Technical Article

A Brief History and the Evolution of Low Slope Roofing …or…

By William A. Kirn, RRC Technical Director National Coatings Corporation


Once upon a time, about 100 years ago, all low slope roofs (read: industrial, commercial, schools, office buildings, factories) were constructed very similarly. The only option might have been whether the roof was asphalt (based on the dregs of the crude oil refining process) or coal tar (a byproduct of coke used in basic steel making). Roof decks were wood planks on heavy wood timber supports. Insulation was rare or non existent as heating energy costs were minimal, and there was virtually no air conditioning.

Since then there have been dramatic changes not only in roofing options, but also in roof and structural deck designs and insulation types and amounts. This paper will focus first on the evolution of deck and structural components and the issues this has created, then insulation, and finally roof systems. As will be shown, the changes in roofing materials have complicated the roof selection and decision process.


As stated above, roof decks, especially in the east and mid west portion of the US had typically been constructed of thick wood plans, between 1½ and 2 inches thick (actual). These were usually supported by very large 10”x 10” thick beams. This type structural design has a dramatic effect on the stresses placed on the roofing system. This heavy wood construction has very little thermally induced expansion and contraction. It is however, susceptible to expansion and contraction due to moisture entrapment and swelling. Thus, moist air rising to the underside of the wood structural support and deck will cause some expansion and contraction. This is offset by the usually black roof and minimal insertion transmitting significant heat into the deck, causing it to dry out and contract.

The wood planks also afforded some very minimal insulation (The R value of wood is 0.8 per inch thickness). By contrast the insulation value of polyisocyanurate insulation, widely used today, is 5.6 per inch thickness.

Currently the typical low slope commercial roofing design uses fluted sheet metal decks, which are usually puddle arc welded and installed over light weight bar joists. This light weight construction, while being acceptable to building code bodies, has considerably less rigidity of the earlier used wood timber and plank deck construction techniques. These light weight metal deck systems allow for more deflection of the roofing assembly when the roof is loaded with snow, standing water or subsequently installed equipment such as satellite antennas and air handling equipment. These deflections are transferred to the roofing membrane above and create stresses on the membrane at attachment points and seams. A cyclic action is caused as the weight of snow and rain causes the roof to deflect, then when the water evaporates, the weight is lifted. This also causes stresses on the roofing membrane above.

Since steel has a higher coefficient of expansion and contraction than wood, the modern roof deck will expand more than the wood decks. This also puts additional stresses on the roofing membrane, especially at roof transitions, edges and parapets.


While drainage may not be immediately considered when reviewing the evolution of the low slope roofing industry, it does indeed have an impact. In the early part of the 20th century, all roofs had significant slope and drains and gutters received significant consideration during design. However, as design practices changed and the costs of designing and building sufficient slope to drain the water quickly off the roof were considered, the amount of slope decreased. This allowed greater contact of water with the roofing membrane and also added additional load to the roof. The increased weight further deflected the already light weight deck.


Low slope roofing insulation became more prevalent during the 1960’s and forward as increasingly more buildings became air conditioned and as the cost of energy, both for heating and cooling rose dramatically. This had a dramatic effect on the longevity of roofing materials. Before roofs were heavily insulated, heat was easily transmitted through the roofing material, deck and into the building. Since all roofs were either black or covered with stone ballast, they experienced tremendous heat build up, but this heat was quickly transferred to the building interior. Thus the roofing material was not heated as hot or as long as it currently is, now that significantly more insulation is present. The insulation retards the rate of heat transfer through the membrane and into the deck and building cavity below. The fact that heat was easily transferred into the building kept the roofing materials from being degraded by the heat.

Currently insulation levels may exceed R30 as specified by code or because of the building’s use and geographic location. This means the heat that was previously transferred into the building now remains trapped in the roofing membrane. This trapped heart can have a detrimental effect on the roof’s service life and longevity.

To further complicate the insulation issue, there is now an option where the insulation is installed on top of the roof waterproof membrane. This system is called IRMA (inverted roofing membrane assembly) or PMR (protected roofing membrane).


As was stated above, roofing membranes were built-up; meaning plies of roofing felt adhered with molten asphalt or coal tar bitumen. The roofing felt was an organic (cloth) mat saturated with asphalt. Typically 4-5 plies of felts were installed over the wood deck. Until the 1960’s, all roofs were either asphalt or coal tar.

In the 1960’s in Europe scientists began testing polyvinyl chloride (PVC) as a new type roofing material. The PVC had advantages as it could be heat or solvent welded, making field seams watertight.

Similarly, again in the 1906’s and in Europe, research was being conducted to improve the performance of asphalt by blending in synthetic polymers at levels of 10-20%. Conceptually this was to incorporate the durability and low temperature flexibility of APP and SBS modifiers into the molten asphalt. This technology was exporter to the US and was used commercially in the US in the early 1980’s.

In the US, a synthetic thermoset rubber polymer technology called EPDM was used for pond liners and other liquid holding sites and as bladders and tank liners for water and other chemicals. This material, besides being an excellent waterproofer, had durability properties that would allow it to be used as a roofing membrane.

In the 1960’s chemists developed polyurethane foam that could be applied on-site using advanced plural component spry technology. This material had excellent insulation and waterproofing properties. It was discovered that it could be applied directly to roof decks as well as expositing roofs, making it an ideal option for re-roofing. The foam could be applied at varying thicknesses to change the slope of the roof and eliminate ponding conditions.

The most recent roofing system is a variant of the PVC roof, however using polymers incorporating ethylene and propylene. This is a thermoplastic and has similar installation requirements as PVC.


There was a time when roofs were merely installed and, after leaking, were torn off and re-roofed. However, today’s facility manager realizes that roofs are a very expensive asset and like other building assets, can be maintained. This can be done by following a regimented maintenance schedule of periodic inspections, and repairs as necessary. Coatings can be applied to the roof when installed or during its life to prolong its serviceability. Over the past 20 years developments in waterborne acrylic technology have enabled any type roof to be a candidate for life extension using these maintenance coatings.

The emergence of technically trained professional roof consultants through the Roof Consultants Institute provides a unique service to the building owner/facility manager. The designation RRC, Registered Roof Consultant, identifies these individuals as being fully knowledgeable about all types of roofing. Moreover they are required to maintain the highest ethical standards in conducting business and are also obligated to continue their technical education through additional CEU course work.


As has been shown above, the architect, consultant, specifier and contractor are offered many choices for new and re-roofing. Interestingly, the traditional built-up roof that had been used for over 100 years still holds about 30% of the low slope roofing market. The decision is further complicated by what type, where and how much insulation should be used. All these options make it difficult to make the “right” decision. When faced with a reroofing decision, it is not uncommon for a building owner to get six different quotations for six different roofing systems, with each system being presented as the “best” option.


To summarize the history of low slope roofing; there has been a shift away from asphalt to non-asphaltic chemistries and technologies. Once, all roofs were either asphalt or coal tar based. Now the majority of newly installed roofs are not asphalt. The emergence of various systems based on rubber or plastics industry related chemistries will continue to be refined as these early generation products evolve.

Another complicating issue is the warranty. The building owner/facility manager must not confuse the wording on a warranty document with service life. While a warranty may look impressive, it does not absolve the building owner/facility manager from his/her maintenance responsibilities.

Building owners, the ultimate customer in this value chain, will push for greater durability, longer service life and seek out ways to maintain their existing roof inventory. The concept of life cycle cost determination has become a valuable tool when applied to roofing system options. While one roof may cost more initially than another, its cost over its entire life, (the definition of life cycle cost) becomes the overriding consideration to the building owner. Moreover, the concept of sustainability is also becoming a key factor in the roofing decision process. Consider the analogy of the car’s crankcase oil. Suppose there was no way to change the oil, and once the oil “broke down” the engine would seize up and the engine would need to be replaced. However, if an innovative engine manufacturer developed a technique for changing the oil, the fill tube and the drain plug) then the engine’s life could be greatly extended. The same is true for roofing. If the roof can be maintained/sustained it will last considerably longer and have dramatically lower life cycle costs than one that cannot be maintained.

The future of maintenance will involve two key factors. First the building owner, specifier, consultant community must become more sophisticated in their involvement in the roofing decision process. They must ask more incisive questions than, “How long is the warranty?” The better question would be, “How does this roof last?” and “How does this roof wear out?” Their focus should be on the life cycle costs rather than the first, or installed cost. This will drive the roofing system manufacturers to provide total roof maintenance systems with accompanying documentation that clearly defines the costs and building owner responsibilities. Roofing decisions will be made on proven durability, not manufacturers’ warranties, and on the ability of the roof to be maintained, just like other assets such as HVAC, and elevators.


In some respects the future is already here. Building owners have discovered the benefits of light colored, reflective roofs as means of reducing the air conditioning electricity usage to achieve cooling on the building. The US EPA, through their ENERGY STAR® program has established minimum solar reflectance values of new and weathered roofing materials. These criteria can assist the building owner/facility manager in selecting roof systems and maintenance coatings that do reduce air conditioning costs.

A recent innovation in roofing has been the introduction of “green” roofs, where the roof is actually the base for a garden. These may be very simplistic with potted plants and small flower beds, or may be quite elaborate with large trees and complete agricultural ground cover. The benefits of this system are reduced storm water run-off, insulation provided by the earth in the flower beds and solar reflectance from shrubs and trees. While this sounds quite beneficial, there are several drawbacks that must be addressed. The roof structure must be able to support the “dead load” of the plantings, not only when newly installed, but years later when the seedlings have reached maturity. The structural load will also include trapped moisture, leaves and other organic debris. The root system of the plantings must not damage the underlying roofing membrane

While this sounds interesting, it must be remembered that there is still a waterproofing membrane under the plantings. This system must include a method for inspecting and repairing the underlying roof.

Photovoltaic systems comprised of thin panels that absorb sunlight and generate electricity have been placed on low slope roofs to provide some or all of the electricity required for the building. This is another example of using the roof as a support for other functions. Again, the structural issues must be addressed and the roof membrane must remain accessible.


Roofing and the technology behind the roofing products has changed rapidly in the past few decades. Building owners are becoming better educated and more sophisticated in their decision-making. This will drive the other participants in this industry, architects, consultants, specifiers and contractors to offer more tangible as well as intangible value to the building owner/facility manager. However, the fundamental requirements for the roof still remain the same: to keep the building interior, equipment and occupants dry. This has been true since the first roof.