF contractors,
took away the option of SPF, leading us to consider
lightweight insulating concrete. A structural engi Photo 4--Medical clinic.
neer was retained to review the building structure. He determined tractor reviewed the roof system to identify possible cost savings
that after the loose gravel from the coal tar roof was removed, and options to avoid hot asphalt fumes, which would be particu
lightweight insulating concrete would add a minimal, acceptable larly problematic on a medical facility. The contractor proposed
amount of weight to the structure. a lightweight insulating concrete system combined with an APP-
On this project, the underlying roof was left in place, and modified membrane torched down over a nailed base sheet. The
lightweight insulating concrete was installed over it. The system system met the owner's budget requirements and eliminated
included EPS board placed in a stair-step configuration to create unacceptable hot asphalt fumes. Later, the owner successfully
an effective slope-to-drain contour. Edges were raised to accom used the same system on the rest of the building's 10-story roofs.
modate the added thickness. In most cases, curbs were raised to Examination of the roofs in 1999 found them in good condition.
accommodate the insulation thickness as needed. On most of the
roof, 1/4-inch per foot slope for drainage was obtained. In a few
locations, the slope was limited by the chapel's steep copper roof.
6 Interface July 2001
Photo 5--Eastdale School
Eastdale School, Ontario, Canada
You would think after performing thousands of condition
reports and preparing the same amount of design drawings and
specifications, one would rarely come across a situation that pre
sents a unique problem. Well, after 17 years, it happened.
Our firm was called in by a school district to evaluate the
extent of deterioration of the roof assembly on a building that
was over 30 years old. The roof was the original four-ply asphalt
BUR. The owner was not experiencing many leaks. However,
due to the age of the roof, the owner thought it might be pru
dent to perform some preventive maintenance to extend the life
of the roof indefinitely. The owner thought it also prudent to
retrofit the roof with additional insulation, as it was believed the
roof did not have good thermal resistance.
We arrived on site to find the roof completely under water,
so our initial investigation was limited to some wading around
and interior investigation. One thing we noted was that there
were relatively few roof drains. The existing deck varied from
area to area and consisted of poured-in-place concrete, metal
deck, wood deck, and concrete slab. The underside was plaster
ceiling. What little waterstaining existed was limited to around
openings. Some time later, when the roof surface was dry, we
returned to complete our evaluation. Infrared scans, core-cuts,
and probes all indicated saturation above all decks. The continu
ity of the decks and a good vapor barrier had been hiding a
secret for some time. Very little preventive maintenance had ever
been performed on this roof.
Roof replacement should be easy! Remove and replace.
Drainage calculations showed that the size of the rainwater lead
ers was adequate to get all the water off the roof; however, they
were few and far between. The distance between drains was as
much as 125 feet. The idea of adding more drains was thwarted
by an inaccessible ceiling space, fire-walls, and a fire-resistant
mineral fiber no longer used in building construction.
Random core samples confirmed that the existing poured-in
place concrete deck was very uneven with deviations up to two
inches. Introducing rigid tapered insulation would have yielded
July 2001 Interface 7
an irregular surface with unsatisfactory results. Furthermore, in Herb Cannon
order to achieve any success with rigid insulation, it would The Cannon Group
require a slope of at least 1/4-inch per foot. The insulation thick
ness would have been from 3 inches at the drain to 19 inches at Terminal Market at Hunts Point
the high point. The costs were rising. In 1994, the 35-year-old roof of the Terminal Market at
It didn't take long to conclude that the optimum solution was Hunts Point, New York, sustained severe damage due to high
lightweight insulating concrete. The existing roof was removed winds. The roof consisted of a poured gypsum deck, three-ply
to the deck. After deck repairs, a new vapor barrier was mopped built-up roof, polyisocyanurate insulation, and a mechanically
to the deck to serve as a temporary roof. Lightweight insulating fastened PVC membrane. An emergency removal of the PVC
concrete was pumped onto the roof surface where it would find membrane and polyisocyanurate insulation was necessary.
its level. The stair-stepped polystyrene would be installed at a Removal was followed by the installation of an SBS-modified
1/16-inch per foot slope. The top coat was installed and finished bitumen sheet to secure the interior of the building. Then the
smooth. After four to seven days, fastener withdrawal tests were building owners, the New York City Economic Development
conducted to determine if the fastener held a minimum 40 Corporation and the Hunts Point Terminal Produce Cooperative
pound load. The venting sheet was fastened to the concrete sur Association, Inc., began considering a full redesign for the facili
face and the base sheet was installed. Then a 24-hour rainstorm ty's 750,000-square foot roof. There were several circumstances
arrived. To everyone's delight, the water was draining off the unique to Hunts Point that had to be considered when evaluat
roof like never before. Even after the cap sheet was installed, the ing possible roof systems:
thickness of the overlaps had little effect on the ability of the The location of the facility, on the bay across from LaGuardia
roof to shed water. The project was completed to the satisfaction Airport, meant that the roof would continually be subjected
of the manufacturer, contractor, consultant, and the building to high winds.
owner--all within budget. The roofs of the 75-foot wide x 200-foot long bays did
not have adequate slope.
The three-ply, built-up roof contained
material that, per New York State regu
lations, either had to be removed or
The market, which sees approximately $4
billion of produce business per year and
operates 24 hours a day, 7 days a week,
could not afford to be shut down for
Lightweight insulating concrete offered a
solution for each of these concerns:
The finished monolithic deck, with its
encapsulated insulation board, is resis
tant to high winds.
The lightweight insulating concrete sys
tem solved the ponding water problem
by creating proper slope-to-drain.
It was determined in a structural review
that the lightweight insulating concrete
system could be poured directly over
the existing assembly. Pouring over the
old built-up roof encapsulated the felts.
This was acceptable according to New York
State regulations in lieu of abatement.
By eliminating the need for tear-off, the
market was allowed to remain in operation
during the entire project. The lightweight
insulating concrete pour took an average
of three days per bay.
Following the lightweight insulating con
crete pour, the project was finished with a base
sheet mechanically fastened to an 1-90 wind
Photo 6--Terminal Market at Hunts Point uplift fastening pattern. A two-ply, torch-applied
8 Interface July 2001
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