Pros and Cons for MB Roofing
|A few manufacturers focus on quality and provide a superior product.||Loses some advantages if applied using normal hot asphalt.|
|Easily repaired and modified.||Commodity manufacturers may not produce a reliable product.|
|Robust||Torch down installation requires open flames and related safety issues.|
|Available with a variety of cool coatings.||Cold fluid adhesives may have high VOC content.|
|Phased two-ply installation can allow contractor to temporarily “dry-in” but completion of the cap ply near the end of construction provides owner a new roof that has not been damaged by construction activities.|
The single-ply family of roof membranes is composed of thermoplastic and thermoset products. Single-ply sheets are factory-fabricated and installed in a single thickness. Single-ply membranes are relatively easy to install on steep or complex roof slopes. In comparison to BUR or MB membranes, they are also very lightweight (except for ballasted systems). However, because there is only one layer of waterproofing, they do not offer the reliability of multiple layers.
There are six primary methods for securing single-ply roofing systems to the roof deck or other substrate:
۱٫ Fully Adhered: The membrane is adhered in a continuous layer of adhesive, preferably to a insulation cover board. Fully adhered roof systems typically have the highest wind uplift resistance and
۲٫ Ballasted: The membrane is loose-laid over the substrate and then covered with ballast to resist wind uplift. Ballast can either be large aggregate (for example, 1-1/2 or 2-1/2 inches nominal diameter, depending upon design wind speed), concrete pavers weighing 18 to 25 pounds per square foot (psf), or specially designed lightweight interlocking concrete pavers weighing approximately 10 psf [49 kg/m²]. Ballasted systems are limited to a maximum slope of 2:12. Ballasted systems should conform to ANSI/SPRI RP-4. Finally, ballasted systems should not be used in high wind or hurricane areas because the ballast tends to become airborne, causing massive damage to adjacent buildings.
If crushed aggregate is specified, a stone-protection mat between the membrane and aggregate should be specified to avoid puncturing the membrane. A stone-protection mat is also recommended when smooth aggregate is used because some sharp fragments are often among the smooth aggregates. Also, aggregates sometimes fracture into very sharp pieces after they have been installed. It is also a conservative practice to specify a mat to protect against abrasion and puncture from fragments during paver installation. A somewhat thinner mat is normally sufficient for paver-ballasted jobs.
NRCA recommends designers consult specific membrane manufacturers’ recommendations for their acceptable aggregate types and ballast application rates.
Generally, loose-laid, ballasted roofing systems are not recommended, or at least, discouraged, for three reasons: 1) a leak is very difficult to locate because the water runs freely in numerous directions and for longer distances under the loose-laid membrane; 2) the membranes tend to pull at their perimeter ties, raising and stretching the membrane around the parapet copings and base flashing areas where the membrane then thins and punctures easily; and 3) the ballast can puncture the roof membrane. For these reasons, fully adhered systems or mechanically attached systems are preferred over loose-laid.
physical performance and are typically considered the highest performing method of installation for single ply roofs.
. Mechanically Attached: The membrane is loose-laid except for a discrete rows of fasteners. There are a variety ways of fastening and fabricating seams with this method, as described in A Guide for the Wind Design of Mechanically Attached Flexible Membrane Roofs which is available from the National Research Council Canada.
Mechanically attached systems may not be suitable for buildings in high wind zones. Specify an air barrier on the deck (in conjunction with a vapor retarder in cold climates with high humidity interiors or with membranes or on new concrete decks) to prevent wind pumping (see discussions above under air barriers and vapor barriers).
To avoid tear propagation in the event that the membrane is torn, it is recommended that only reinforced membranes be specified for this attachment method.
Because of the mechanical stresses focused on the edges of the sheet and the air pumping which acts to pull interior air into the roof assembly and cause condensation problems, mechanically fastened membranes are not typically considered the best choice.
۴٫ Electromagnetic Induction Welding: A thermoplastic roof membrane is bonded to fastening plates without membrane penetration or a fastener line at membrane sheet seams. The fastening plates are factory coated with the same material as the roofing membrane and an induction welder tool is used to bond the roof membrane to the plates from above the membrane.
۵٫ Loose-Laid Air-pressure Equalization System: The membrane is fully adhered around the roof perimeter, but elsewhere the membrane is only loose-laid. This system should only be used over an air-impermeable roof deck or over an air barrier. To compensate for minor air leakage between the membrane and the deck/air barrier, air-pressure equalization valves are installed at prescribed intervals. The valves are one-way: they allow air underneath the membrane to vent out, but outside air is prevented from flowing through the valve and underneath the membrane. As with mechanically attached systems, it is prudent to only specify reinforced membranes for this attachment method. This type of system is susceptible to wind blow off if the vents fail to operate or future roof penetrations are cut through the deck/air barrier and left unsealed. These systems are typically not recommended due to shifting of the insulation board that commonly occurs and the difficulty in detecting leak locations due to the free flow of water underneath.
۶٫ Protected Membrane Roof (PMR): See the Modified Bitumen (MB) section above.
Single-Ply Roofing Materials
Thermoplastic materials do not cross-link, or cure, during manufacturing or during their service life. Field-fabricated seams are typically welded with robotic hot-air welders. Hand-held, hot-air welders are used to weld seams at flashings and penetrations. Thermoplastic membrane seams are typically extremely reliable when properly installed, resulting in a very low incidence of seam failures. These sheets are normally around 5 to 12 feet wide [1.5 to 3.6 m]. Some manufacturers weld the sheets together in the factory to form large sheets that are then welded together on the roof.
Primary membrane types in this category are:
Polyvinyl chloride (PVC): PVC membranes are among the oldest single-plies still available. If in contact with polystyrene insulation, the polystyrene will cause the plasticizers in the membrane to leach out. To avoid such membrane embrittlement, a separator sheet needs to be installed between the membrane and the polystyrene. To avoid membrane damage, a separator is also needed to isolate PVC from asphalt and coal tar products. The ballasted attachment method is not recommended because fine dust particles from the ballast or particulate fall-out from the atmosphere may leach plasticizers from the membrane. PVC membranes are available in a wide variety of colors and can even be printed with building names and logos. This membrane is sometimes selected for steep-slope roofs where a strong or unique color is desired. PVC is naturally a brittle material and must be modified with plasticizers to be suitable for roofing. Some early formulations of PVC suffered from plasticizer leaching out over time and experienced catastrophic failures. Select PVC membranes that have been manufactured for many years to verify their stability.
PVC and PVC alloys are resistant to animal fats and grease and are a good choice for roofs with kitchen exhausts.
PVC Alloys or Compounded Thermoplastics (also referred to as PVC blends): These membranes are related to PVC membranes. They are primarily compounded from PVC, but they have additional polymers that provide somewhat different physical properties. Only a very small number of manufacturers make these products. The primary types of membranes in this category are: copolymer alloy (CPA), ethylene interpolymer (EIP) and nitrile alloy (NBP).
Thermoplastic Polyolefin (TPO): TPO membranes are a relatively new roof membrane in the commercial roofing market and have seen several reformulations in the past decade. They are typically white in color and, as a thermoplastic, the seams are heat welded. Since they are new, long-term performance is unknown at this time.
TPO is the latest thermoplastic membrane introduced into the marketplace. It was commercialized in North America in the early 1990s. It is formulated from polypropylene, polyethylene or other olefins. Unlike PVC and PVC blends, TPO membrane do not rely upon plasticizers for flexibility, so embrittlement due to plasticizer loss is of no concern. TPO membranes are typically white, and are available in sheet widths up to 12′ [3.6 m]. NRCA suggests designers specify 60-mil-thick or thicker TPO membranes.
Ketone ethylene ester (KEE): This membrane is also referred to as a tripolymer alloy (TPA), and the polymer is known by the trade name of Elvaloy. KEE sheets are similar to PVC.
Thermoset materials normally cross-link during manufacturing. Once cured, these materials can only be bonded together with a bonding adhesive or specially formulated tape. Primary membrane types in this category are:
Ethylene Propylene Diene Monomer (or Terpolymer) (EPDM): EPDM is a synthetic rubber sheet. As of 2005, EPDM enjoys the largest market share of the single-plies in service in North America. EPDM membranes are extremely resistant to weathering and they have very good low-temperature flexibility. However, EPDM is susceptible to swelling when exposed to aromatic, halogenated, and aliphatic solvents, and animal and vegetable oils such as those exhausted from kitchens. On portions of roofs where the membrane may be exposed to these materials, an epichlorohydrin membrane can be specified over the EPDM as discussed below. EPDM membranes are suitable at airport buildings, provided liquid fuel is not spilled on the membrane.
The sheets are typically available in widths of 10, 20 and 45 or 50 feet [3, 6 and 14 or 15 m], and lengths up to 200 feet [61 m]. Hence, on large roofs with very few penetrations, this type of membrane can be very economical to install. Most EPDM sheets are black, although white sheets are available. White sheets, however, are not nearly as resistant to weathering as black sheets. EPDM is typically non-reinforced. Note that reinforced sheets can begin to delaminate very quickly if water gets to the scrim because of abuse or simply from wear. Therefore, reinforced sheets are only recommended for mechanically attached and loose-laid air-pressure equalized applications. Reinforced sheets also offer some increased resistance to puncture and tearing when used in fully adhered and ballasted applications, where non-reinforced sheets are vulnerable to physical damage, especially if rounded, graded to 3/4″ minimum size, river-washed ballast is not used. If a rigid insulation cover board is included as a substrate the non-reinforced sheet is preferred.
In fully adhered applications, typically a contact adhesive is applied to the substrate and the sheet. After the adhesive dries, the sheet is mated with the substrate. Another method of application uses fleece-backed EPDM, which is set in low-rise sprayed polyurethane foam adhesive. As there may be issues with a fleece-backed system set in asphalt, designers are advised to consult with the manufacturers before specifying or detailing fleece backed system with asphalt.
Field seams are formed using either a liquid-applied adhesive or specially formulated tape. The latter is recommended. Although tapes offer performance advantages over liquid-applied adhesives, the contractor still needs to exercise care in cleaning the EPDM prior to tape application, priming the EPDM and diligently executing the seam work as recommended by the manufacturer.
EPDM roof membranes provide predictable serviceability in roof systems in all climates. The minimum sheet thickness should be 60-mils if reinforced and 90-mils if unreinforced. All lap seams shall be fabricated with 6-in. (150 mm) seam tape and stripped-in with self-adhering, semi-cured EPDM cover strips.
EPDM sheets are resistant to the effects of UV radiation and are very durable. Seaming technology and adhesives have improved reliability with the use of tape-applied adhesive. Tape applied seams should be used. Properly constructed EPDM systems are now providing 30 years or more of service life.
Epichlorohydrin (ECH): This sheet is similar in appearance to EPDM. ECH, however, is resistant to hydrocarbons, solvents and many greases and oils, so it can be used in areas of the roof that are exposed to chemical discharges that are harmful to EPDM. Because of its permeability, the ECH manufacturer recommends placing ECH over an EPDM membrane. Because it is so specialized, ECH is seldom used. Only one manufacturer produces it in North America.