Letters

Support for self-adhered membranes

This is a follow-up to "A contractor's insight," January issue, page 28, and "Will self-adhered roof systems stick?" March issue, page 31. Both articles provided thorough analysis of the evolution and challenges faced by self-adhered roof membranes.

Self-adhered membranes were developed as a solution to [potentially problematic] issues with conventional application methods and the consequent problems of higher insurance premiums, code regulations, fire hazards, environmental pollution, etc. Self-adhered membranes do not have any of these application-related issues or environmental drawbacks. However, they have some unique issues that deserve special attention.

Compared with products installed via conventional application methods, self-adhered membrane installation is cleaner and generates minimal waste. For example, a typical 25-square (225-m²) job may generate less than 20 pounds (9 kg) of release liner to be disposed of or recycled. Contrast that with fumes, fire hazards, emissions, and empty buckets or cans that are associated with conventional application methods. Removing release liner from a membrane's backside requires no more attention than torching a roll or mopping a membrane. There are inherent risks associated with all application methods that must be addressed using recommended safety practices.

Roof shrinkage or snapback is critical for polyester-reinforced self-adhered membranes. Recognizing this, several manufacturers have addressed the issue by using polyester reinforced with fiberglass yarns in all their self-adhered membranes. Such membranes possess vastly superior dimensional stability when compared with membranes with standard polyester reinforcement, thus drastically reducing the chance for potential roof shrinkage or snapback.

T-laps, junctions, rooftop penetrations and other details can be addressed successfully using proper application guidelines. But membrane overlaps are the most difficult problems associated with self-adhered materials.

Self-adhesive compounds do not adhere well to granule-surfaced base sheets; therefore, most self-adhesive base sheets are manufactured using polyolefinic film on the top surface. Self-adhesive compounds bond well to such base sheets. Another innovative approach to the manufacturing of self-adhesive membranes is the use of a dual-compound. This process allows the use of standard APP- or SBS-modified bitumen compound on a membrane's weathering surface without the need for reformulating these compounds to be "sticky."

Self-adhered membranes are not inferior to other products with respect to fire ratings. There are SBS-based self-adhered membranes that have achieved Class A fire ratings at slopes of 3-in-12 (14 degrees) by using uniquely formulated self-adhesive compounds. Typically, higher fire ratings are achieved with dual-compound self-adhesive membranes versus full self-adhesive membranes. This is because there only is about 0.016- to 0.024-inch- (0.4- to 0.6-mm-) thick coating of self-adhesive compound that potentially can flow versus a full thickness of 0.12 of an inch to 0.18 of an inch (3 mm to 4.5 mm) of self-adhesive compound that is more prone to slippage, thereby negatively affecting fire ratings.

Whereas standard fire-retardant additives potentially could interfere with adhesion characteristics of a self-adhered membrane, special fire-retardant additives are available that do not have these drawbacks and work well for self-adhesive application. Interestingly, there are nonfire-retardant APP-based self-adhered membranes that have Class A fire ratings.

When self-adhered membranes are manufactured using properly formulated compounds that are sticky at low temperatures but pose no problems related to rooftop traffic in warm climates and possess unique features to address roof shrinkage, lap formation, roof details, etc., such membranes are a perfect solution to the problems associated with other installation techniques.

Shaik Mohseen
Polyglass USA
Hazleton, Pa.

Comments about roof ventilation

Congratulations on publishing "Balancing ventilation," February issue, page 42. It is an excellent and long overdue look at this important component of any efficient roof venting system. The media and advertisers generally only make passing references to the need for high-level exhaust venting to be balanced by low-level intake venting. However, I cannot entirely agree with the article's statement, "It's nearly impossible to have too much" intake venting.

I have seen cases where misguided designers specified a number of intake louvers in excess of code requirements and the installed exhaust venting that caused severe problems.

The inward pressure on the windward side can combine with negative pressure on the leeward side causing wind shear across the attic floor and sucking rain or snow through the windward vents. At the same time, on some ridge vents, air flow can be reversed allowing weather penetration.

This problem is more common on simple gable roofs if the wind is at right angles to the ridge and fewer, larger soffit louvers spaced further apart have been used. This concentrates air flow and, incidentally, can leave large areas of an attic unventilated. Visible symptoms are snow deposits on the attic floor immediately inboard of the soffit and below the ridge.

When louvers are placed far out on a rafter tail, rain, unlike snow, falls mainly on the top of the soffit before reaching the attic floor and can drain and dry before being noticed.

I admit to never having encountered this problem where exhaust vent capacity met code requirements and continuous eave venting was two times or even three times as much.

Continuous ridge vents matched to continuous soffit/eave venting generally—but not always—is the best venting system option. When considering field/pot vents combined with intake soffit louvers, my advice is to consider using smaller capacity units more evenly distributed than fewer larger, more widely spaced units. If it is a hipped roof, consider mounting some smaller exhaust vents on hips rather than bunching them on the rear slope.

Apart from eliminating the problem described earlier, a roof system will be more efficient with less risk of unvented dead air spaces.

Michael Keogh
Keogh Consultants Inc.
Campbellford, Ontario, Canada

Support for NFPA

Thank you for "Demystifing the insurance industry," February issue, page 46, and NRCA's Associate Executive Director of Technical Services Mark Graham's article "Concerns regarding NFPA 5000," February issue, page 96. The juxtaposition of these articles has caused me to appreciate a reason for the National Fire Protection Association (NFPA) pushing its code in the face of the International Building Code.

Graham is critical of NFPA 5000 because the basis for it is loss mitigation rather than life safety. He justifies his criticism with an illustration of a provision that is not "technically justified" in the context of a building code. This is a circuitous argument. Further, the public may not appreciate what professionals know—building codes establish the lowest permissible standards for construction. Codes do not necessarily represent good construction standards, materials or practices. It is laudable of NFPA to advocate higher standards that are cost-effective to mitigate losses to increase profits and reserves and control costs of insurance—to the benefit of all building owners, tenants and taxpayers.

There also is good reason for outspoken criticism of many roofing professionals. In my area, it is difficult to find a three-tab shingle roof system that is installed according to a manufacturer's installation instructions. Procedures as simple as cutting off starter shingles, installing shingles with seal strips down and nailing below the seal strip routinely are ignored. And it is difficult to find a manufacturer's identification on shingles. I wonder why?

Professional Roofing is an excellent publication. I keep clippings for reference from almost every issue.

Ken Zenzel
AMER Building Inspection Services Inc.
Virginia Beach, Va.

Clarifying metal slope recommendations

Some points in "Metal slope requirements," March issue, page 60, need clarification. The term "standing seam" has been used liberally by the metal roofing industry. Many raised seams have been classified as standing seams when they are not seamed at all. Specifiers should require standing-seam roof systems with true double-lock, machine-seamed (not crimped) seams and factory-applied mastics.

Roof slopes of 1/4-in-12 (1.2 degrees) will perform well if the proper roof profile is specified. I have been involved with the installation of several hundred thousand square feet of standing-seam roof systems at slopes of 1/4-in-12 (1.2 degrees). These roof systems have performed well with no slope-related problems. A difference of 1/4-in-12 (1.2 degrees) of slope, particularly in a reroofing situation, can be extremely important when window heights, rooftop units, flashings, etc., are affected.

The statement that NRCA's 1/2-in-12 (2.4-degree) minimum slope recommendation for metal roof systems is "prescriptive" whereas NRCA's recommendations for low-slope roof systems are "performance-based," leads readers to believe metal roof system specifications cannot be performance-based. Positive-drainage requirements and roof deflection issues should be considered in all roof system designs. I have witnessed considerably more ponded water instances with 1/4-in-12 (1.2-degree) specifications for low-slope roof membranes where excessively large crickets or stripped-in edge conditions have prevented positive drainage than with all the 1/4-in-12 (1.2-degree) standing-seam roof systems I have observed. Proper design details in all roof systems are critical for roof system performance.

NRCA should expend more effort in educating building owners and contractors about the differences in roof profiles and performance requirements of the various metal roof systems available.

John L. Hequembourg
Office of Administration for the State of Missouri
Jefferson City, Mo.