The Roofing Square

April 9, 2008

In the roofing world, the term “roofing square” refers to a roof area of 100 square feet (or 9.29 m2). This is further defined as the number of square feet of material (e.g. 216 square feet of asphalt shingles or 108 square feet of underlayment are needed to allow for double coverage or side- and end-lap, and still cover 100 square feet of roof area).

In the roofing world, the term “roofing square” refers to a roof area of 100 square feet (or 9.29 m2). This is further defined as the number of square feet of material (e.g. 216 square feet of asphalt shingles or 108 square feet of underlayment are needed to allow for double coverage or side- and end-lap, and still cover 100 square feet of roof area).

In the world today, the term “back to square one” is defined as “back to the beginning, to start again.” Google tells us that the most widely reported suggestions for the origin of this phrase are BBC sports commentaries, board games like Snakes and Ladders (also known as Chutes and Ladders), and playground games like hopscotch.

The convergence of these two definitions is especially relevant right now. Back during the oil embargo of 1973-1974, the construction industry took what could be considered heroic efforts to conserve energy. For roofing, this meant two approaches: dramatically increased thermal efficiency, which, fortunately, was accommodated by cellular foams, and solar panels installed on roofs. 

The solar panel effort was, unfortunately, a big flop, at least for whole building heating and cooling. Most of those plumbing nightmares are long gone, never providing the energy savings or payback promised. (This is not to say that solar panels for heating hot water are not viable. In some countries, only non-fossil fuel can be used for heating domestic hot water.)

Here we are again, although there are differences for sure. Thin-film photovoltaic (PV) panels are remarkably efficient, appear to be durable and relatively uncomplicated, and their costs are coming down (or at least not rising as fast as heating oil and gas).

Are we back to square one? There is really only one way to be sure we will succeed this time around: install pilot installations on buildings. No, not the whole roof, but of sufficient size to gain knowledge under the exact same environmental conditions (e.g. foot traffic, exhausted fumes or debris, hail impact) as other existing buildings.

Many building owners have found that they can learn as much from a 40-foot by 40-foot bay (or 1,600 square feet) installation as from a 100,000-square-foot project. (This is large enough for a contractor to mobilize his crew, yet small enough that a failure is not catastrophic.)

How would you undertake such a demonstration project? First, select a roof area that is not peppered with penetrations, or HVAC that requires constant servicing. If your existing roof membrane is a single-ply membrane, you could attach the panels directly to the membrane or to extruded plastic battens that have been welded to the membrane.

At the 23rd RCI Intl. Convention & Trade Show, which occurred recently in Phoenix, a comprehensive review of the status of solar panels was presented. The presenter, Michael Gumm, president at Seminole, FL-based Corporate Roof Consultants, cautioned that many of the photovoltaic (PV) panels currently in vogue will increase thermal load on the bottom side of the panels, and this could accelerate aging of the roof membrane (in other words, cook it). He noted that the heat might even be sufficient to soften bituminous and modified-bitumen roofs to the point of potential slippage. (It’s possible that there are just too few PV-to-asphalt installations to know just yet.) Roofs should have sufficient R-value insulation under the roof membrane to reduce the heat transfer from the PV modules in order to reduce cooling cost in warmer climates.

Gumm mentioned some good results from first installing a combination foam-cement product on the existing roof membrane. The foam side of the panels is 2-inch-thick extruded polystyrene, which is an excellent insulator (R=5.0 per inch), and the top side is a 3/8-inch-thick latex-modified (freeze-thaw resistant) cement topping. One such product has a tongue-and-groove interlock to increase wind-uplift resistance.

n effect, laying these panels on top of your existing roof would increase the thermal resistance of your roof assembly by an R-value of at least 10. Secondly, this would separate the photovoltaics from the existing roof, eliminating the danger of premature aging of the membrane; should the photovoltaics flop and need to be removed, they would cause no damage to the underlying membrane.

These combination panels only weigh 4.5 pounds per square foot, which is not much of an increased load, yet the interlock provides good wind-uplift resistance. For high-wind areas or building corners/perimeters, metal straps can be screwed into the cement topping (see photo No. 2), or 2-inch-thick cement pavers could placed over the composite panels. Validation of protected membrane roofs (see photo No. 3) can be found in many U.S. Army Corps of Engineers publications

When the roof needs replacement, the composite panels (see photo No. 4) can be lifted and reinstalled. In fact, their thermal resistance can be warranted even when recycled.

Where the photovoltaics require wiring, the wires are laid on top between the composite panels, or the foam under the panels could be kerfed to lie flat over the wires (see photo No. 5). Gumm points out: “Commercial roof applications are complex. Besides array layout design, cabling layout, running the wiring to the combiner boxes, and incorporating lightning protections and disconnect switches, adding the inverter and grounding can be challenging. While DC current is safer than AC current, most commercial systems array strings will be running voltages up to 600 volts DC, so the system must be carefully designed.”

Why should you consider such a test roof? Will you miss out on something good, or should you wait for “proven” products? Part of the answer lies in the fact that we all need to be doing something to reduce reliance on fossil fuel and to join the green revolution. If you act now, it’s very likely that you could gain community involvement and goodwill, and possibly qualify for incentives from the government or energy suppliers.

Best of all, you will have a jumpstart on learning what works (or not) on your building. If the performance is good, you will have a good feel for the potential payback of photovoltaics if they are installed on the rest of your roof. You will be able to compare the durability of the membrane on the test sections to the remaining roof areas to see if life is shortened or extended, and to plug this into a validated life-cycle analysis. “The cost of a roof system with a photovoltaic is expensive; the average installed cost of a commercial low-slope roofing and photovoltaic system can average between $72 and $110 per square foot. The average cost per watt installed on low-slope commercial applications is between $8 and $9 per watt DC. The high cost of the photovoltaic system can be reduced by 30 percent with federal tax credits, and many states have an energy-incentive rebate program to reduce the PV system cost even more,” explains Gumm.

With adequate testing on your own buildings, perhaps we can avoid looking back 10 years from now and finding ourselves, once again, at square one.

Consult the following resources for more information:

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