S
P
E
C
I
A
L
S
E
C
T
I
O
N
Use four inches of flashing for leak-free solar
G
enerations of builders have installed asphalt shingle roofing on homes with chimneys, side walls and other structures that introduce a vertical surface above the roof line. To preserve the integrity of the roof and prevent water leaks, roofers tend to install flashing next to these structures with at least a 4-in. overlap on the horizontal plane. As a manufacturer of roofing and solar photovoltaic systems, CertainTeed recommends that when installers affix modules to the roof, they treat the anchoring system like any other vertical rooftop structure and provide at least a 4-in. horizontal overlap. Why 4 inches? The specification comes from the latest edition of the National Roofing Contractors Association Roofing Manual: Steep-slope Roof Systems, published in 2013. (NRCA will release a new version of the manual next year.) Though there isn’t necessarily empirical data showing the lateral distance that water can travel on a sloped roof, NRCA members have 45 years of accumulated knowledge and experience since the first publication of the manual. They know weatherproofing for shingle roof systems about as well as anyone. The minimum recommendation of a 4-in. horizontal overlap, also known as
a side lap, applies to sloped roofs with a pitch of 4:12 or greater. Water can travel a greater lateral distance on lower slopes. If the pitch is less than 4:12, check with the roofing system manufacturer and be prepared to use flashing with a wider side lap. For standard, rack-mounted photovoltaic systems, it’s always a good idea to read the product specifications and installation instructions prior to purchasing flashing to ensure that it is sized properly. With CertainTeed’s Apollo II integrated solar roofing system, there’s no need to search for compatible flashing. The product includes flashing designed with a 4-in. side lap. Because there are many applications in asphalt shingle roofing that call for a 4-in. side lap, it is relatively easy to select appropriate flashing products for this purpose. The same cannot be said about selecting proper adhesives
for sealing the roofing material and the flashing. When choosing among the many available adhesives, focus first on products that are compatible with asphalt shingles. It will do no good to purchase a high-quality adhesive if the manufacturer can’t vouch for its performance with your chosen material. For flashing with asphalt shingles, including the Apollo II flashing, use an elastomeric, a tri-polymer, a urethane or a roofing mastic that meets the ASTM D4586 Type II standard. This still leaves an endless variety to choose from, but you will have narrowed down the options to those adhesives that can support a leak-free installation for the lifetime of the system. SPW
CHRIS FISHER
PV PRODUCT DEVELOPMENT LEADER, CERTAINTEED
Size an array according to battery capacity
B
atteries used in DC coupled PV systems are typically rated at STC and at a discharge rate over a time period of 20 hours (C/20). The C represents the battery capacity and the 20 represents the number of hours of discharge time. The C rate can also describe the charge rate to return the discharged capacity to the battery. Generally, a C/5 rate, replacing the capacity in 5 hours, is commonly used when the objective is to quickly charge the batteries while utilizing inverter/chargers with generator or grid AC inputs. PV nameplate performance, however, is never achieved largely due to the impeding factors such as system
Consult your battery manufacturer if in doubt of the optimal charge rate, absorb time and voltage set points of your battery bank.
NABCEP section 3-16_Vs3SB.indd 35
3 • 2016
SOLAR POWER WORLD
35
3/7/16 6:56 PM