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Published in Maintenance Solutions (April 1995)
The Latest Generation of Roofing Options
Tougher expectations among users force changes in technology

By Jeff Evans

Fresh out of college with a degree in construction administration, I began my roofing career in 1977 with a contractor specializing in a "new revolutionary product," a single-ply ballasted roof membrane.  Knowing little about roofing in general, I was moderately successful in selling the new membrane to costumers fed up with perceived performance problems of built-up roofing, the other option at the time.

Roofing has changed dramatically since then and continues to change daily.  Over the past 20 years, buyers' choices have multiplied many times over.  For example, single-ply roofing has progressed from a new product into a major player in the roofing market.

The built-up roofing industry, out of favor for some time because of real and perceived problems, has re-established its visibility as a capable performer by improving its base components and developing better flashing and accessory products.  Built-up roofing manufacturers and contractors also have educated the roof-buying public of essential design principles, such as eliminating ponding water, to reduce the inappropriate use of built-up materials.

For maintenance managers, engineers, facility managers and others involved in purchasing roofs today, two words best describe the state of the industry:  choice and change.  Compared to the 1970's, when the only decision a roof buyer had to make was how many plies were best, the 1990s present a dizzying array or choices.  Following is a review of some updates in each generic product line.

Built-up roofing
The main improvements in built-up roofing have been the shift to fiberglass felts and the use of rubber-modified asphalt flashing materials.  Fiberglass-ply felts now come in two types, the widely used Type IV felt, generally considered the commodity felt, and the newer but stronger Type VI felt.  Both have worked in well-designed applications, and at this point the Type VI felt is positioned as the premium felt for those buying upscale.  Type VI felt systems generally are required to qualify for longer roofing system warranties.

Modified bitumen systems
Modified bitumens - sometimes referred to as mod bits - are a family of asphalt membranes made of a rubber or plastic additive and combined with asphalt, fillers and felt reinforcement.  Modifiers improve the cold flexibility of the membranes over conventional blown asphalt and are formulated to improve elasticity of the roofing material.

Typically, mod bit membranes are made in sheets 3 feet - 1 meter - wide and resemble rolled roofing.  Two primary modifiers are used:

  • styrene butadiene styrene (SBS), a rubber-like substance
  • atactic polypropylene (APP), a waxy plastic-like modifier.

APP modifieds are generally torch applied, while SBS membranes are applied in hot asphalt.  Cold adhesives for both are now on the market, which may help in places where installers can't use torches or hot asphalt.

Mod bit systems are usually multiple-ply systems; two plies are a recommended minimum.  Again, there are many choices available to the roof designer, depending on the needs of the application.  Hybrid systems have become popular, using one-three plies of fiberglass felts in hot asphalt, followed by a finish ply of modified bitumen.  If these don't offer enough choices, several roofing manufacturers supply kegs of modified asphalt to allow a modified bitumen application similar to built-up roofing, alternating layers of hot-modified asphalt with reinforcing felts of fiberglass or polyester.

Modifieds have the same general sensitivity to ponding water as asphalt built-up membranes, so sloping to drain is a needed design feature.  They tend to be fairly durable and generally have performed well over the past 10-15 years in the U.S.

Single-ply systems
Single-ply systems consist of one layer of an elastic/plastic membrane, usually 45-60 mils thick and installed in a number of ways:

  • generally ballasted - laid dry but held in place by gravel or pavers
  • fully adhered - glued in place
  • mechanically fastened - anchored to the roof deck with fasteners.

Chemically, these membranes are made from synthetic rubber or plastic, sometimes incorporating a layer of reinforcing fabric or mesh.  More commonly, they are known by their primary polymer component makeup:  EPDM, PVC, CSPE, PIB, EIP, etc.  Single-ply roofing membranes, adapted from other uses of elastomeric membranes materials, such as pond liners, have excellent elongation.  Here are some things to look for and to avoid in selecting single-ply systems:

  • Ballasted single plies have a low initial cost but are hard to find leaks on and are not well suited to roofs with heavy or frequent traffic.  Adding 1 ton of gravel to the top of a structure for the sole purpose of holding two squares of roof in place seems a bit odd, although it's a viable option when seeking the least costly roof.
  • Fully adhered systems are dependent on the roof insulation to which they are adhered to for wind uplift resistance.  Our experience has led us to avoid using fully adhered single plies on roofs with heavy traffic, not so much because of threat of puncture but because often, the insulation is crushed and damaged.  Once the insulation is damaged and the adhesive bond is lost, there is little holding the roof in place.  This is especially true with polyisocyanurate insulation in the thicknesses of less than 2 inches.
  • Mechanically fastened systems have been in this country for nearly 20 years, but they became more prominent in the early 1980s and continue to grow within the single-ply arena.  They rely on attachment to the roof deck by means of fasteners, bars, plates or other clamping devices.  As with fully adhered roof systems, compression of insulation from heavy roof traffic also can cause problems, such as tenting of the membrane over fastener heads.  This often results in membrane punctures.  Mechanically fastened systems also must be carefully designed to accommodate high wind.  Research is under way to identify what constitutes sufficient fastener spacings in high-wind exposures.

We've witnessed a peculiar result of wind flutter that occurs in some mechanically attached roof applications.  A roof membrane that billows or flutters in winter conditions draws warm, moist interior air to the underside of the roof membrane, where it can reach dew-point temperature and condense.  Infrared moisture detection scans on roofs that had not leaked still revealed wet insulation.

Our educated hunch is that the repetitive billowing of the membrane contributes to this moisture pickup, especially on air-permeable roof decks, such as steel decks.  An air barrier below the roof insulation reduces wind flutter and subsequent moisture infiltration.

Metal roofing
Metal roofs have come a long way from the leaky metal sliding panels that were caulked together and screwed to the structural frame.  Today's metal roofs are attached to the structure with concealed clips, which slide to accommodate expansion and contraction.  Panel sealants still are used at end laps and some side lap locations, but the sealant is subject to less movement stress and often is shielded from exposure to ultraviolet light.

Metal roof systems can be used in new roofs and some reroofing situations, but they always require a minimum 1/4 inch per foot slope.  When buildings are too wide to accommodate a single slope from ridge to eave, metal roofing designers must turn to using an interior common gutter, a practice we don't recommend,

We also discourage using metal roofing on facilities that expect to add new roof penetrations with some frequency, since the new units must resort to caulk and exposed fasteners, the very items that caused the first generation of metal roofs to leak.

Market changes
The roofing industry is dynamic, with new innovations, materials, tools and regulations cropping up on a regular basis.  Over the past 10 years, the rubber roofing market went from eight manufacturers of EPDM rubber to two.  The number of roofing manufacturers continues to increase, while the number of companies leaving the market also increases.

The consequences are predictable.  If maintenance managers were dissatisfied with built-up roofing, the spotty performance of some single plies has evoked similar dissatisfaction.  To those fortunate enough to be working with the right unproven single-ply products and are enjoying years of watertight service, maybe it was solid investigation and sound decision making, or perhaps it was other factors.  Many others have not enjoyed such success.

Over the past five years, there has been a major settling of this market, leaving fewer, more successful companies and membranes to deal with.  Maybe we can be thankful for this small upside.