"So what is the best roof system in the world?"  Without a doubt, this is the most frequently asked question in the roofing industry.  And with at least ten product categories in this $11.5-billion market, the range of choices can easily confuse a building owner or facilities manager.

There is a method, however, to selecting roofing that goes beyond flashy advertising, following market statistics, or simply guessing.  The parameters include:

• Owner's objectives.  Rarely will an owner say, "I don't care what it costs, just build me the best roof."  Generally, budget, life expectancy, and esthetics will all play a role.  For example, one owner was interested in selling a strip shopping center with a roof that was in poor condition.  The owner knew a savvy buyer would use the roof's condition as leverage during negotiations, so his goal was to put on the least expensive roof available, but one with a ten-year warranty, that would be recognizable as new.
• Code requirements.  Codes cover energy conservation, fire and wind resistance, and equipment mounting.  Building codes are not very restrictive in terms of system selection, but in certain instances, they may affect choice.  For example, in high wind zones, gravel surfacing or ballast on high-rise roofs may not be permitted.
• Insurer's requirements.  Many owners are either insured by a Factory Mutual company or by an insurer that follows FM requirements.  They test roof membranes with respect to wind, fire, and hail resistance, and review the performance of the insured's roofs with respect to claims.
• Site conditions.  The roof designer must take into consideration the building's height for wind design; the higher the roof, the higher the wind speeds it will be exposed to.  Building height also affects material transportation to and from the roof--in particular, pumping or hoisting hot asphalt for built-up or hot-applied modified systems.  There are height limits for pumping asphalt, and if transporting asphalt to the roof is difficult, one must be concerned over safety and with keeping the asphalt hot enough.
  Roof location on site, relative to other structures, is also important.  Freestanding buildings with plenty of ground access area create few problems.  But roofs that are landlocked by other roof areas, pipelines, or high tension wires require an analysis of how material handling will be affected.
• Building use.  Interior occupancy may have a huge impact on roof selection.  For example, some food processing plants do not allow the penetration of steel decks with insulation or membrane fasteners, often for sanitary and cleaning reasons.
  In one unusual case, a cheese producer had to consider odor control/absorption during construction when selecting a roof for its plant. The fear was that customers might taste asphalt or solvent in their aged cheese.  In other cases, buildings that house swimming pools or locker rooms may have interior humidity that could require a vapor retarder and a special level of insulation.
• Roof deck construction.  Attachment of the roof insulation and/or membrane is directly influenced by the deck type.  There are screws, rivets, adhesives, nails, and various types of ballast materials that have been developed over the years to address roof attachment, depending on the type of deck.
• Climatic exposure.  Some roof membranes by virtue of their color or reflectivity are more suitable for areas with intense sunshine and high summer temperatures.  For example, selecting a black EPDM roof membrane for a roof in Minnesota probably won't increase the building's energy usage over the year, but a black roof surface in Arizona will undoubtedly increase roof temperatures and interior cooling loads.  Likewise, gravel-ballasted built-up roofs are more durable in hail-prone areas than smooth surface built-up roofs. 
  The climate the roof must endure during construction, such as cold weather, is also a factor.  Temperatures much below 40 degrees require special producers such as insulating hot asphalt piping.
• Chemical and traffic resistance.  Most office and warehouse roofs are not subject to chemical fallout, but many manufacturing plants vent chemicals (e.g., chlorine or petroleum distillates) that can be harmful to the roof.  Most roofing manufacturers offer a chemical resistance list as a guide for the roof-selection process.
  Meanwhile, foot traffic can create problems if the roof membrane and insulation are not suitable.  For example, single-ply roofs can suffer punctures and compression of underlying insulation, and fasteners can protrude through the membrane due to heavy roof traffic.  Shopping mall roofs, for instance, may be better suited to modified bitumen or built-up roofing, if traffic cannot be confined to walk pads.
• Water drainage.  Naturally, sloped roofs offer the best drainage.  But in the past, many owners preferred dead-level roofs, so they could build new floors as needed.  Drainage on buildings with flat roof decks may be improved by adding slope through tapered insulation, adding drains, or through a combination.  But these drainage features can add cost.
• Structural capacity.  Roof loads become a factor when a second roof is built, where a ballasted single-ply system is used, or where heavy walk surfaces or platforms are added.  A structural engineer can ensure that load capacity is not exceeded.

Several other criteria can enter the discussion.  Cost, product availability, experienced contractors, maintenance issues, warranties (ten years at minimum), and esthetics are all part of the mix.

In short, there is no one answer to the question, "what is the best roof in the world."  Rather, the best roof in the world is the one you carefully choose after reviewing and studying the appropriate selection criteria.