Lessons from the rooftop

Roofing experts share the most common errors they witness

When a roof system doesn't provide the level of protection expected or things simply go wrong, a project is investigated and the problem's cause determined. Because roofs have been around since humans began constructing shelter, there is not much roofing experts have not seen, and it is disheartening to realize many problems could be avoided by not repeating the lessons we already have learned.

There have been many attempts to develop resources that educate roofing professionals about common problems and how designers, contractors and building owners can avoid them. Likewise, we have avenues where researchers, technical experts and others share data, experience, and the methods of their investigations and discoveries regarding issues in our industry.

These avenues include symposia, educational sessions, regional conferences and magazines.

One benefit of this type of cooperative spirit is the ability to seek guidance from a host of experts. Somewhere in our collective past, we learned helping others avoid problems or solve a particular problem is to our benefit and knowledge not shared is knowledge wasted.

In this spirit of cooperation, I posed the following question to several roofing experts whom I hold in high regard for their roofing knowledge: "As a part of trying to educate and share collective experience, are there one or two major problems that when you see them, you think: 'If only I had a megaphone and could caution contractors, designers and building owners not to do?'" My question was deliberately open-ended and didn't provide any limits on the type of response being solicited. Following are some lessons the roofing professionals have learned.

Newton's law of gravity

Simply stated, Newton's law of gravity defines the attractive force between all objects that possess mass. Understanding the law of gravity, one of the fundamental forces of physics, offers profound insights into the way our universe functions.

In roofing terms, if a material is on a slope and you can "see the slope" (including vertical surfaces), it probably needs to be fastened or, over time, it will slip, slump, slide or otherwise move where it is not supposed to. Think of base flashings, asphaltic membranes on slopes, leveling of sealants and fillers, etc., and you will see the law of gravity is not suspended for roofing materials.

These same forces can be magnified by other dynamics, such as a material's weight and what adhesive and/or waterproofing material was used to install a particular product.

For instance, an SBS polymer-modified bitumen membrane installed in Type III asphalt on a slope as low as ¼:12 (1.2 degrees) can slip if there is too much asphalt; the membrane is a dark color that will experience higher temperatures that approach asphalt's softening point; there is a complex cricket design that increases the slope routinely; or any combination of these reasons.

The three forms of water

As you know, water exists as a solid, liquid or gas. In the roofing industry, we generally understand a roof membrane is supposed to keep liquid water out of the rest of the roof system and building. We also understand liquid water can do much to negate most materials' insulating abilities and cause many materials to fail prematurely. And we know when liquid water changes to a gas or solid, it expands in size.

Experience tells us Newton's law of gravity works on liquid water, too. If there's a way for liquid water to get closer to the ground, it will find that path and make its way down. (And, yes, in its solid form, if water's mass exceeds its strength and attachment, it, too, will be pulled to earth).

Most experts I spoke with cautioned about water and the need to understand the principles of moisture control in roof systems. If we build water into a system or allow it to accumulate in the system, it will want to go somewhere and can cause harm. Blistering, downward drying into the interior and condensation within the system each can be the result of not understanding moisture control or the principles of volume, temperature and pressure as they relate to water vapor.

Care must be taken to not allow water into a roof system during installation, including avoiding a skip in a mopping of asphalt that could introduce minute amounts of air, water in materials that were improperly stored or water from the construction process. Likewise, understanding building use and occupancy-generated moisture and making provisions to accommodate and/or control that moisture are necessities for most roof systems.

There is no simple solution to moisture control, such as always installing a vapor retarder. Although a vapor retarder serves a valuable purpose when properly designed and installed, many roofing professionals do not recommend using one unless there is an explicit need. Primarily, this is because by its nature, a vapor retarder easily can serve as a second waterproofing layer, which makes leak detection difficult and can allow a large amount of insulation to become saturated from a small roof leak before building occupants are aware there may be a problem.

The weakest link

The next maxim is an easy rule most roofing professionals know but sometimes forget to practice when thinking about how their actions ultimately affect roof system performance. The bottom line is a roof system's wind resistance is only as good as its weakest link. When designing roof systems and performing load calculations, some consider this rule to be the reason the industry uses a safety factor of two—to provide insurance for the weakest link. Yet time and again, this rule goes by the wayside and problems arise.

Edge metal is critical to a roof system's wind performance, and how well edge metal performs often determines roof system performance. Properly attaching edge metal at a roof's perimeter can avoid serious damage and water intrusion. To avoid inadequate fastening, Dick Canon, president of Canon Consulting & Engineering Co. Inc., Moore, S.C., requires all edge metal to be pre-drilled so the fastening pattern requirement is met.

Although some might consider this to be an unnecessary or extra step, it certainly makes fastener placement and spacing easy once the edge metal is on the job site and ready to be installed.

Membrane attachment, insulation attachment, and deck design and securement also can be weak links when improperly executed. Each roof system component must be installed correctly, perform as intended and be maintained as recommended to avoid being the weakest link.

More is not always better

All roofing materials have a purpose and should be installed as intended. This certainly applies to roof system components. Similarly, many roofing materials' performances do not follow the adage "more is better." Examples abound within our industry, but too often, good intentions, lack of understanding and lack of attention can cause parties involved in a problem to try to state or even prove more is better.

For example, overdriving fasteners ("more" depth) is the quickest way to doom a fastener, and if you cup a plate so it tears or punctures the facer on the insulation or base sheet, you already have failed the fastener.

In addition, installing a heavy mopping of asphalt—perhaps because the asphalt is not at the proper application temperature—can cause slippage problems. Setting a hot-air welding machine to too high a speed or too hot a temperature also are examples where "more" can cause membrane seaming problems. Another example is installing a coating over an already compromised membrane, which only results in a coated compromised membrane.

Communication 101

A case can be made that communication among everyone involved with a roofing project has improved. When effective, this has led to projects running more smoothly. Unfortunately, a common theme in construction projects is a lack of communication among trades. And this lack of communication often is most harmful when other trades perform work before or subsequent to roof system installation.

Carpenters who fasten wood nailers with a nail gun without using enough fasteners or using the wrong fasteners is just one example—and it goes without saying that roof system designers need to verify nailers are properly fastened to meet ANSI/SPRI ES-1, "American National Standard for Edge Systems Used with Low Slope Roof Systems," requirements for edge metal attachment.

Another example is the disconnect between the Steel Deck Institute's deck attachment requirements and the requirements to be compliant with a specific FM Global RoofNav listing. And it's critical (to the performance of the wall-roof interface) for appropriate details and proper materials to be installed in the correct order on an exterior insulating finishing system wall a roof system flashes into.

These examples illustrate what Jeff Evans, vice president of Benchmark Inc., Cedar Rapids, Iowa, says is so important to a roof system's performance: A roofing contractor and consultant (if present) must begin communicating early with the designer, general contractor, building owner and other trades to ensure work that interfaces with the roof is done properly.

Roofing 101

NRCA Associate Executive Director of Technical Services Mark Graham sums up his experience with roof problems as a basic lack of understanding of what used to be covered in the one-day roofing technology seminar offered by the former Roofing Industry Educational Institute (now a part of NRCA University).

The lack of understanding about the basic principles of moisture control, material mechanics (such as coefficients of expansion and contraction), deck deflection, use of a deck and/or insulation as a substrate, asphalt equiviscous temperature and how it relates to field application, etc., may explain why the roofing industry unfortunately "recycles the errors of the past."

Roofing is a science

Is there a shortage of fundamental education, or are we too busy to stop and ask "why" so we understand how to avoid common problems? The experts I spoke with agree designers, consultants, contractors, material suppliers and other roofing professionals do not want to cause harm. Each error, cause and problem identified can be critical to a roof system's success, and none of them should be new to roofing professionals; yet we see these issues repeatedly.

So we must remind ourselves that though it is not rocket science, there is a science to roofing and we need to pay attention to the details and make sure requirements are met.

Helene Hardy Pierce is vice president of technical services, codes and industry relations for GAF Materials Corp., Wayne, N.J.


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