From Rags to Riches

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 Asbestos was withdrawn due to health issues and several versions of glass fiber mats were introduced for both hot built-up roofing and as shingle underlayments.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