Roofing system (or roofing membrane) degradation is largely driven by physical impact (foot traffic, hail, etc.), solar radiation, and extreme temperature fluctuations. The longevity of a roofing system can be increased by protecting it from these elements.
Our roofing system is protected from the elements by the drainage layer and the insulated roof pavers, except along the parapet. Our solution was to cover the parapet with XPS insulation (or pink board). Because XPS insulation will degrade when exposed to sunlight, I cladded the whole parapet in aluminum.
It is a similar principle to the insulated pavers, with the XPS insulation at the bottom, which in turn is protected by the thinset layer atop.
The XPS insulation along the parapet is basically an extension of the coping nailer. The aluminum cladding is fastened to the coping nailer, and riveted together at the seams. The cladding is installed from the bottom of the roof to the top so that the overlap at each seam is pointing downstream.
The steel parapet coping will be attached to coping nailers on the inside of the parapet, and coping cleats on the outside. Because the steel coping will be held by the coping cleats, there will be no visible fasteners on the outside of the parapet.
I fit the coping cleats to the various sections of the parapet edge, and anchored them into the parapet top with masonry anchors. In the process, I kept checking the distance between the outer edge of the nailer and the cleat to make sure that it fit the width of the steel coping I had ordered.
The roof was waterproof since we installed the membranes and flashing. The same cannot be said for the parapet, which still needed a coping. We originally had clay tile coping on the parapet…
…but I opted for steel coping this time round to reduce maintenance and water infiltration issues.
The steel coping is secured by a cleat on the outside of the parapet, and as such has no visible fasteners. On the inside of the parapet, it needs to be fastened to a nailer that is anchored along the top edge of the parapet.
We used a pressure treated stud for the nailer, cut it to length, clamped it to the top edge of the parapet, and anchored it about every 24 inches.
We finally had a photovoltaic array on our roof. The solar cells on each module produce direct current (DC). Yet the electricity we use in our buildings is alternating current (AC). To convert the direct current into usable alternating current, we need an inverter.
The direct current from the solar array flows to the inverter, where it is changed to alternating current. The inverter is feeding the electricity into our electrical panel (or load center), from where the alternating current can power electrical loads in our home, such as the heat pump, fridge, dishwasher, vacuum, lights, the TV and radio, our PC’s and laptops, etc. It also feeds electricity back into the electrical grid, which is a process I described in the blog post “How to connect a solar system”.
In other words, our photovoltaic array is not functional, unless it is connected to an inverter.
I had saved some wall space for the inverter in the basement next to our load center. I also had installed a conduit run from the basement up to the roof during construction when we had all the walls open so the big stretch in the middle was already in place. That came very handy, because the electrician just had to install a little bit of extra conduit in the basement and on the roof.
The inverter got wired up from the roof side and connected to the main load center. It also got connected to the internet via our router. That way I can monitor our solar production in real time by logging into the online interface of our inverter.
This sexy little white box has some additional safety functions built into it, such as an automatic safety disconnect, which is an important feature if you have a grid tied system like ours.
The inverter can tell in an instant if there is a power outage. If so, it automatically turns off (or disconnects) and no longer feeds any electricity from the photovoltaic array into the load center and into the electrical grid. This is important because we don’t want to electrocute any line workers fixing the grid during an outage because we are still feeding power into it.
The same safety consideration applies to any maintenance or repair work on the photovoltaic array. By turning off the inverter, we can draw down the power in the roof array and won’t run the risk of getting shocked by a jolt of DC current.
This covers the practical aspects of the inverter installation, which is only half the story. The other half is about connecting to the grid or not and the administrative and contractual implications. These are described in detail in the blog post “How to connect a solar system”.
If you find any of the solar terminology confusing, you find answers in the previous blog post “Solar lingo”.
We had been actively preparing for this moment for a long time. The installation of the 8.58 kW photovoltaic roof array with its 26 modules is a massive step towards our goal of achieving net-zero-energy. So, let’s break down the installation process into digestible pieces.
In preparation for the roof solar array installation, we had to fix various items on and around the roof (see “The roof project” for more information). Elements that were directly related to the solar array were the solar blocking to anchor the array to the roof, and the solar posts installation that will support the array, which we carefully coordinated with Lisa Albrecht from All Bright Solar.
The morning of the installation a crate with our 26 solar modules was dropped off, which were carefully unpacked and hoisted onto the roof. It was not really much of a workout, as the modules themselves were rather light.
Up on the roof, the crew began to mount the railing system to the previously installed solar posts, and laid out the wiring of the array. Next the crew set the modules onto the rails, carefully lined them up and bolted them down. Once they were attached, the crew made the final wiring connections and cut off the rail ends to fit.
Now let’s talk about the hidden elements that went into this installation, such as building code requirements. If you look at the photovoltaic array layout, you may ask why we only had four modules in a row, while there would have been space for six modules.
Chicago building code requires a minimum of 36 inch clearance around the array. My understanding is that this is a fire safety provision, i.e. to make roof access to fire fighters easier.
How about the other direction? Why didn’t we space the rows closer together? Why didn’t we increase the angle of the modules to get a better solar yield?
The answer is: 21. Like December 21st – or the winter solstice. To prevent the modules from casting shadows onto each other, the row spacing and angle of the modules was determined by the sun’s angle at winter solstice.
Are we ready yet to catch some photons? Well, hold your horses! For the photovoltaic array to actually work, we need an inverter, which will be the subject of the next post.
If you find any of the solar terminology confusing, you find answers in the previous blog post “Solar lingo”.
