Various Roofing Options

Based on the abuse it receives and the key role it plays, a roof system must be chosen very carefully and be matched to the specific needs of the building. A factor which places even more importance on the correct roofing decision is the rising costs of new and replacement systems. A reasonable “ball-park” figure for the installation of a roof is $3-$5 per square foot. The U.S. Air Force, for example, has recently indicated that their roofing costs are in the range of $3-$9 a foot (D.M. Firman, “Maintenance Needs: An owner’s perspective,” Professional Roofing, 8/90, Page 23). The $9 figure is not unreasonable if factors such as size, complexity, tear-off of an old roof and asbestos handling costs are involved.

The building owner can be at a decided disadvantage, on the other hand, when it comes time to choose a replacement roof system. Although unfortunately not always the case, a roof is expected to last approximately 20 years. An owner who finds it necessary to replace a 20-year-old roof is faced with an overwhelming choice of systems. Additionally, most of these systems have entered the market in the 20 years since the last roof was purchased. Plant maintenance personnel are often placed in the position of choosing the replacement roof system. They are expected to be experts in all phases of building and equipment maintenance and repair. Understandably, their concentration is on day-to-day problems and on immediate and highly visible needs such as keeping production running and addressing those components of a building which are observed daily. The roof is truly “out of sight, out of mind” until it leaks and begins causing problems to the interior of the building. By the time leaks occur, in most cases the roof, or at least a major portion of it, must be replaced and the owner or the owner’s representative is faced with an important and expensive decision.

BUILT-UP

The major types of roofing options available in the marketplace today include built-up, single-ply and modified bitumen. Built-up roofing (BUR) is by far the oldest low slope category of roofing membrane with beginnings that can be traced back to the mid-1800s. The first of these roofs were installed using coal tar. Asphalt roofs entered the market in the 1880s and today greatly dominate the built-up roof segment of the U.S. market.

A built-up roof, as the name implies, is “built” or assembled in several layers of the job site. The roofs are applied by alternating layers of molten bitumen (tar or asphalt) and some type of reinforcing material. The reinforcing materials are referred to as roofing felts or plies and over the years have been made from rags, paper, asbestos, glass and, most recently, polyester. There are also felts available which use combinations of materials such as glass/polyester. In simple terms, the felts provide the strength or integrity to the built-up roof, while the bitumen (coal tar or asphalt) provides the waterproofing characteristics. Built-up roofs are typically installed in three or four-ply configurations; although in the early history of roofing, five or six-ply assemblies were more typical.

Built-up roofing membranes have lost a considerable amount of market share to some of the newer systems over the last 15 to 20 years. In the last two or three years, however, BUR systems have begun to reclaim some of the market. This is occurring because the newer membranes are demonstrating that they too are not without problems if improperly applied or specified.

SINGLE-PLY

Of these newer products, the single-ply materials have made the greatest market penetration. Their growth began in the early 1970s and was accelerated by the energy shortages of that time and the consequential price increases of asphalt and coal tar. Single plies are generally classified as either rubber products or plastic membranes. The rubber membranes are materials such as EPDM (ethylene-propylene-diene monomer), neoprene (chlorinated butadiene) and butyl (polyisobutylene). These are synthetic rubber products which are formed into rolls and cured or vulcanized in the factory. The EPDM products dominate the rubber roof market and are produced or marketed by several very large companies which have their roots in the tire or inner tube industries.

Another type of single-ply membrane commonly available uses plastic polymers as the base for the finished product. The most well-known are those formulated with PVC (polyvinyl chloride), but there are others such as the “polymer alloys” (proprietary blends of plastic polymers) and the CPE (chlorinated polyethylene) materials. These roofing products, just as the rubber membranes, are manufactured in sheet form, but do not undergo the curing process. They rather are processed at high temperatures or high shear rates when they are liquids or semi-solids. They are then pressed into sheets to form the final material which is delivered to the job site. They, of course, are formulated in such a way that they do not become too brittle at low rooftop temperatures or too soft under high temperature conditions. The general chemical term for the behavior shown by these materials is thermoplastic (having the ability to repeatedly be softened when heated or hardened when cooled).

There is a third classification under single-ply and it is filled by one product: Hypalon (chlorosulfonated polyethylene). This is a hybrid material which is formulated with curing agents during manufacture of the sheet material. These chemical agents, however, do not cause the Hypalon to cross link or cure until about six months after the sheet has been produced. Thus, it is manufactured as a thermoplastic product, but cures to have rubber characteristics after being installed on the roof.

Single-ply roofs are installed by three general methods. Those techniques are fully adhered, loose-laid/ballasted or partially adhered. The fully adhered procedure involves the use of some type of adhesive which is hot- or cold-applied to the roof substrate and sometimes also the bottom side of the single-ply. The sheet is completely adhered at all points of the field of the roof.

The loose-laid application consists of rolling out the single-ply materials and then seaming the adjacent widths and lengths together. The roof is fully covered in this way and this formed blanket is then attached at the perimeter or edge of the roof. The single-ply is then completely covered with either gravel or concrete pavers to hold it in place and protect it from ultraviolet rays from the sun, roof traffic, etc. The significant issue with this method of application is that the supporting deck must be able to carry the 10 to 24 pounds per square foot of gravel or pavers which are used.

The third method of installing single plies is the partially adhered technique. As the name implies, the sheet is attached only at specific points. There are a number of ways to accomplish this. Sometimes batten bars are screw-attached at the edges of the sheet. The adjacent sheet is then seamed over the top of this bar to seal over the holes made by the screws. Another method is to attach the insulation with screws having large heads which can be adhesively attached to the bottom side of the single-ply. Yet another method is to sue mechanical fasteners which are designed to accept the sheet and a locking device that holds the sheet in the head of the fastener. The advantage of the partially adhered technique is that the need for heavy weights of ballast is eliminated.

MODIFIED BITUMEN

Modified bitumen roofing is a third major type of membrane which is being used in today’s market. This roofing system has some elements of built-up, as well as single-ply. Modified bitumen membranes are typically installed in two plies and sometimes in as many as three or even four plies. Because the modified bitumen products are based on asphalt, they are compatible with many of the materials used in standard built-up roofing. Thus, systems are now appearing which will employ one or more plies of asphalt roll roofing with a covering of a modified bitumen sheet.

Styrene-butadien-styrene (SBS) is one major type of modifier and it imparts rubber-like characteristics to asphalt. Sheet goods and hot melt adhesives made from SBS asphalt have the following advantages:

  • Excellent cold temperature performance and handling characteristics
  • Outstanding fatigue (cycling) resistance
  • Very good elasticity and strength properties

The other major modifier being used in roofing is atactic polypropylene – better known as APP. Whereas SBS produces rubber characteristics, APP develops plastic properties in asphalt. APP-modified sheet goods are extremely tough and have very good traffic resistance. Among the other advantages of APP are:

  • Very good high temperature properties
  • Excellent resistance to ultraviolet light
  • Improvement in low-temperature properties versus standard asphalt

When comparing the characteristics of each modifier, it should be apparent that SBS materials are the preferred products in cooler regions of the United States and the APP modifies are more often used in the warmer climates. The APPs are typically installed using a torch application. However, concern over open flames of the rood has prompted some manufacturers to offer the options of installation with cold adhesives or marginally effective, electrically powered heating elements. SBS sheets, on the other hand, can be applied using hot asphalt, cold adhesive, or torch methods.

Both SBS and APP materials give excellent performance when properly specified and installed. Just as in all roofing situations, the key is to match the product with the requirements of the particular job situation.

The usage of modified bitumen products has grown slowly but steadily since their introduction to the U.S. in the mid-1970s.

OTHER ROOFING SYSTEMS

Other systems are important and fill specific needs in the marketplace. They represent about 6 percent of the installed roofing market with the three major membranes accounting for the remaining 94 percent. Of these other membranes, the three systems that make up the great majority of the 6 percent figure are metal roofing, cold-process built-up and sprayed-in-place urethane. Each of these types has specific applications and, of course, its own set of advantages and disadvantages. For example, sprayed-in-place urethane is highly successful in dry, warm climates of the U.S. because the weather conditions are ideal for its application. Its use is limited in the northern climates because there are only a few days each year which fit the necessary installation criteria for temperature, humidity and wind. Cold process built-ups are used primarily in the western states because of contractor preference and also in situations where small areas of roof need to be installed. These membranes eliminate the need for equipment set-up as the adhesive cab be applied by brush or squeegee.

Metal roofing is used most often on commercial buildings or in applications where specific aesthetics are required. This system is showing some market growth as the products have been improved and its “metal shed” image is being overcome. Metal roofing can be costly compared to the other options which are available.

PERFORMANCE

Now that the choices available have been reviewed, an evaluation of the performance of the various systems is certainly in order. As can be expected, an industry as large, complex and diverse as roofing presents some difficulties from the standpoint of analysis. Fortunately, the National Roofing Contractors Association (NRCA), by far the largest of the roofing contractor groups, instituted a roof survey among its members in 1974. This program, known as Project Pinpoint, was to provide an “early-warning procedure for the identification of problems experienced with roofing materials in low-slope roofing applications." Project Pinpoint Analysis: Trends and Problems in Low-Slope Roofing, 1983-1988, NRCA Special Report, 1989. As a secondary purpose, the data would provide information on the types of systems being used and the frequency of use of each of them.

The data that will be discussed will focus on the years 1983-1999.W. Cullen, “NRCA’s Project Pinpoint tracks roofing’s ups and downs for eight years,” Professional Roofing, 9/91, Pages 22-26. Probably a good starting point is to present the trends which are being noted in the usage of the various membrane types. In terms of the single-ply products, as EPDM significantly dominates this segment of the market, the NRCA data shows this membrane as a separate listing. The remaining single plies are included in the “Other” membrane category.

This information is quite informative and, although there are some fluctuations, identifies some very definite trends:

  • Built-up roofing membranes showed a very steady decrease in usage (with the exception of 1987) until it reached its low point in 1988. The last two years reported in the survey, however, show a reversal with built-up roofing staging a comeback.
  • EPDM systems, on the other hand, seemed to reach its peak in 1988 with a loss in market share in the last two years reported. It appears that all three of the other membrane categories claimed some of EPDM’s market in 1989 and 1990.
  • Modified bitumens show a slow but steady growth throughout the period 1983-1990.
  • The “Other” systems category reported in this survey also appears to be gaining a greater market share. Some of this may be due to a switch from EPDM to single-ply products which are formulated from other types of polymers. Other industry reports indicate that systems such as standing-seam metal roofing are showing greater market strength.

Project Pinpoint provides much more data than simply the market shares of the various membranes. It also gives some insight into the problems which contractors are observing with either their own installations or those of others.

The below table summarizes the ratios of problems identified in a particular year with the number of jobs reported in that same year for built-up and single-ply systems. Note that the problem jobs and the jobs installed are two separate pieces of data. The problems may not necessarily be on a job installed in the reporting year or even on a job installed by the reporting contractor. The ratio simply gives a warning or indicates a concern regarding the direction of the performance for a particular membrane. The specific types of problems being observed will also provide additional useful information, and this will be reviewed as well.


Table: PROBLEM-TO-JOB RATIOS

Membrane Type

1983

1984

1985

1986

1987

1988

1989

1990

 

Built-Up

0.5

0.6

0.7

0.8

0.9

0.9

1.0

0.9

Single-Ply

0.5

0.8

0.7

1.2

1.8

1.1

1.3

1.5

The trends observed in the table are again relatively clear. The built-up systems appear to have a relatively stable problem-to-job ratio. The single-ply systems, in contrast, seem to show a trend to increasing problems or performance difficulties.

The types of problems which are being experienced by single-ply and built-up membranes in decreasing order of occurrence are:

Single-Ply

1. Lap Defects

2. Flashing Defects

3. Membrane Shrinkage

4. Puncture and Tear

5. Wind Related

6. Blistering

7. Embrittlement

Built-Up

1. Inter-ply Blistering

2. Membrane Splitting

3. Wrinkling and Ridging

4. Flashing Related

5. Membrane Slippage

6. Wind Related

In the Project Pinpoint survey, lap defects are consistently the number one problem associated with single plies. The roofing industry has been overwhelmed with studies on why laps fail, proposals for quality control measures, studies on how adhesive application rate influences bond strength and analysis of every other variable in the field seaming process. This attention to single-ply field laps gives emphasis to the key role this application step plays in the success or failure of an installation. The manufacturers themselves have spent a great deal of time and research effort on adhesive development and the industry is now on its second generation of lap cement chemistry. The early adhesives were neoprene-based materials while the newer products are based on butyl compounds. Some manufacturers are also specifying the use of self-adhering tapes as the seaming material to eliminate the chance of misapplying liquid adhesive.

The other problems on the single-ply list will not be discussed in detail. Some are clearly material problems such as membrane shrinkage and embrittlement. Others, such as flashing defects and wind-related failures, can be attributed more to application errors or overly sensitive application criteria. The embrittlement situation is at least partly the result of manufacturers of the PVC materials not completely understanding the applications in which their membranes would be placed. PVC is not compatible with either coal tar or asphalt. Any contact between these materials causes as loss of plasticizers (those chemicals which make plastic flexible) from the PVC and results in a hardening of the product.

The problems with built-up roofing are basically the same issues which have been associated with this type of membrane since its beginnings. Most of these can be associated with misapplication or an improper specification for a given job situation. Interply blistering, for example, can be due to such causes as moisture in felts, skips or voids in the application of bitumen or not having proper contact of felt and bitumen. Blistering became a significant issue when plastic roof insulations such as urethanes, isocyanurates and phenolics were introduced to roofing for greater building energy conservation. The interaction of hot bitumen with these insulations was and is still not completely understood.

Splitting is typically the result of installing a membrane which is too weak and inflexible for an application where significant movement is occurring or expecting a roof to hold structural components of a building together. The remaining problems listed can also be diagnosed as a combination of poor application and/or simply choosing the wrong material for the specific requirements of a job.

In overview, each of the various roofing options available have important application and can be used to address particular problems and job situations. Difficulties are usually encountered when a single membrane is viewed as a universal solution and is installed in circumstances unrealistic to its performance capabilities.

Ken Brzozowski is a co-owner and Chief Technical Director of American Roofing System Corporation, located in both Massachusetts and Ohio. A member of AIPE, Dr. Brzozowski has published numerous articles on roofing, waterproofing, paints and coatings.

Originally published in the September/October 1994 issue of AIPE Facilities.