Figure 34. Racking
Solar racking is fairly straight-forward, standard, and reliable.
There are positive attachments that lag screw directly into the rafter in most cases. The racking system is designed in manufacturer software to fit your roof and the built to order with some minor field customization.
No matter the roof material, there is a solar rack for it. There are multiple kinds of racking systems for just Spanish tile, ranging from older hooks to full tile replacement. But once the attachment point is determined, solar rail will run along the roof.
Figure 35. Positive Attachments
Module clips secure the module frame, the clip itself being bolted to the rail at just the right module height for a secure fit.
This piece is called an L foot, as it looks like an L. A bolt fits into this side channel and then a nut gets screwed down to clamp the
module frame between the rail and the clip.
As far as industry jargon, end clips go on the very end of the array and mid clips go between two modules. This little notch becomes a spacer in between two panels. There is some nuance in racking selection. I prefer racking systems whose rails have a wide U channel to lay cable into, whereas other installers prefer a more exposed wire which is better for rainwater run-off. That’s another discussion for another day.
Figure 36. Cable Management
But cable management is one of the more important jobs in solar installation and there are different tools for that job. Zip ties are useful for bundling cables together and keeping those cables away from pinch points and sharp metal edges of the rack itself. But there are also metal module cable clips that are worth having bags of because these actually accomplish a different goals of keeping the module MC4 connector whips tucked up underneath the module frame when installed.
Sometimes the clips are simply there to be an extra hand during a particularly challenging rooftop install. A zip tie is a more permanently secured solution. Both have their place on site.
Figure 37. Underneath the Solar Array
The overall goal of cable management is that you can look underneath
the solar array and not see any loose cables touching the roof or hanging down. A clean undercarriage is a sign of a good installer.
Figure 38. Array Skirts and Snow Gaurds
Other material items to be aware of snow guards, which are
little clips dot the array, creating little grips for the snow to hold onto to prevent avalanches off of the roof. That’s only used in areas of very heavy snowfall.
These are array skirts, and they prevent pigeons, squirrels, and debris from getting under the array. Most installers do not install these but I recommend an array skirts, even a cheap one, if it is identified that leaves can fall on the roof, or other obvious signs that birds or squirrels are a concern. It is cheaper to install than fixing a problem afterwards.
Figure 39. Flashing
The standard solar roof layout for shingles is flashed attachment points staggered across all the rafters of the roof, roughly 4’ on center. This distributes the load well and makes good use of the strength of the solar rail.
Figure 40. Lag Screws
Finding the rafters for the attachment points is part of the trade, but sealant and flashing works great on near misses. There are techniques to avoid unnecessary holes in the roof, but you do not want your installer to be someone who isn’t already comfortable fixing a hole in the roof!
There are some non-flashed systems that sit on top of the shingles, but they require more precision to install, and so are really for advanced installers.
Figure 41. Metal Roofs
These are clips for standing seam roofs, which are not the only metal roofing option. Positively attaching to a metal roof with screws sounds crazy, but it my favorite option, because a clip will only clamp to the top of that metal roofing panel. But a positive penetration that screws into the rafter is directly attached to the structure itself.
There are products for routing conduit across the roof, including metal roofs.
Figure 42. Rail-less Racking
This alternate racking design is called a rail-less and unlike traditional rail-based systems, it is commonly installed in landscape, which can look better or fit better under the right circumstances. I like this system when the location of the rafters cannot be determined, or the shingles are particularly weak, such as on cheaper roofing systems like foam-sealed trailer homes. But expert installers prefer it for cost advantages, despite the extra precision needed to install.
The rail on site can be useful for squaring the system, or as a stepping area on a slanted roof, or to position and handle the modules during the install. I recommend starting with rail-based systems.
However, there are even systems designed to attach directly to the roof decking, not needing the extra strength of a rafter attachment. So again, there are many solar racking options available for almost all roof types strong enough to support a few extra pounds of dead load.
Figure 43.Pull Strength
The racking manufacturer will provide engineering data, and sometimes that data can be very detailed, such as if they have their own sizing software available. For this particular mounting bracket, with the
safety factor of three considered, the allowable pull strength is 180 pounds and the pull strength of lag screw into the rafter is six hundred and thirty pounds. Yes the lag screw size and number of fasteners is important, but ultimately the decking has enough pull strength to hold solar in some circumstances, but not all. Attaching to the rafter is so strong you do not need to worry over it.
Figure 44. Live and Dead Loads
Staying in the interior of the rooftop, the racking software shows a force of a hundred pounds whereas if the edge of the roof in this other zone shows a 300 pound uplift force. Revisiting the pull strength of the attachment, the 180 pound uplift force (with a safety factor of three) for the decking is fine for the center of the roof, but not for the edges which would require a rafter attachment.
Advanced solar designers will consider this, and have attachment points closer together along the corners and edges of the array, but space things out to a maximum allowable span in the interior of the array.
Figure 45. Positive Attachment vs. Concrete Ballast
Many commercial racking systems cover the roof in concrete. Other racking systems positively attach to the roof. I’m an outspoken fan of commercial racking that gets rid of the concrete because covering the entire roof with loose concrete blocks does not seem like
a good long-term plan. A building’s maximum allowable load is often an assumed number, and the vertical load of earthquakes on this loose concrete has not been well studied.
These systems can weigh as much as 10 pounds a square foot
in certain spots and without the concrete they can weigh less than three pounds a square foot. So don’t be afraid of poking a hole
in a commercial rooftop – it’s something that can add structural benefit to the building and something a roofing contractor should know how to do.
Above the Ground Racking
Figure 46. Above Ground Mounts
Here are other kinds of non-metal racking systems, useful for metal-free rooftops or on landfills. Here is an alternate racking system for above the ground installation, for concrete pours. This can be useful for rocky terrain as well.
Screws + Piles
Figure 47. Ground Screws and Helical Piles
In rocky terrain or in other cases where foundation concrete is to be avoided, there is a ground screw which can be driven with a Bobcat with good reach and hydraulics. In sand or clay, a helical post can be a better fit. These mounts do not save any money compared to concrete but can improve logistics for the right site
Figure 48.Concrete-free Ground Mounts
Residential ground mounts are typically installed across two rows of support posts instead of one row, as you would find on a utility
Figure 49. Fixed Utility Ground Mounts
The one post methodology is easier for maintenance but most residential ground mounts need the scaffold style foundation because the foundation depth of single post systems can be over the capabilities of a Bobcat.
Figure 50. Uneven Terrain
There’s different ways to level uneven terrain, ranging from
laser finders to string and a bandsaw.
Single Axis Tracking
Figure 51. Tracking
Single-axis tracking is a standard practice at the utility scale.
This is where the bifacial solar panels discussed before, the kind that collect sunlight underneath, have the most value.
Design Your Solar Roof