What the Three Little Pigs taught us about roof top PV

Since the early 1800's people have been telling the tale of the Three Little Pigs to illustrate the importance of building something right, with inherent strength, for ultimate safety and protection. The house of straw and the house of sticks couldn't stand up to the attack of the big bad wolf, but the strong house of bricks stood.

Another classic is the biblical parable of the two builders. The first man built his house carefully on a solid foundation of rock. When the floods and rain came, and the wind blew, the house stood. The second man built his house quickly without digging the foundation deep to solid rock. When the storms came, the house fell.

So what do we learn from these stories as it relates to rooftop PV systems? Build it strong, and understand the foundation. In the case of rooftop mounting systems, the foundation is the existing roof framing. And this is the first place people seem to blow it. In order to build a proper rooftop photovoltaic mounting system analogous to the brick house on a foundation of rock, the racking needs to be strong and firmly mounted to the structural members in the roof. But what we're seeing in the industry is a widespread lack of understanding in terms of roofing and building structures, leading to many rooftops getting the straw and stick house built on sand variety of PV systems. An example of this "straw house" method of putting solar on commercial rooftops is the very common ballasted systems. Ballasted PV systems are the most prevalent in the industry. Ironically, most people don't know that they are potentially harmful to the roof. In fact, the NRCA (National Roofing Contractors Association) recently published "Guidelines for Roof-mounted Photovoltaic System Installations", in which they specifically state they do not recommend using ballasted PV systems. For more of my views about ballasted systems, please see The Roof Penetration Myth blog post.

"The NRCA (National Roofing Contractors Association)....specifically state they do NOT recommend using ballasted PV systems"

As the commercial rooftop solar industry matures over the next few years, we will see the engineering community become a very big part of it. I know it is a challenge when there are no engineering dollars allocated on the front end (believe me, I know), but that is where we will also see certain companies mature and others fade away. Good companies will learn to work with the engineering community, develop internal tools for estimating, and learn to sell the customer on the benefits of paying for engineering up front when possible. Other companies (and there are a lot of them out there right now) will gloss over some very critical aspects of projects and get burned badly. Many will disappear like the house built on sand.

6 things ballasted PV racking vendors don't want you to know

Racking (mounting) systems for photovoltaic and solar thermal systems fall into two basic categories: ballasted and penetrating. Here is a brief explanation of each:

• Ballasted - The word ballast simply means "weight added for stability". The concept is that a mounting rack, usually some type of metal frame, is arranged on the rooftop and instead of fastening it to the roof structure, it is held down by weights. The ballast (commonly concrete blocks) need to be heavy enough to resist wind, snow and seismic forces.
• Penetrating - This term applies to any type of system where the waterproofing membrane of the roof system is penetrated to make the attachment to the structure. Wind, snow and seismic forces are resisted by the mechanical attachment to the building structure.

Here's what the guys selling ballasted systems don't want you to know:

1. When they say "non penetrating", they really mean "mostly non penetrating" or "sometimes non penetrating. Ballasted systems do require mechanical attachment to the structure to comply with code if they are located in a seismic zone.
   
2. Future roof repairs and maintenance will be outrageously expensive. It would be great if roofs never needed to be worked on, but the reality is that every roof needs some maintenance and, unfortunately, occasional repair work. Roof work is expensive enough with a wide open roof. Now imagine paying someone to remove thousands of pounds of cinder blocks and disassembling solar arrays just to figure out where a roof leak is originating. Elevated racking systems can be easily worked around to keep the roof clean and well maintained.
   
3. Future re-roofing? Forget about it. You can't replace the roofing with a ballasted PV system in place. Well, actually you can, but nobody would pay the price. The labor to remove and replace the system and all the ballast blocks would be prohibitive. With a properly installed elevated racking system, re-roofing is very feasible.
   
4. Roof penetrations are a good thing. Every roof has penetrations. Every skylight, air conditioner, plumbing vent, electrical conduit, fan unit, etc. is a roof penetration. The roof penetration just has to be done right. Roofers know how to do it. The reason for the bad feeling in the industry about roof penetrations is because of all the poorly installed (and leaking) penetrations installed by people with no roofing expertise. Don't be afraid of roof penetrations, just do them right.
   
5. Ballasted PV systems block water flow. Good drainage is critical for a healthy and long lasting roof. The way ballasted racking systems sit directly on the roof with the cinder blocks spread out to hold them down, water cannot flow in the direction and velocity intended in the original design. Dirt and debris will build up around the obstructions and accelerate the deterioration of the roof membrane.
   
6. Elevated PV racking systems give you higher power output per square foot of roof. Ballasted systems need unobstructed roof areas because they can't span over pipes, vents, skylights and other rooftop equipment the way a racking system can. This can radically limit the number of modules you can fit on a given roof. Also, being so close to the roof and at a low angle, the modules will build up more heat which lowers efficiency. Elevated racks allow air flow keeping the modules cooler.