Design Considerations

Section Contents:

Product Characteristics
Technical Data
Maintenance Requirements

Product characteristics. Vinyl roofing membranes can often be distinguished from some other low-slope roofing products by their light color. Vinyl roofs are typically white, gray 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.

At the same time, vinyl membranes also offer color versatility, enabling designers to incorporate patterns, logos or a variety of colors into the vinyl roofing membranes. Specific color 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.

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.

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. In fact, vinyl roofing systems can be designed to meet or exceed the wind uplift requirements to obtain Factory Mutual (FM) approval.

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.

Vinyl roofing systems offer a wide range of application techniques such as mechanically fastened, adhered, ballasted and "green" roof systems. (See "Installation.") 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. More information on the specific technical performance of a vinyl roofing membrane can be obtained from the manufacturer (see "Resources").

In 1999, engineering firm Simpson Gumpertz & Heger Inc. conducted a detailed analysis of the technical performance of vinyl roofing membranes as compared to thermoplastic polyolefin (TPO) and other thermoplastic membranes such as acrylonitirile butadiene (ABC) and ethylene-propylene (EP). Test data on 31 different performance criteria (according to ASTM D-4434) for 13 membrane samples were collected and assigned performance values on a scale of 100 to allow for comparability.

When averaged together, the performance ratings of seven of the eight vinyl samples included in the analysis outperformed the other materials, with up to a 36 percent better performance rating than the TPO samples and up to a 57 percent better performance rating than EP.

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. Most vinyl roofing membranes are sealed using hot air, although the methods of sealing seams may vary depending on the individual roof conditions.

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. This ensures applicability to almost any roofing situation. 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. The following design considerations should be kept in mind for the three main application methods:

•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 colorful 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 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 colors. 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 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 slopes no greater than 2" in 12", 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.

Building height and location can also contribute to the design of the ballasted system. High degrees of wind uplift resistance can be produced by altering the ballast pattern, either by increasing the ballast weight or stone size, or by adding positive attachments at the more vulnerable corners and perimeters.

Vinyl membranes can also be installed in re-roof (tear-off) and re-cover roof construction, over existing built-up roofs, some metal roofs and existing single-ply membranes. In re-cover construction, if the existing roof is dry and sound, the vinyl roof system can eliminate the cost of disposing of the original roof and the existing insulation layer can be re-used.

Fleece- or felt-backed vinyl membranes are ideal for use over minor imperfections such as ridges or splits that may have formed in an existing smooth-surfaced roof system. For re-roofing applications, the fleece or felt serves as a separation or cushioning layer between the vinyl membrane and a pre-existing, smooth built-up, other single-ply or cap-sheet roofing surface, as well as concrete decks or certain insulation. The fleece- or felt-backed membranes can be mechanically fastened or adhered directly to selected substrates with adhesives.

Cost. Installed costs for vinyl roofing membranes vary according to installation technique, ranging between $1.28 and $1.89 per square foot, according to R.S. Means 1999 data. These costs are extremely competitive with alternatives and offer benefits in lifecycle costs as well, due to vinyl's durability and low maintenance requirements. For example, installed costs for EPDM (rubber) range from $1.02 to $1.61 per square foot, elastomeric roofing membranes (i.e. Hypalon) range from $2.83 to $13.15 per square foot, modified bitumen roofing ranges from $1.29 to $2.78 per square foot and built-up roofing ranges from $1.42 to $2.45 per square foot.

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.