Breathable Underlay Types
There are three basic types of underlay used beneath roofing materials:
•asphalt-saturated felt;
•rubberized asphalt; and
•non-bitumen synthetic.
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 cellulose base.
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
Slope Limitations
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.
Fastening
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 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 for synthetics.
FELT UNDERLAY FAILURE
Asphalt-saturated felt may fail for a number of reasons:
Poor Quality
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 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 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.
UV Exposure
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.
Installation Damage
When the roof-covering material is being installed, the underlay takes a beating and may be damaged by footfall or other materials.
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 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 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.
RUBBERIZED ASPHALT
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 different requirements:
•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.
Polymer-Modified Bitumen
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 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 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 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.
Selvedge Edge
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 SYNTHETICS
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.
Advantages
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.
Disadvantages
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 products.
•Many synthetic underlays don’t meet current building code requirements
•Use of these underlays 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 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 problems.
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
Slope limitations will vary by manufacturer. Some specify a greater overlap for low-slope roofs, and some don’t.
Roof Edges
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
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 inspection.

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 buildings.
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 ventilation.
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 robust.
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 condition.
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 perms)
• Waterproof, four ply material.
• Approved by: Miami Dade 09-0915.10 NOA and ICC-ESR 3017
• Suitable for Non Ventilated cold and warm roof applications.
• 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 installation guidelines.
Great popularity has been gained by added innovation in roofing membranes.
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 the structure.