Vinyl Roofing

Vinyl roofing membranes and other single-ply technologies have brought about revolutionary change in roofing specification and installation over the past 30 years, offering a clean, quick and safe alternative to built-up roofs with greater design flexibility. This article will address vinyl's contribution to roof performance, design, installation and building energy efficiency, and will also discuss the manufacturing processes and environmental performance of vinyl roofing membranes.

Overview

Designers of commercial and industrial buildings with low-slope roofs - such as sports arenas, schools, manufacturing facilities, office buildings and hospitals - face one of the toughest structural challenges of all: designing a roof system that withstands wear and tear and the toughest of weather conditions while remaining watertight.

About seven to ten percent of all commercial and industrial roofs today are covered by a vinyl roofing membrane. At the most basic level, vinyl roofing membranes are weatherproofing sheets that utilize flexible vinyl as a primary component, and all fall into the "single-ply" family of roofing membranes.

Types of Roof

Most vinyl roofing membranes on the market today differ slightly. Nearly all are internally reinforced with a woven fabric (polyester or fibreglass) which improves the strength and stability of the membrane. Vinyl membranes are appropriate for use in any of the three main roof applications, depending on building specifications:

• Mechanically attached systems, in which the membrane is affixed to the roof deck, typically with a screw and metal plate or fastening bar. The fasteners are then covered and sealed against outside weather.
• Adhered systems, in which the membrane is "glued" to the roof deck or suitable substrate using either a solvent- or water-based adhesive.
• Ballasted systems, in which the membrane is secured to the roof system using weight, typically roofing pavers, blocks or stones.

Flexible and lightweight, vinyl roofing membranes have a proven and tested performance history that goes back more than 30 years in the global industrial and commercial roofing marketplace.

History

1960’s

Historically, commercial and industrial buildings with low-slope roofs have been covered using built-up roof technologies, using layers of hot asphalt and roofing felts. The advent of single-ply roofing technologies in the United States came in the mid-1960s, with vinyl membranes arriving shortly thereafter in the early 1970s. The concept was attractive to roofing contractors and building owners, because single-ply technologies offered a clean, safe, energy-efficient and cost-effective alternative to built-up roofs, as well as greater design flexibility with slopes and roofing aesthetics.

1970’s

In that same time period in Europe, single-ply roofing membranes were being used widely and successfully. As the industry grew, European companies and their American counterparts made advances in formulation and installation techniques. It was this transfer of technology between continents in the early 1970s that really began to drive the expanded use of vinyl roofing systems in the United States. Soon after, in the mid- to late-1970s, prices for asphalt began to rise sharply, further encouraging specifiers, contractors and building owners to seek alternatives to built-up roofs.

Today

Today, some of the first vinyl roofs installed are still in service. The knowledge base and performance record that have grown during the vinyl roofing industry's nearly 40-year history give it an added advantage today over many newly developed competitive single-ply systems.

Design Considerations

Product Characteristics

Vinyl roofing membranes can often be distinguished from some other low-slope roofing products by their light colour. Vinyl roofs are typically white, grey or tan, and hence tend to reflect heat away from building rooftops and reduce the amount of heat that enters a building. Research by both the U.S. Department of Energy (DOE) and Environmental Protection Agency (EPA) shows that reflective roofs may lower a building's air conditioning utility expense by as much as 40 percent in some climates.

Colour

At the same time, vinyl membranes also offer colour versatility, enabling designers to incorporate patterns, logos or a variety of colours into the vinyl roofing membranes. Specific colour matches with other building elements, such as weathered copper or brick, are also possible. Vinyl membranes are designed to repel dirt, maintaining aesthetic appeal and maximizing reflectivity.

Performance

Vinyl roofing membranes can be designed to resist wind uplift, structural movement and harsh outdoor elements; in addition, vinyl's inherent durability and the strong reinforcement fabric used in such membranes further enhances vinyl's performance. As compared to newer membrane materials, vinyl roofing membranes have a well-documented performance history which demonstrates consistent good performance under outdoor conditions.

Welding

Most vinyl roofing membranes are sealed using hot air, which can be compared to welding metal. The process joins two rolls of membranes at the overlap areas (without using another dissimilar material) to create one monolithic layer that covers the entire roof. The heat-welded, permanently fused seams on vinyl roofing membrane form a watertight seal, and their peel strength can be critical to the roof system's long-term performance against wind forces. Properly designed reinforced vinyl roofing has the ability to resist severe wind forces.

Weather Sensitivity

The heat-welding process also makes the vinyl roofing membrane less weather-sensitive during installation than many other roofing systems. Even some properly designed roof systems can exhibit some moisture accumulation during the wet winter months (wet cycle), but proper roof design will also allow for drying out during the summer months. The vinyl roof will typically aid in this drying-out process, which can maximize the thermal performance of the roof insulation. A reduction in moisture buildup can reduce the potential for the corrosion of fasteners and steel roof decks.

Fire

Vinyl has inherent fire-retardant properties which provide self-extinguishing characteristics which can significantly reduce flame spread in the event of a roof fire

Technical Data

High standards for technical performance of vinyl roofing membranes are critical to ensure a sound, watertight building. Vinyl roofing systems are subjected to a battery of tests designed to measure thickness of the membrane, tensile strength, water absorption, thermal stability, seam strength and other performance criteria.

Installation

Vinyl roofing membranes can be fabricated in a variety of widths and lengths to meet the exact dimensions of a roof. For most installations, though, there is still a need to join roofing sheets by creating seams. The performance of any single-ply membrane roofing system is highly dependent on the strength of its seams, one of the most obvious entry points for rooftop water.

When specifying vinyl roofing systems, it's important to consider which application method is appropriate for the building, taking into consideration the roof deck, insulation, slope, local building codes and desired aesthetics, in addition to other factors. Barrels, domes, steep slopes and large expansive flat roofs can be accommodated. Single-ply vinyl membranes can be adapted to specific site conditions on an individual installation. For instance, steeply pitched roofs can require different installations - mechanically fastened near steeply sloped edges and adhered sections for lower sloped sections.

Application Methods

The following design considerations should be kept in mind for the three main application methods:

Mechanically Fastened

Mechanically fastened roof systems are most popular over insulated substrates such as steel and wood decks, which can accept fasteners. These systems are typically light in weight and, because the membrane is exposed, can take advantage of a smooth or even colourful appearance to satisfy aesthetic needs in the roof design. Mechanically fastened roofs provide positive attachment of the membrane to the structure and are particularly adaptable to common rooftop conditions.

Vinyl roofing membrane can be mechanically fastened directly into the structural deck or structural framing in many configurations. Steep slopes or irregular shapes can be easily accommodated. Significant development in in-seam and metal plate fasteners has enhanced uplift performance, benefiting the wind uplift ratings of reinforced vinyl roofing systems. New fastener installation devices provide efficiencies in speed and ease of application, as well as installation reliability.

Adhered

Adhered roofing systems are also lightweight and smooth-surfaced and they are flexible so they can absorb normal structural movement without compromising the system's adhesion. They are often used on high-profile facilities that have unusual shapes, forms and colours. Adhesion to the substrate is achieved by using a water-based or low-solvent-based adhesive on both the substrate and the membrane.

Adhered systems are typically installed directly over a compatible rigid insulation board that has been mechanically fastened or adhered to the structural deck or another suitable substrate

Ballasted

Ballasted roof systems consist of a roofing membrane attached at the roof perimeter and held in place with ballast. Ballasted systems are usually limited to roofs with shallow slopes and are only suitable when the structure is capable of withstanding the added weight of the ballast or pavers. The weight-bearing capacity of the structural deck and support should be evaluated to include dead loads, such as existing roof systems, without encroaching on live-load capacity. The ballasted system often uses rigid insulation board. The board is placed below the membrane in a standard ballasted system, or above the membrane in an inverted roof assembly.

Ballast can be stone, concrete pavers, lightweight interlocking pavers, or a combination of these. Pavers provide added protection to the vinyl membrane from abuse or excessive traffic and may be attractive surface materials as well. The ballasted system can be cost-effective due to the minimal need for fasteners and adhesives.

Cost

Installed costs for vinyl roofing membranes vary according to installation technique. These costs are extremely competitive with alternatives and offer benefits in lifecycle costs as well, due to vinyl's durability and low maintenance requirements.

Maintenance Requirements

According to a study conducted in 1996 by engineering firm Simpson Gumpertz & Heger Inc., the average lifespan of a vinyl roofing membrane - including both reinforced and unreinforced systems - was approximately 14 years. However, real-life experience among reinforced vinyl roofing manufacturers places that estimate much higher, with some vinyl roofing membranes lasting more than 30 years. When damage occurs to vinyl roofing - such as a tear, cut or seam opening - it can be spot-repaired without recoating, resurfacing or resaturating. Rather, the membrane need only be cleaned and then repaired with the same techniques used to weld the seams during installation. The process for regular maintenance and repair (when necessary) is greatly simplified. While vinyl roofing systems require little maintenance, it's recommended that all roof systems be inspected twice a year for damage, plugged drains and weathered sealants, and after each heavy storm.

Selection and Specification Guide

Relevant standards

ASTM D4434: Specification for Polyvinyl Chloride Sheet Roofing is the only nationally recognized standard created specifically for and applicable to all vinyl-based roofing membrane. For this standard, ASTM set a minimum 5,000-hour criterion for weathering performance. This is based on an accelerated weathering test that subjects the material to cycles of exposure to a xenon arc light source to simulate sunlight, water spray and elevated temperatures. All of these factors can degrade roofing membranes.

The minimum requirement for a vinyl sheet under this test criterion is that it can't show any evidence of cracking or crazing and only negligible evidence of discoloration after 5,000 hours of exposure.

Code Information

In general, vinyl roofing membranes far exceed the minimum requirements of model and local building codes for many performance criteria, including fire propagation and energy efficiency. Proper design of vinyl roofing systems can also enable these systems to meet even the most demanding code requirements for performance criteria such as wind resistance.

Manufacturing

Raw materials

All vinyl products are made from a unique combination of vinyl resin and various additives which give these products their particular properties. Every formulation is different, and most are proprietary. Some of the additives commonly used in vinyl roofing membrane include:

• Plasticizers, which give vinyl roofing membranes flexibility to ensure long-term durability, weldability and reparability. Plasticizers are selected for their stability and resistance to degradation by UV light. Tests have shown that 20-year-old applications of vinyl roofing membranes have retained 80 to 90 percent of their original plasticizer content.
• Stabilizers, commonly used in reinforced vinyl roofing membranes to improve heat and UV light stability.
• Pigments, which, added during the manufacturing process, give vinyl a range of colours.

Processing / Fabricating

Once the additives have been combined with the resin, the resulting material is called vinyl compound and is in liquid, pellet or powder form. The nature of the vinyl compound allows versatility in the production process, enabling manufacturers to meet many of the unique performance requirements of buildings. In the next stage of manufacturing, vinyl roofing membrane is created through one of two different processes:

Spread Coating

Vinyl compound in a liquid form known as plastisol is mixed and uniformly applied onto the reinforcement layer. This layer may consist of either a non-woven glass mat or a polyester fabric, and is fused at high temperatures. There are no pressure rollers used to form the sheet in this application method.

Calendering and Extrusion

These processes use rollers to create a vinyl membrane under heat and pressure. The materials' reinforcement is incorporated by laminating two calendered or extruded films around a polyester or fibreglass scrim.

Increasing the thickness of the sheet typically improves the weathering properties as well as tensile strength, tear resistance and puncture resistance. Finished sheets of vinyl roofing membrane are wound onto rolls, ready to be shipped to the construction site.

Environmental Considerations

Recyclability

Vinyl roofing membranes can be and are being recycled into such second-generation products as speed bumps, parking curbs and asphalt patching material. Nearly all vinyl roofing manufacturers utilise post-industrial recycled roofing scrap which has been generated in their manufacturing processes in their products. In other cases, because of the high performance standards for the product, little or no amounts of recycled content are used. As processing technologies improve, however, there will be expanded opportunities for the use of recycled material in this industry.

Energy Efficiency

The relatively light weight and compactness of vinyl roofing makes for energy-efficient transportation of the product to the installation site. The manufacturing process for vinyl roofing membranes is also more energy efficient than that of some alternatives. But vinyl's most notable advantage in energy efficiency comes in the use of the product. Because they are typically light in colour, vinyl roofing membranes are known as "reflective" roof surfaces, reflecting sunlight and radiant heat away from a building, helping the structure to stay cool and reducing energy use for air conditioning.

Air Quality

The light colour of vinyl roofing membranes has also been shown to have a positive impact on air quality. Researchers at NASA found that summertime urban air temperatures can be greatly reduced by using such light roof surfaces as compared to dark-coloured roofing materials. Decreases in urban air temperature can substantially improve air quality, since smog is the result of photochemical reactions that are triggered by air temperature increases.

Resource Conservation

Lightweight vinyl roof systems help reduce the need for steel and wood which are needed to support heavier roof system options.

Other Environmental Considerations

The versatility of vinyl roofing membranes allows it to re-cover an existing roof, which is both an environmental and financial benefit. A built-up roof can be left in place, overlaid with insulation and topped with a vinyl roofing membrane. Conversely, disposal of a built-up roof could mean high disposal costs, greater material volume in landfills, increased labour requirements and possible exposure to asbestos in the old roofing system.

Finally, vinyl roofing membranes can also be found as a component in "green roofs," which consist of a multi-layer soil and drainage system upon which vegetation can grow on urban roofs. Green roofs, in turn, improve the energy efficiency of buildings, enhance the look of urban rooftops and promote healthier air in such area.

Source: The Vinyl Institute

For more information on this source please visit The Vinyl Institute