How I see it

Editor's note: The following views are those of the author and not necessarily those of NRCA or Professional Roofing magazine.

Roofing professionals rarely get to walk over a functional roof for the sole purpose of observing how it is doing. We may walk across a roof we helped design and install, but ordinarily, we are there for another reason and do not have time to stop and ponder its performance.

Time goes by all too fast; in fact, we are more than halfway through the first decade of the 2000 millennium. What changes have occurred in roofing technology since 2000? A number of new products now are widely used, such as low-rise adhesive foam and self-adhering membranes. There also is a single-ply membrane that was introduced in the early 1990s—TPO. How is this material doing?

Our time-honored single-ply roof systems—EPDM and PVC—are adapting to the use of adhesive foam. Built-up roof (BUR) systems using asphalt or coal tar have lost popularity in some quarters, but more than a few building owners have nothing but BUR systems and continue planning for new ones. Polymer-modified bitumen systems are now mature; their performance is accepted and acknowledged. Self-adhering modified bitumen systems are catching on. There are few true equals when specifying these products. Metal and spray polyurethane foam- (SPF-) based roof systems also have gained market share; we even see a wider acceptance of residential metal roofs. SPF is in much wider use than 10 years ago; there seems to be a heightened interest for this system among a number of roof system designers.

There is a "green roof" revolution under way, which, in some cases, borders on marketing delirium. As roofing professionals, we are being told to follow the instructions of a "green roof specialist," who, it turns out, is a landscape architect with virtually no roofing or waterproofing knowledge.

I believe we are building roofs that, on average, last longer than anticipated. My field observations seem to confirm the following:

1. Many building owners are more knowledgeable about roofing matters.
2. More building owners understand the need for roof system maintenance and have seen the benefit of roof asset management.
3. Roofing contractors, on average, are installing better roof systems.
4. Manufacturers have retrenched, cutting costs in light of excess capacity and intense competition. Some have done so while maintaining the technical edge with their products; others have not.

Following are some additional observations about specific roof systems and roofing materials.

Low-rise adhesive foam

Low-rise adhesive foams are liquid materials dispensed with a wand in beads onto a roof's surface or sprayed over the entire surface to be bonded. The liquid is either a single- or dual-component polyurethane. The materials have limited rise (about ¼ of an inch [6.4 mm]), which is intended to bridge small potential gaps between the substrate and board product placed on it, should a gap occur. The cell structure is not that important because in many applications the cells are crushed and application depends on the polyurethane liquid to cure and tightly bond the materials together.

Low-rise foams not only are used to bond board products to a deck, they also are used to bond board products together. Another popular use is to spray low-rise (dual-component) adhesive foam and roll a fleece-backed membrane into it before curing.

Moisture is needed to cure a single-component material; this is a slow process when compared with a dual-component material. However, there are distinct advantages to each because the cure rate of a dual-component material can be affected by a cold deck. The heat of reaction is important because it helps complete the cure. Formulations can help minimize the temperature concerns for a dual-component material.

Single-component polyurethanes have a small portion of unreacted isocyanate, which cures with water (vapor) in the air. A single-component foam adhesive chemically cures but, as I mentioned, depends on water in the air. Therefore, this reaction can be slow.

Dual-component polyurethane adhesives cure in minutes; their spray formulation provides maximum coverage over almost all substrates. The material will maintain its strength if moisture penetrates the roof system once cured.

Single-component materials are easy to handle and do not require sophisticated dispensing equipment. There typically is a manufacturer's specified distance to maintain between rows of dispensed adhesive foam. Quite often, a single-component material dispensed with a wand is to be dispensed 12 inches (305 mm) on center. I highly recommend roofing manufacturers "line" their facers and cover boards to make single-component materials (and for that matter, mechanical fasteners) easier to place.

A cured single-component material has a moderate shear modulus, which means it firmly bonds materials together in a nonbrittle manner. Dual-component materials tend to have a higher shear modulus. I'm not saying this is good or bad, but I like the shear modulus of single-component materials. It allows a small amount of shear (horizontal) movement in a roof system, which can spring back or recover.

Asphalt-attached systems have an extremely high shear modulus, which, of course, ends up a brittle failure. Mechanically fastened insulation has a shear modulus that depends on the compressive force of the stress plate and screw fastener. This shear modulus is like the adhesive foams except it is nonelastic and will not allow recovery. All in all, single- and dual-component foam adhesives are a step in the right direction.

Self-adhering membranes

Self-adhering membranes have opened the door for profound change in the industry. I see a lot of things that have gone wrong with some current self-adhering products, such as laps opening up and loose membranes. However, a number of manufacturers have done their homework and understand the limitations of their particular self-adhering membranes.

At this time, installing a self-adhering modified bitumen membrane on a cool day (40 F to 50 F [4 C to 10 C]) may be touchy with some products. A number of products require 40 F (4 C) and rising to be installed properly. The nasty part of this statement is defining "rising." Many in the northern states may start the day at 38 F (3 C) and see it warm to 43 F (6 C). This limits a number of self-adhering products to be installed later in the day. These limitations are impractical for a lot of contractors. Pre-heating rolls helps; even a light torch has been known to work in some situations, but all the safety rules for torch welding must be followed. Self-adhering compounds are not made for torch welding per se, but a light torch heating of a base sheet can help. Caution is advised; I only recommend this method under unusual circumstances.

The best thing you can do to improve a self-adhering membrane's peel strength is provide heat. Typically, solar exposure to heat helps treat the material. My company has assembled and tested a variety of self-adhering modified bitumen membranes and found that for most, installing them at 55 F (13 C) to 70 F (21 C) results in dependable adhesion. The benefit of heat—a warm day or two—markedly improves peel strength.

Late fall may be too cold for installing some of these products in the northern states. Contractors and designers should heed the temperature limitations on the products. Problem jobs I have investigated typically have cold conditions present somewhere in the job history.

Polymer-modified bitumen

Recently, two APP-modified bitumen manufacturers have been using polyethylene either in the modified blend itself or as a burn-off sheet. Polyethylene basically is a contaminant when introduced to a blend of APP-modified bitumen and asphalt. Laps that are torched together initially will not remain firmly welded; a sticky soft material will be found at the lap interface in short order. This happens not only with torch-welded installations; cold-process systems will exhibit the problem because the laps need to be heat-welded.

Most in the roofing industry thought manufacturers recognized this during the 1980s. Now, we are somewhat perplexed to see our manufacturing colleagues introduce a known contaminant into their mix. Historically, burn-off sheets were thin polypropylene materials. The problem with thin polypropylene sheets is their propensity to tear easily on the manufacturing line.

Some APP-modified bitumen manufacturers have resorted to using high-density polyethylene (HDPE) material as their burn-off sheet. HDPE cannot be burned off like the thin polypropylene sheet. In fact, to burn off an HDPE sheet completely, you would risk damaging the polyester reinforcement. If you have lap joints in an APP-modified system that part easily, check the material safety data sheet to see whether polyethylene is being used instead of polypropylene.

TPOs

My company has inspected a number of TPO roofs that are in their eighth year of performance. We have noted the membranes are taut throughout these roofs but not necessarily tenting or pulling at the flashings. In these installations, we found no cracks or excessive weathering. However, we did note ponded water exhibited some chalkiness when disturbed by hand. It appeared these roofs easily would go beyond a 10-year warranty; their condition at 15 years of age will be interesting.

We have seen some 3-year-old TPO materials exhibit small stress fractures adjacent to the leading edge of the lap joint or random cracking across a sheet's field. In these instances, we have determined a high ratio of polypropylene to polyethylene was present in the TPO blend. Another TPO sheet exhibited welding problems because the laps would open up and not remain watertight. Of interest were the laps on the north side of a penthouse did stay watertight. Our analysis showed that, in this case, we had the reverse situation with the high ratio of polyethylene to polypropylene. It was discovered the polyethylene had a melting point within range of rooftop temperatures expected on a hot summer day.

The industry may need to carefully re-examine lap-welding techniques. It appears that by simply folding the fly sheet over itself, damage to the material may occur if a TPO membrane is creased. A gentle radius should be used in this process. Do not crimp the sheet by placing pails of mechanical fasteners on it. I saw one manufacturer's TPO sheets installed on numerous roofs with no signs of splits at the laps. That same manufacturer's TPO sheet in the hands of a less-experienced contractor ended up with a series of small splits near the lap joint. These roofs were comparable in age but not date of manufacture.

I believe some TPO products will last a long time. The industry probably needs a midcourse correction with a small portion of this product along with a close review of field performance and lap-welding technique.

Green roofs

Green roofs are interesting; our colleagues in Europe have used them for decades to help control stormwater runoff. The U.S. previously has adopted and taken an interest in other European roofing technology, namely polymer-modified bitumen and PVC membranes.

Green roofs turn a previously unused building element into a working space—one that ostensibly helps a building pay its dues for using the environment. This is a good tradeoff. However, we as an industry must insist good roof system design, construction, inspection and maintenance practices be followed.

Too many contractors who install green roofs blindly have neglected the factual time-proven principles of roofing and waterproofing in the name of environmental good. My company recently was confronted with a large number of green roofs that had no wind-uplift rating or fire-resistance classification.

For example, one roof we observed is filled with plastic trays that weigh 10 pounds per square foot (psf) (48.8 kg/m²) soaking wet and less than 2 psf (9.8 kg/m²) dry. Green roofs actually offer a new frontier for design; a number of membrane manufacturers are providing long-lasting membranes, flashing details and accessories to overcome the different challenges a green roof presents.

Unfortunately, a number of promoters of green roofs suggest the choice of membrane material and design considerations are irrelevant because a green roof protects a membrane. They conclude by saying any membrane will do; the need for flashing details still is not understood or recognized by more than a few of these advocates.

If you are contemplating installing a green roof, consider the challenges of a plaza waterproofing project. But instead of pavers, assume blended soil or containers loaded with blended soil are almost on top of your membrane. Partial soil erosion occurs because of windy conditions typically found on roofs. A source of water is needed; drains may become plugged. If a green roof serves as the roofing ballast, what resistance to wind uplift do you have when the soil dries? What about the fire-resistance rating for all the plastic trays partially filled with dry soil?

I expect future editions of The NRCA Roofing and Waterproofing Manual to have a section about green roof systems, including design and construction. Until that occurs, I predict the roofing industry is going to see a number of failures.

Moving ahead

The roofing industry once again has "pulled up stakes" and is on the move with slow but profound change, including new techniques and material modifications. This is a work in progress and will be for the rest of the decade.

René M. Dupuis is president and owner of Structural Research Inc., Middleton, Wis.