There are three basic types of underlay used beneath roofing
•rubberized asphalt; and
One of the most common types of underlay used in
residential, steep-slope applications is black, ashphalt-saturated
felt paper. Felt underlay may be made from either organic or
fiberglass substrate, although the organic is much more
common. It's called "organic" underlay because it has a
Felt underlay is water-resistant, but not waterproof. It’s
available in two thicknesses: 15-pound and 30-pound.
Fifteen-pound felt has a perm rating of about 5, although
this number can rise in high-humidity conditions.
Thirty-pound felt is more resistant to damage during
installation of the roof-covering material, and will protect
the roof longer if it should somehow become exposed to
weather. The difference is obvious, once you see them
together. Thirty-pound felt is much thicker and stiffer.
INSTALLATION OF FELT UNDERLAY
In low-slope roofs, which include 2:12 up to 4:12, felt
courses should overlap a minimum of 19 inches. This will
provide a double layer of underlay across the entire roof.
In steep-slope roofs (4:12 and steeper), the upper courses
of felt underlay should overlap lower courses by at least 2
inches. You can see the difference between the underlay
overlapped 19 inches on the roof to the right and overlapped
2 inches on the roof to the left. In Figure 1 the lower roof
is low slope with a 19-inch overlap and the upper roof is
steep slope with a 2-inch overlap.
Felt is usually fastened with staples, but in high-wind
areas, plastic windstrips may be used along the edges to
Felt may also be attached in high-wind areas using plastic
caps. Plastic caps offer better wind resistance than
staples, and help prevent leakage through the holes made by
Edge Metal Laps
Felt underlay should overlap the edge metal at the eaves and
be overlapped by edge metal on the rakes. This is also the
case for rubberized asphalt underlay, but not necessarily
FELT UNDERLAY FAILURE
Asphalt-saturated felt may fail for a number of reasons:
A number of ASTM standards exist which offer specifications
for asphalt-saturated felt.
Many manufacturers produce asphalt-saturated paper labeled
“Underlay,” “15-lb.” or “30-lb.,” which do not comply with
any standards, and which are often saturated to a lower
level than an ASTM-compliant underlay. These underlays
typically absorb water more readily, and fail sooner. Water
absorption can cause wrinkling as the product expands. These
wrinkles may telegraph through to roof-covering products,
such as thinner asphalt shingles.
Water from the felt may be absorbed by the roof deck, which
can cause problems with expansion and contraction of the
You won’t be able to tell by looking whether a product
complies with any standards, but if you see what looks like
premature failure or distortion of the underlay, it may be
caused by sub-standard underlay.
Loss of Volatiles
Over time, volatile compounds in the asphalt will dissipate,
and the underlay will become more fragile and
moisture-absorbent. This will happen more quickly when felt
is exposed to heat. The source of heat may be a warm
climate, a particular type of roof-covering material, or
poor roof-structure ventilation.
Anywhere felt underlay is exposed directly to sunlight, UV
radiation will accelerate its deterioration. These
poorly-bonded shingles were attached with staples on a home
located in a high-wind area.
When the roof-covering material is being installed, the
underlay takes a beating and may be damaged by footfall or
NO MORE ASPHALT FELT
In the future, asphalt-saturated felt underlay will probably
be used less and, by 2014, it will likely no longer be
installed at all. Asphalt is basically the residue left over
from the process of refining crude oil. As the price of oil
has increased, refining techniques have been developed that
extract the maximum amount of high-quality products from the
These techniques, involving the use of coker units, result
in a residue of powder instead of the sludge from which
asphalt is normally produced. With less asphalt being
produced, an allocation program has been established for
which the asphalt produced each year is allocated in limited
amounts to manufacturers of asphalt shingles and underlay.
Since shingles produce a higher profit margin than underlay
for the amount of asphalt used, most manufacturers are
phasing out asphalt-saturated underlays in favor of
synthetic underlays. Although they fluctuate with raw
material prices, as of 2010, prices for felt and synthetic
underlays were similar.
Various types of rubber-like materials are also used as
underlay and are generally referred to as “rubberized
asphalt.” These typically have adhesive on one side, which
is protected by a peel-off membrane, making them
self-adhering. The rubber-like qualities of these underlays
make them self-sealing, meaning that they seal well around
fasteners, such as staples and nails.
Rubberized asphalt underlays are manufactured to meet
•They may have polyethylene or polyester bonded to the upper
surface to provide non-skid and weather-resistant qualities.
•They may have a polymer film bonded to the weather surface
to improve moisture resistance.
•They may be fiberglass-reinforced.
•They may have a mineral coating on the weather surface.
They may be formulated for use in high-temperature
situations. Some underlays are designed to resist heat up to
250° F without degradation of the adhesive. This allows them
to be installed under metal roofs an in harsh environments.
The asphalt may be polymer-modified.
The terms "modified bitumen" is often used when referring to
asphaltic roofing materails. Sometimes, this term is
shortened to "mod-bit." The term "bitumen" is a generic name
applied to various mixtures of hydrocarbons. One of these
mixtures is the asphalt used in underlay, asphalt shingles,
and built-up roofing. It's a common term in the roofing
To improve various characteristics such as strength and
elasticity, bitumen is sometimes modified using polymers
which give it plastic-like or rubber-like properties,
depending on which process is used.
Polymers are materials made of molecules which are
custom-designed to give the material specific properties.
Polymers are used in many different types of roofing
products to increase their resistance to damage and
You may also hear the term “cross-linked polymer” used.
Molecules in cross-linked polymers actually bond to each
other at the atomic level; they actually share atoms, which
greatly increases the strength of the material.
Rolls of rubberized asphalt underlay may come with a
selvedge edge along one side of the roll. The selvedge edge
is designed to create a strong, watertight seal along the
edges where rolls overlap. The selvedge edge should always
be along the top edge when the underlay is installed in
courses across a roof.
Non-bitumen synthetic underlays are made from polypropylene
or polyethylene. These synthetic polymers are also used to
make a huge variety of other types of products, from
food-storage containers and rope, to long underwear.
Like other underlay materials, the use of synthetics has
both advantages and disadvantages.
Among their advantages include their light weight and high
strength. They are also typically non-skid.
Synthetics are resistant to fungal growth and are
wrinkle-free, since they don’t absorb moisture. Although
they can be designed as moisture-permeable, they are
typically considered moisture barriers.
They’re also very resistant to UV damage and can be left
exposed to weather for periods from six months to a year,
depending on the manufacturer’s recommendations.
As of 2010, there are some concerns with synthetic underlay.
According to the National Roofing Contractors Association:
•To date, there are no applicable ASTM standards for these
•Many synthetic underlays don’t meet current building code
•Use of these underlays may void some manufacturers'
material warranties for certain roof coverings (such as
Concerns from other sources include the following:
•Wicking can be more of a problem than with felt underlay.
Installation along the roof eave is different with some
types of synthetics.
•If the installer fails to read and follow the manufacture’s
installation instructions and instead installs it like they
would if they were using felt, they may create moisture
As an inspector, you are not responsible for identifying the
type of underlay, but it’s a good idea for you to know what
types exist and some of their properties.
Although companies who manufacture synthetic underlay may
also manufacture similar-looking housewrap, housewrap does
not meet roofing underlay requirements. Housewrap installed
as underlay is a defective installation. Underlay is usually
thicker than housewrap. In the photo above, you can see the
difference between the two.
INSTALLING SYNTHETIC UNDERLAY
Slope limitations will vary by manufacturer. Some specify a
greater overlap for low-slope roofs, and some don’t.
To avoid problems from wicking moisture, many synthetic
underlays are designed to wrap around the roof edge and
protect the edges of the roof sheathing. The edge metal is
installed over the underlay at both the eaves and rakes.
Fastening is generally done with plastic caps or roofing
nails. The use of staples is discouraged because synthetics
are not self-sealing.
In summary, roofing underlay is an essential component to
the roofing materials' ability to withstand the elements,
protect a home's interior, and prolong its service life. The
more an inspector understands about a roof's components, the
better he can spot problems and deficiencies during an
Construction breathable underlay
Comparing European and North American roof construction, it
seems rather logical that both have something to offer the
other party. Europeans have built churches, official and
residential buildings using steel, copper, lead and other
similar materials for centuries, yet similar products are
lifting their heads in North America only recently. Moisture
and condensation has been dealt with by creating technically
compatible buildings where air circulation has been the main
feature. Designing appropriate air circulation and correctly
applied roofing and wall materials, have given the total
building a healthy environment where condensation and
harmful bacteria are part of history.
Dry-Tek underlay from Nemco Industries
The Europeans are now also using increasing quantities of
“breather” membranes that are used within pitched roofs and
are referred to as vapor permeable underlays.
“Breathability” is therefore the measurement of the amount
of water vapor, or perspiration moisture, transported
through the barrier layer, into the atmosphere. “Vapor
permeable underlays relate not to the passage of air, but
rather to the passage of water vapor through the membrane.”
Building regulations have required increased amounts of
insulation in the attic space during the last 30 years and
the combination of heating and double glazed windows has
increased air tightness with a consequent increase in the
risk of condensation becoming one of the major problems
within cold roofs.
Ongoing research and progress has resulted in the
development of a range of construction membranes for use in
a wide variety of wall and pitched roof systems to prevent
condensation — most recently witnessed in Vancouver, B.C.,
Canada, where millions of dollars was been spent on roofing
and wall membrane control and repair. Adequate breathability
is the key to all mold and rot issues. A little moisture and
no air circulation will cause insurmountable damage to
As the water vapor in a building passes through a
conventional ceiling and high permeability insulation, it
reaches a cold roof space. The cold reduces the air’s
ability to hold vapor and consequently increases the risk of
condensation forming on rafters and roof timbers, which can
cause mould growth and rot. In hot and humid climates, the
above applies in reverse.
Traditional systems allow water vapor to be trapped, within
the roof space by roofing underlays having low permeability
such as bitumen, or synthetic roofing underlays, making it
necessary to design a means to allow the vapor to escape
from cold pitched roofs. This means of escape can only be
increased by ventilation at the eaves with the use of ridge
In Europe, roofing underlays and breathable membranes have
become an important component of modern pitched roof
construction. Their correct specification, detailing and
installation helps to minimize the risk of water ingress
into the completed building, giving the construction team
greater confidence that new roofs will be reliable and
There are several different test methods, which can be used
to measure breathability. However, there is no global
consensus that any one method can measure the conditions
encountered during different end uses. It is also clear that
a Universal correlation cannot be obtained using different
methods to test the same product; therefore, it is apparent
that a specific test is needed for a specific end use
Nemco Industries Inc. has introduced, into the North
American Markets, a breathable mechanically fastened roofing
underlay with the following features:
• Very strong and robust vapor permeable underlay. (146
• Waterproof, four ply material.
• Approved by: Miami Dade 09-0915.10 NOA and ICC-ESR 3017
• Suitable for Non Ventilated cold and warm roof
• UV Stable for up to four months.
• Hydroponically treated.
• Self-adhesive tape on overlaps preventing water ingress.
• Produced in rolls of 808 square feet
• Must always be installed in accordance with local
Great popularity has been gained by added innovation in
As a result, the addition of a single layer breathable
membrane, a newcomer to the roofing industry, has been
added, that of 3mm high Polypropylene webbing attached to a
breathable membrane, called Dry-Tech. This method of
combining two media into one makes roofing battens
unnecessary, saving costs of labor, as it incorporates
breathability and the creation of an air space into one and
the same installation.
Without space between deck and metal sheathing, corrosion,
mold and mildew can occur as no air circulates freely. In
cold regions condensation is of major concern and in hot
climates the same issue in reverse. Control of moisture has
also prompted metal roofing manufacturers to consider
voiding warranties should no air space be incorporated into