What products have dominated the roofing industry for more than a century while incorporating principally recycled materials and remaining highly cost-effective?
Ask a roofing contractor, and the response would likely be No. 15 asphalt saturated roofing felt. An old-timer might call the product “rag felt,” as rags were actually recycled in the manufacture of these dry felts. Since the rags were principally cotton, the felt was receptive to impregnation with asphalt saturant, and ASTM standards called for 150% of the dry felt weight to be saturant.
With the introduction of synthetic fibers such as polyester, acrylic and nylon, rags were no longer suitable for roofing. The saturation process melted the rags, so the industry shifted to cellulosic fibers such as sawdust, waste paper or newsprint. ASTM reduced the requirement for saturant to 140%, then 120%, with the original specification for saturated felt published in 1925 as ASTM D226: Asphalt-Saturated Organic Felt Used in Roofing and Waterproofing. There are still a few roofing contractors that call the product tar-paper even though the product never had coal tar pitch in it.
Roofing applications generally fell into two categories:
- Steep slopes, where the roof drained by gravity. No. 15 asphalt saturated felt was laid dry on the deck and nailed. The felt was not perforated, as it performed as a secondary waterproof layer. When used as an underlay on tile roofs, the heavier No. 30 felt was used.
- Low slope roofs, generally less than 2 inches per foot, where the felts were perforated to release steam while they were embedded in hot asphalt.
In addition to these two organic products, felts were produced using asbestos or glass fibers. Asbestos was withdrawn due to health issues and several versions of glass fiber mats were introduced for both hot built-up roofing and as shingle underlayments. ASTM Specification D2178 (Asphalt Glass Felt) was introduced in 1963 and covered six different types including steam-blown, continuous strand and wet laid.
With the introduction of polymeric single-ply materials in the 1960s, synthetic fiber woven and non-woven materials provided several properties that make them attractive to the roofing industry. These include non-hydroscopicity, which is extremely important in roofing exposures, rot resistance, tensile and tear resistance, and excellent work-to-break ratios.
This combination of properties has led to the use of non-wovens in many different roofing applications.
The 7 Roles Non-Wovens Play in Roofing
1) Internal Reinforcement
Polymer modified bitumens have been utilized for the last several decades. They consist of a waterproofing (and sometimes adhesive) mass of bitumen, usually asphalt that has been modified by the addition of selected polymers. The polymers frequently include a thermoplastic resin type known as Atactic Polypropylene (APP) or an elastomeric type such as Sequenced Butadiene Styrene (SBS). The waterproofing mass is not designed to carry its own weight, nor is it sufficiently tough to perform the duties of a complete roof membrane.
One use of non-wovens, therefore, is to act as a carrier for the modified bituminous sheet as it is manufactured, transported and installed. Since both APP and SBS modified sheets are frequently applied as a hot melt, heat resistance and dimensional stability are important parameters. The APP sheets are frequently applied by open torch melting, and stability under variable heat exposure is important. To achieve the combination of extensibility and heat stability, several commercial roofing sheets incorporate two (or more) fiber scrims (or webs) combined with polyester mats. Some modified bituminous matrix materials are designed to be self-adhesive. U. V. stabilized polypropylene has been used as a topside carrier for these systems.
Most applications for commercial (low slope) roof systems use more than a single layer. Puncture resistance is enhanced and this second layer doubles the waterproofing reliability. The base layer may be an unmodified base sheet, frequently glass mat reinforced or it may be similar in construction to the upper ply. Since most modified bitumens do not have adequate long-term weather resistance, this is handled by opaque, factory applied roofing granules, metal foils, or by field application of various coatings.
Reinforced Thermoplastic Sheets such as Polyvinyl Chloride (PVC), Chlorinated Polyethylene (CPE), Hypalon®, (CSPE) and Thermoplastic Polyolefin (TPO) are also found. Non-wovens are usually used in mechanically fastened or partially fixed membranes because of their greater resistance to localized stresses such as by use of mechanical fasteners. In cast PVC systems (Plastisols), the carrier is needed for production as well as resistance to stress.. Glass fiber mats are sometimes used in cold-applied, fully adhered systems where dimensional stability and shrinkage resistance are of paramount importance.
Thick backing of polyester fleece is used both as a reinforcement and as a chemically compatible back surfacing, permitting the polymeric membranes to be laid directly into pouring of hot bitumen or as a divorcement from existing bituminous roof membranes. In synthetic elastomeric systems such as Ethylene Propylene Diene Terpolymers (EPDM) and Neoprene, (Chloroprene) mats are sometimes incorporated in high stress applications such as in mechanically restrained systems.
2) Fire Protection/Wind Resistance
Roofing membranes can be subjected to external fire exposure tests, following ASTM procedure E108. The roof membrane is tested for its ability to prevent burn-through, important on wood decks, as well as its surface burning characteristics. When roof systems have a heavy gravel surfacing, such as a gravel surfaced build up roof system, they usually receive Class A fire ratings on both combustible and noncombustible decks. The behavior of reinforcing mats within the membrane are of little consequence.
However, most of the reinforced single ply systems are used in unballasted applications.
Glass fiber mats are usually superior to polyester reinforcements, especially in burn-through resistance. The selection of core, surfacing and underlayment must be appropriate in order to meet even Underwriters Class B rating (lower than A).
The mats also play a part in wind resistance. Current wind tests, such as the FM Class I uplift test or the UL Class 30, 60, 90 apparatus do not accurately evaluate fatigue and flutter, which happens on real roofs. The mats are extremely important in controlling stretch and in handling the localized stresses that occur around bars or fasteners. Many single ply manufacturers have had to develop their own engineering data on reinforcements and membranes, and tailor their applications to the derived data. Usually this requires many more fasteners or bars in the corner and edge locations of the roof surface.
3) Filter and Hold-Down Applications
Some unique roofing systems consist of a waterproof membrane covered by a water-resistant thermal insulation. The insulation, in turn, is ballasted for wind protection and fire resistance as well as UV screening. These inverted roof systems are generically called Protected Membrane Roofs (PMR).
Non-woven mats have successfully been incorporated into these systems. Between the ballast and thermal insulation, the mat keeps the boards together, even if the boards float due to temporary ponding. This function is described as "rafting" the boards. The mat also serves as a filter course, preventing small stones from dropping through the joints of the insulation, which prevents their nesting. The mat also keeps silt from getting under the boards promoting vegetation growth or clogging the drainage channels that are designed to allow the insulation to dry out and retain its thermal value.
Thick pads of fleece have also been incorporated as a protecting padding, to be laid directly on top of single ply roof membrane systems when it is impossible to obtain smooth, river-washed rock as a ballast. In this case, crushed stone is accepted as a ballast material when applied over the thick cushioning pad. In both of these applications, rot and U.V. screening are of paramount importance.
4) As an Underlayment
Layers of non-wovens are incorporated as slip-sheet materials, to underlie single ply membranes. The underlay can serve to bridge gaps, to cushion against rubble or point stress areas in the decking, or to resist the effects of fasteners backing out of the substrate. Examples would include where single ply systems are laid directly on top of poured concrete or plywood roof decks without a divorcing layer of roof insulation.
Fleece has also been used as an underlay for wood "duck" boards, concrete walkway planks, and treated wood "sleepers" which supporting conduit, pipe or other rooftop equipment. The fleece provides a cushion against the cutting or crushing pressure of such units. In addition, the fleece serves as a wick, drawing water out of the paver and allowing it to dry out.
5) As Coating Reinforcements
Non-woven materials have been used in many applications as reinforcements for solvent or water based cold process roof resaturants or recoating systems. Throughout the country, there are a great number of bituminous built up roof membrane systems in advanced stages of their life cycles. Coatings based on bituminous substances, elastomerics or polymerics are sometimes applied to these membranes to seal the surface of the membrane and to provide additional weather protection.
While these coatings sometimes are internally reinforced with fibers, their bridging ability is extremely limited and the resultant coating is not particularly durable. Synthetic fiber mats which have both flexibility and conformability have been adopted by many roofing contractors in systems which combine the mat with the various coating systems.
In some cases, resurfacing existing roof with unreinforced coatings is interpreted by the building codes as just a coating, rather than a new membrane. This allows restoration of some of the physical properties of the roof membrane without resorting to a complete tear-off. This code interpretation circumvents the limitations of most roofing codes which limit a building to two or three overlaid membranes. Both polyester and polypropylene are suitable reinforcements for such cold process coatings, since no heat resistance is required.
6) Reinforcements for Hot Bituminous Built Up Roofs
The application of hot buildup roofing incorporates petroleum asphalt or coal tar pitch, very cost effective waterproofing and adhesive agents. These products serve to both glue various elements of the roofing system together and to waterproof it as well.
In order for a bitumen to be cost effective, it is normally melted and applied at temperatures which frequently exceed 400 degrees F. and can at times reach 500. At these temperatures, most synthetic fiber mats lose their strength and tend to distort or swell when applied. The lamination of non-woven mats with non-woven or woven glass fibers appears to show promise in permitting hot BUR applications.
Since hot built up roofing is a very competitive business, it remains to be seen whether multi-ply glass fiber reinforced non-woven membranes can compete against conventional reinforcements such as fiber glass or organic felt. It is deceiving to compare only the tensile strengths or elongations of membranes made up of bitumen and reinforcement. While a two-non-woven mat membrane may, at first glance, appear to have superior physical properties to three or four plies of conventional BUR, BURs frequently serve as a work platform. They need to have a great deal of puncture resistance to withstand wheelbarrows or other rolling loads. In addition, workmanship is always variable and the redundancy of a 3- or 4-ply system allows the skips or errors of one layer to be covered by another layer.
The low temperature brittleness of conventional roofing bitumens also works against the success of a hot non-woven BUR system since a rupture of the bitumen would allow leakage into the roofing system, even if the reinforcements themselves did not shatter or rupture. It is possible that internal reinforcements in the modified bitumen systems provides a more logical application of the advantages of non-woven since the factory modification of the bitumens can more closely approximate the high performance of the mat itself.
7) Reinforcements in Hot-Applied Bituminous Flashing Systems
One system that does not fit this outline was a product called “reinforced asbestos base flashing.”
In this case, a woven burlap material called osnaburg fabric (a coarse fabric woven from flax yarns originating in Osnabruck, Germany) was treated with asphalt and surfaced with asphalt coated asbestos felt. Woven materials are strong and flexible, and the asbestos felt on the weather side provided UV resistance.
The Final Verdict
The growth of non-wovens in the roofing market has been remarkable. They have established dominant positions where water resistance and high tear strength are important.
The engineering aspects of each application are dictated by the characteristics of the entire system, including exposure, surfacing, polymers used, localized stresses, and more. The compatibility with older bituminous technologies, as well as with newer ones like single plies and PMRs, promises a bright future in the fundamental business of shelter.
More information on non-wovens is available from INDA, the Association of the Non-woven Fabrics Industry.
Asphalt vs. Coal Tar Pitch Roofing
How the two popular bitumens measure up across the years.
Avoid Roofing Condensation Issues
Beware of situations that can lead to costly repairs.
Top 3 Considerations for Rooftop PV
The biggest obstacles to a solar retrofit.