Most home builders are familiar with spray-in-place insulation
products, which have become increasingly popular in recent
years. But not many are aware that spray polyurethane foam
(SPF) is also used as a roofing material. Spray foam roofing
has been around for more than 30 years; it's used mainly to
reroof existing flat roofs on commercial and residential
Our company does SPF roofing and insulation in Northern
California, on both residential and commercial projects. And
since homes with flat or low-slope roofs are common in our
area, and many of them have old, leaky built-up roofing (BUR),
residential reroofing is a considerable chunk of our
In this article, I'll talk about reroofing a low-slope roof
with SPF. The project I'll focus on began when the homeowners
decided to install a photovoltaic power system. The PV
contractor told them there was no way to attach mounting
brackets to an old built-up roof like theirs without creating
leaks. He recommended reroofing and suggested using SPF; the
owners called us.
For a project involving a new roof, the process would be nearly
the same as the one I'll describe here except with less prep
Polyurethane, a plastic, was invented prior to World War II. It
was first used for roofing in the 1970s, when U.S. military
officials recognized its value as a lightweight, waterproof,
all-in-one covering and insulation. Since then, its use has
spread to the private sector.
Polyurethane foam is formed when two liquid components are
mixed at a 1-to-1 ratio inside the tip of a specialized spray
gun. It contains isocyanates, polyols, catalysts, and a
non-ozone-depleting blowing agent that causes tiny bubbles to
form in the mixture as it exits the gun. Bubbles continue to
form until the material has expanded to 30 to 50 times its
Dense closed-cell foam. Roofing foam
is similar to the foam used to insulate walls, but it differs
in density and permeability.
Open-cell wall foam is soft and permeable to moisture.
Closed-cell wall foam is denser and impermeable to moisture.
Roofing foam is denser still, and hard enough to walk on.
Because of its closed-cell structure, it is impermeable to
water and air, and is also a vapor retarder.
Foam as Roofing
As with any roofing system, foam has its pros and cons. Chief
among the former is its ability to conform to shapes that are
difficult to flash and waterproof with conventional materials.
Here's a brief overview of the product's other advantages, as
well as its limitations.
Excellent R-value. Many roofs, even
with rigid insulation above the sheathing, or batt or blown
insulation in the cavity below, permit significant heat gain.
This is especially true of BUR; most built-up roofs act like
giant solar heat collectors.
SPF roofing, however, resists solar heat gain; with the right
coating, it can be both Cool Roof-rated and Energy Star-rated.
Roofing foam has an R-value of about R-6.5 per inch of
thickness; a typical application is around 1 1/2 inches thick
(though some parts of the roof get more foam than
Long service life. Cured foam retains
its flexibility over time and is chemically very stable. The
only thing that can damage it — aside from being
punctured by a sharp object — is ultraviolet (UV) light.
A properly applied and maintained SPF roof should last the life
of the building.
Maintenance involves renewing its UV-resistant top coating
every 10 to 15 years. With upgraded systems, the recoating
cycle can be extended to 20 years.
Unlikely to leak. Most roof leaks
happen when water gets in through an open joint or seam in the
roofing material. If the roof contains several plies — as
BUR does — water can get in at one place, migrate
horizontally between layers, and come through the ceiling
somewhere else. Such leaks can be difficult to locate and
An SPF roof is a thick monolithic membrane; it has no seams,
joints, or plies. The only way it will leak is if a hole goes
all the way through it.
Limitations. SPF works well on sloped
roofs, but since it's not as attractive as shingles, people
tend to install it on low-slope roofs where it can't be seen
and where shingles would be a poor choice.
Also, foam roofing can't be installed year-round in all
climates. This is because at the time of application there must
be very little or no wind and the roof surface needs to be
absolutely dry and above 50°F.
Installing a spray-foam roof is not the cheapest option, but
future energy savings and a longer, more trouble-free service
life should offset its higher initial cost.
In our area, the cost to roof or reroof an average residence
with SPF runs between $5.50 and $10 per square foot.
Inspecting the Existing Roof
The first thing I determine when I'm inspecting an existing
roof is whether it should be stripped or swept. It's surprising
how many multiple-overlay built-up roofs we find — even
though they impose an excessive load on the structure and in
some areas aren't allowed by code.
In cases where there are multiple roof layers, it's best to
strip back to the sheathing. This lightens the load and
provides a smoother substrate for the foam.
If there is only a single layer of roofing, it's often possible
to sweep off the gravel, prep the surface, and apply a layer of
foam. This allows the building owner to avoid the mess and
expense of a tear-off, yet still end up with a lighter
When we inspected the roof of the home shown on these pages, we
found leaks, ponding, and an area with rotted sheathing. But
since the roof was just one layer, we decided to repair the rot
and install SPF over the existing membrane.
Ponding. In older buildings, sagging
and settling can lead to significant ponding. With many roofing
systems, this problem is difficult to repair — but with
SPF, low areas can be built up by applying more foam.
To find the low spots, I survey the "topography" of the
existing roof with a laser level. I measure the elevation at
various locations and draw a map to show our applicator where
the high and low spots are so that he will know where to add
more foam to get the roof to drain (see Figure 1).
Figure 1. The author uses a laser level to
measure the elevation at various points on the roof (above) and
then records the information (right). The map tells the
applicator where the low spots are so he can build up those
areas with a thicker layer of foam and thereby prevent
If we simply applied the same thickness everywhere, the low
spots would remain and water would continue to pond.
Prep work is extremely important, especially when foam is
applied over an existing roof. Some roofing contractors have
been known to show up, spray foam all over everything, coat it
once, and call it good. Although that kind of installation may
provide temporary relief from leaks, the homeowner will
experience problems over time.
For a proper job, the roofer must prep the surface, repair
damaged sheathing, replace sheet metal and flashings as
necessary, deal with drainage issues, and apply foam and
Surface prep. To achieve a good bond, all loose material must
be removed from the existing roof. That means sweeping up and
hauling away any loose gravel. Sometimes areas have been built
up with asphalt and gravel; if that material is coming loose,
we scrape it up with a spudding tool (Figure 2).
Figure 2. To prep the roof, workers sweep
up and remove loose aggregate (top left) and scrape up poorly
bonded areas of asphalt and gravel (top right). Once the roof
is clean, they spray on a primer to make the foam stick better
(right). The vertical posts — which were just installed
— will be used to support solar panels.
Once the major debris is gone, we clean the membrane by
sweeping up any accumulated silt and then dislodging the
remaining dust with a gas-powered blower. If the roof doesn't
leak, we power-wash it.
Sprayed foam will adhere to just about anything, but it's still
a good idea to prime the surface of the existing asphalt. We
use an airless sprayer to apply a special adhesion building
primer to the surface of the roof.
Sheathing repairs. When we find rot, we strip that portion of
the roof and replace framing and sheathing as necessary.
If a Class A fire rating is not required for the roof, we prime
the sheathing and apply foam over it. When a Class A rating is
required, we nail DensDeck (Georgia Pacific,
1/4-inch fiber-impregnated gypsum board, over the patched area
before priming and applying foam. There's no need to put
DensDeck on BUR because foam over BUR is a Class A
Sometimes we can reuse the existing sheet-metal vents, but we
always install new foam stop along the edge of the roof.
Edge metal. Edges are one of the weakest links in
any roof. To beef them up, we use a commercial-gauge metal foam
stop that's hemmed and kicked out to drip at the lower edge
Figure 3. Before SPF is applied, the
existing gravel stop is removed (top) and replaced with a new
stop designed for foam. The crew uses polyurethane sealant and
at least two nails at every lap (bottom) to prevent the joints
from flexing and cracking. Next, the stop is nailed 9 inches
o.c. to the edge of the roof.
We apply urethane sealant where the pieces lap, interlock the
overlaps, and use at least two fasteners at each joint. This
prevents the edge metal from lifting and causing the membrane
to crack in a critical area.
Vents and fans. If we strip the roof, the
existing roof jacks may need to be replaced; but if it's a
sweep job and the jacks are in good condition, we leave them in
place and entomb them in foam. Any unsatisfactory vents are cut
out and replaced.
Existing roof-mounted vent fans can present a problem because
the foam could raise the surface of the roof high enough for
water to run into them. We usually build a curb around each fan
and install a custom-made vented cap over the top; that way,
the old fan can remain in place and be serviced or replaced in
Never let a roofer foam in the fan without considering the
potential for leaks or the possibility that the fan will
someday need to be replaced.
Drains and scuppers. Most of the
homes we work on have scuppers or built-in metal dropout drains
that pass through the overhang and connect to downspouts below.
We replace the scupper linings at the same time that we replace
the edge metal.
We also cut out and replace the dropout drains. They may be
rusting through, and we want a clean new surface for the foam
to adhere to (Figure 4).
Figure 4. Although some sheet metal can be
reused, scuppers (top) and drains should be replaced so the
foam has a clean surface to bond to. Here, a roofer cuts out
the membrane around an existing drain (middle); it's replaced
with a new dropout fitting (bottom) that feeds to a downspout
below. The new fitting will be nailed to the deck and primed
before the foam is applied.
Counterflashing. In certain
circumstances, foam is self-flashing when lapped up onto
adjacent vertical walls. In most cases, though, it's necessary
to counterflash the roof-to-wall junction so that any water
that gets behind the wall cladding is redirected onto the roof
Figure 5. Foam is self-flashing when
applied to waterproof substrates, but water can still get
behind wall cladding. Therefore, the crew breaks out this
stucco, slips counterflashing behind the building paper, and
laps it over the foam below (above). Where the foam runs up
onto the wall, the roofer laps over it with flashing that tucks
behind the gutter (right).
Occasionally, for reasons of cost and practicality, we might
make an exception on a reroofing job: If the roof butts to a
low wall protected by an overhang, or if a short stucco-clad
chimney projects from the roof, we may lap foam onto them
without installing flashing. The building owner would have to
agree to this and be responsible for keeping the siding
Skylights are a notorious source of leaks. In many cases, we
need to raise the curbs so that the skylights are lifted above
the top of the foam.
Never let the roofer foam onto the skylight flange. The
skylight should be an easily removable fixture that sits above
the flow of any water or rain splash.
When we reroof a building, we remove the skylights, build up
the curbs (if necessary), foam up to their tops, and then
reinstall the skylights over foam weather-stripping. The roof
foam is self-adhering and self-canting, so there's no need for
curb flashing or cant strips (Figure 6).
Figure 6. The crew takes skylights off the
curbs before applying foam. This curb will be too close to the
surface after the foam is applied, so the roofers extend it
with a wood frame (top). Then they mask the opening and
waterproof it with a layer of foam (middle). Once the rest of
the roof has been foamed and coated, they remove the masking,
apply weather-stripping to the curb top, and reinstall the
It's not uncommon for hvac units or other equipment to be
installed on flat or low-slope roofs.
Placing the equipment on sleepers applied directly to the roof
is asking for trouble and should be avoided at all costs.
The correct method is to put equipment on boxed curbs with
sheet-metal cap pans. The idea is to separate the equipment
from the roofing system so it can be replaced without damaging
the roof, and the roof can be maintained without moving or
removing the equipment.
The foam can be sprayed when the prep work is done and the
weather conditions are right. The roof deck must be dry and the
air should be relatively warm. There should be little wind. If
too much moisture is present, it could disturb the chemical
reaction that creates the foam, resulting in a degraded product
that leaks. Excessive wind could increase overspray and
negatively affect the texture of the foam surface.
The temperature of the surface to be coated should be warm
(above 50 degrees). Applicators must be willing to shut down
operations when conditions aren't right.
To handle overspray, we mask skylights, sidewalls, shingles,
and anything else we don't want foam on. Masking the edge metal
allows us to apply foam up and onto the edge. We spray detailed
areas like edges and roof jacks first and do the field later
Figure 7.The applicator pulls up
on the masking paper while spraying foam up and onto the edge
metal (top). After spraying such detail areas as edges, curbs,
and roof jacks, he sprays the main body of the roof (middle). A
worker uses a grinder to bring the foam flush to the edge metal
(bottom) and to make it slope properly into scuppers and
Since the foam cures quickly, we can walk on it within a couple
Grinding and detailing. After the
foam has been sprayed, we use a grinder to bring it flush to
the edge metal and to make it slope smoothly into drains and
scuppers. In places where the foam is too high or doesn't look
right, we grind it down.
Detailing usually involves applying polyurethane caulking to
critical areas where the foam meets another material.
Coating and Granules
The final step is to apply an elastomeric coating to the foam
to protect it from being degraded by UV rays. Without this
coating, the foam would eventually wear away. Achieving the
specified 24-mil to 28-mil coating takes two coats and 3 to 3.5
gallons per square.
Because grinding opens up the cell structure at the surface of
the foam, we precoat any areas that were ground.
Base coat. The coating is applied like a heavy
paint; we cut in the detailed areas with a brush or roller, and
spray the field with an airless sprayer (Figure 8).
Figure 8. To prevent UV rays from
degrading the foam, the crew coats the surface with a
protective elastomeric coating. The base coat goes on first and
dries overnight. Then the top coat is applied; while it's still
wet, a worker broadcasts a layer of mineral granules onto it
(top). The finished roof (bottom) is a monolithic waterproof
membrane with an R-value of about R-6.5 per inch of
The base coat is tinted, so if we miss any spots with the top
coat we'll be able to tell. The tint also acts as a recoat
indicator, because it'll show through when the top coat wears
Top coat and granules. After the base coat has
cured (usually overnight), we spray on a top coat; before the
top coat dries we embed a layer of mineral granules.
In addition to creating a more consistent appearance, granules
offer several functional advantages. They speed evaporation by
wicking water off the roof; they increase resistance to
abrasive wear; and they prevent birds from pecking at the foam.
The better-quality granules provide additional resistance to UV
Like most SPF roofs, ours usually go 12 to 15 years without
recoating, a process that entails power-washing the roof,
applying a primer, and applying one or more coats of top-coat
material. Recoating a 2,500-square-foot roof costs $3 to $4 per
square foot.James Morshead is the senior project
manager/technical director for American Services Co. in Dublin,