|Roof Underlayment –Why It Is
When a roofer first walks onto a job, unless he’s tearing off an old
roof-covering material, he’s faced with a bare roof deck. The first
component to be installed on the roof is roofing underlayment.
Roofing underlayments are manufactured with different properties designed to
meet the needs of homes in different climate zones. An underlayment
that works well under metal roofing in a hot, humid place like New
Orleans, Louisiana, may not
work well beneath wood shakes in a cold, dry climate like Jackson,
Wyoming. The different types of roof-covering materials may also
have specific underlayment requirements.
Although roofing underlayment is typically required in new construction by
building codes, in the past, roof-covering material manufacturers
haven’t always required it on slopes of 4:12 and steeper.
Purposes Of roof underlayment
Most roof-covering materials are not waterproof, but
water-resistant, and are designed to be installed over a waterproof
or water-resistant membrane of some type. “Underlayment” is the
general term used to describe these membranes. Even though the
roofing underlayment is the first material to be installed on the roof deck,
the roof-covering material -- the shingles, tiles, metal or slate --
is the primary barrier against roof leakage. Underlayment is a
secondary barrier. Water-resistant roofing underlayment may allow the
passage of moisture vapor, but will prevent the passage of water in
its liquid form. Waterproof underlayment will prevent the passage of
both liquid water and water vapor. Waterproof roof underlayment is
typically used on parts of the roof that are more likely to leak or
suffer moisture intrusion. This includes penetrations in areas where
roof-covering materials change or end, and low-slope sections of
roof. It’s not unusual to use combinations of underlayment on a
The permeability of roofing underlayment is the extent to which it allows
the passage of water vapor. Although all underlayments are designed
to prevent the passage of moisture in its liquid form, they can have
different levels of resistance to the passage of water vapor.
Underlayment permeability ratings are provided by the manufacturers,
and are less important in roof underlayment than they are in
housewrap. Underlayments with a perm rating of 1 or less are
moisture barriers. Underlayments rated above
1 are moisture retarders.
Underlayment provides temporary protection of the building interior
and the roof deck before the roof-covering material is installed.
Ideally, the roof-covering material would be installed as soon as
possible, but in the real world, the roof may be protected by only
the roofing underlayment for days, weeks, or sometimes months.
Protecting the building interior is especially important when an old
roof-covering material is being replaced and the home interior is
finished. During that time, the roof underlayment may be under attack
from weather elements such high winds, UV
radiation, and precipitation. It also needs to resist the wear and
tear that occurs when the roof-covering material is being installed.
Preventing Chemical Degradation
Underlayment also provides a layer of separation between the roof
sheathing and the roof-covering material.
Newer homes use plywood or an engineered panel called oriented
strand board (OSB) for roof sheathing
For many years, pine and fir boards were used as sheathing, and many
older homes still have these boards in place. Resin pockets in these
boards can react chemically with some roof-covering materials, such
as asphalt shingles. In these situations, missing roofing underlayment can
cause accelerated deterioration and premature failure of the
Underlayment materials are available for wood roofs which increase
their resistance to fire. In fact, without special roof underlayment,
wood shakes and shingles cannot achieve a Class A fire rating, which
is the highest available.
Factors Affecting Underlayment
A number of factors can affect the performance of roofing underlayment and
determine which types are appropriate:
For purposes of determining optimum roofing material, depending on
location, climates in North America can be separated into two basic
hot or cold dry climates and;
hot or cold humid climates.
Hot and dry climates will affect bituminous roofing underlayment by
accelerating the loss of volatiles.
In humid climates, older felt underlayment will absorb more moisture
which, in turn, can be absorbed by the substrate, causing it to
expand. In cold climates, roof underlayment will become brittle and more
easily damaged by footfall and impact.
Each of these climate types should have roof underlayment installed which
has performance characteristics compatible with that particular
Some designs shed runoff quickly. Some have design features which
may actually trap runoff and expose the roofing underlayment to more
Manufacturers produce underlayment of different types for use with
the different types of roof-covering materials. The use of
roof underlayments that are not compatible with the roof-covering
material with which they’re installed can cause problems.
Roof-covering materials in poor condition which expose underlayment
to weather, especially to UV radiation from sunlight, can accelerate
Missing roof underlayment
Although underlayment is typically required in new construction by
building codes, in the past, some manufacturers have not required it
on roofs of 4:12 and steeper.
Determining whether roofing underlayment was required means finding the
manufacturer’s installation instructions for that particular
roof-covering material, and also finding out what jurisdictional
requirements were in place at the time the
home was built.
Underlayment & Flashing
In most areas of the country, a roof can't withstand exposure to the
elements with just a layer of shingles to protect it. An additional
roof underlayment layer(s) is needed to deter water penetration.
Properly sealing out water before shingling requires these elements:
Laying A Waterproofing Membrane
Start by adding a waterproofing membrane along eaves and valleys.
These are high-risk areas for leaks; valleys due to improper
flashing installation and eaves because of ice dams. We used a
thick, bituminous material with an adhesive backing.
The membrane protection provides extra protection against water
penetration and is also required by code in "Snow Belt" states at
Waterproofing membrane material can be considerably more expensive
than just using felt. Most professional roofers recommend the extra
protection, and for a do-it-yourselfer, it's good insurance against
a leaky roof.
Rolling Out Builder's Felt
Builder's felt (tar paper) is the most widely used roofing
underlayment. It comes in rolls and its thickness is gauged in
pounds. Typically, a roll of 15 lb. felt may cover about 400 sq. ft.
and a roll of 30lb. would cover half that area.
Laying felt is much easier and faster with two people; one rolling,
one stapling/nailing. Position the felt roll flush with the gable
Working toward the far end or valley, unroll about 5' of felt,
square it with the roof edges, press out any wrinkles, and
staple/nail it in place as you go. Proceed at 5' intervals to the
far end. When you run into a valley, angle cut the felt to lie down
the middle of the valley. If you forego the membrane, lay down extra
felt at the valleys.
Overlap the second row of felt on the first row by 2". Work up the
roof this way to the ridge (peak). Leave the ridge exposed and
continue roofing underlayment on the other side. When you reach the ridge,
fold the felt over both sides (overlapping 2") and fasten it into
Once the roof has roof underlayment, it can withstand exposure for a few
days. However, if it gets wet, the felt may wrinkle up a bit and
that increases its chances of tearing by wind.
Roofing Underlayment Types
There are three basic types of underlayment used beneath roofing
•rubberized asphalt; and
One of the most common types of underlayment used in residential,
steep-slope applications is black, ashphalt-saturated felt paper.
Felt underlayment may be made from either organic or fiberglass
substrate, although the organic is much more common. It's called
"organic" underlayment because it has a cellulose base.
Felt roofing underlayment 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
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
INSTALLATION OF FELT UNDERLAYMENT
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 roof underlayment across the entire roof.
In steep-slope roofs (4:12 and steeper), the upper courses of felt
underlayment should overlap lower courses by at least 2 inches. You
can see the difference between the roofng underlayment 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 prevent tearing.
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 the fasteners.
Edge Metal Laps
Felt underlayment 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 underlayment, but not necessarily for synthetics.
FELT UNDERLAYMENT FAILURE
Asphalt-saturated felt may fail for a number of reasons:
A number of ASTM standards exist which offer specifications for
Many manufacturers produce asphalt-saturated paper labeled “Underlayment,”
“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
underlayment. These roofing underlayments 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 deck.
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 roof underlayment, it may be caused by sub-standard
Loss of Volatiles
Over time, volatile compounds in the asphalt will dissipate, and the
roofing underlayment 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 underlayment 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
When the roof-covering material is being installed, the roofing underlayment
takes a beating and may be damaged by footfall or other materials.
NO MORE ASPHALT FELT
In the future, asphalt-saturated felt underlayment 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 crude.
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 roof underlayment.
Since shingles produce a higher profit margin than underlayment for
the amount of asphalt used, most manufacturers are phasing out
asphalt-saturated underlayments in favor of synthetic roofing underlayments.
Although they fluctuate with raw material prices, as of 2010, prices
for felt and synthetic underlayments were similar.
Various types of rubber-like materials are also used as underlayment
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 roof underlayments make them self-sealing, meaning
that they seal well around fasteners, such as staples and nails.
Rubberized asphalt underlayments are manufactured to meet different
•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
roofing underlayments 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 underlayment, asphalt shingles, and built-up roofing. It's a
common term in the roofing industry.
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
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 deterioration.
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 underlayment 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 underlayment is installed in courses across a roof.
Non-bitumen synthetic roofing underlayments 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 underlayment 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
As of 2010, there are some concerns with synthetic roof underlayment.
According to the National Roofing Contractors Association:
•To date, there are no applicable ASTM standards for these products.
•Many synthetic underlayments don’t meet current building code
•Use of these underlayments may void some manufacturers' material
warranties for certain roof coverings (such as asphalt shingles).
Concerns from other sources include the following:
•Wicking can be more of a problem than with felt roofing underlayment.
Installation along the roof eave is different with some types of
•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 problems.
As an inspector, you are not responsible for identifying the type of
underlayment, but it’s a good idea for you to know what types exist
and some of their properties.
Although companies who manufacture synthetic roof underlayment may also
manufacture similar-looking housewrap, housewrap does not meet
roofing underlayment requirements. Housewrap installed as
underlayment is a defective installation. Underlayment is usually
thicker than housewrap. In the photo above, you can see the
difference between the two.
INSTALLING SYNTHETIC UNDERLAYMENT
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
roofing underlayments are designed to wrap around the roof edge and protect
the edges of the roof sheathing. The edge metal is installed over
the underlayment 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
In summary, roofing underlayment 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 inspection.
Breathable Roofing Underlayments
Roof underlayment acts as water barrier installed underneath the
metal roofing system. Essentially, it is a roof underneath the roof.
It protects your roof from condensation that may form underneath the
metal due to the differences in temperatures in the attic and
Synthetic underlayment became popular for use in metal roofing
systems because of its strength and water barrier properties.
However, there was one major fault with many synthetic underlayment
systems used in metal roofing. Most of the systems acted not only as
a water barrier, but also as a vapor barrier, which would result in
moisture being trapped underneath the underlayment. The moisture
would cause the roof deck to rot eventually destroying your roofing
The roofing deck on this shed was originally covered with TAR paper,
an inexpensive, but not very efficient roof underlayment commonly
used in conventional roofing. The roofing deck became damaged and
rotted due to the use of inferior roofing underlayment that did not
provide adequate water barrier protection nor allowed the roofing
deck to breathe. The trapped moisture had no way of escaping and it
caused the rotting of the roof deck.
Breathable Synthetic Underlayment
In order to combat the problem of rotted roof decks due to the use
of vapor barrier underlayments, metal roofing manufacturers
introduced a new product, which offered the same strength found in
of the conventional roofing underlayments, yet it would not act as a
vapor barrier. Therefore, the roof deck could now breathe because
excess moisture would no longer be trapped underneath the
If you are involved in any type of DIY
roofing project, then we recommend you spend
the extra money and install the breathable
roof under layment for your metal roofing
project. We make this recommendation based
on the fact that any roof deck that is not
properly ventilated can fail even with the
best roof protecting it. Choosing a reliable
breathable roof under layment for your
project will make it easy for you to walk on
your roof. It will prevent the tearing
associated with lesser grade underlayments,
and it will help your roofing deck breath
once the roof is installed. The use of the
correct roofing underlayment is no
substitute for the proper roof