Solar blocking

Nope, I am not trying to block the sun, but this is a good reminder that the whole roofing project was happening because we were getting ready to install a photovoltaic array on our roof. And one question to resolve was: How should we attach the solar panels to the roof?

Some systems are weight based, meaning they are not physically attached to the roof structure, but instead weighed down by concrete blocks, for example. This method has the advantage that there is no hardware that penetrates the roofing membrane. The disadvantages are that the roof needs to be able to accommodate the extra weight, and more importantly that the City of Chicago would not permit weight based systems, according to my solar installer.

We needed to come up with a solution to anchor the solar array to the roof joists.

The solar panels are mounted onto rails in rows. There is one rail towards the bottom and one towards the top of the panels. The rails in turn are mounted onto posts, which are anchored to the roof structure. There is a short post towards the front of the panel and a longer post toward the back, so that the panel faces the sun at an angle. (If you struggle with some of the terminology, go to: Solar lingo)

If you look at the plan sheet above, you can see that almost none of the posts (red dots) line up with the roof joists (dotted line going from left to right). So, anchoring the posts directly into the roof joists was not an option. Instead, we planned to indirectly anchor them. We installed and anchored three rows of blocking that was running perpendicular to the roof joists (the dotted line running from top to bottom), to which we could anchor the posts.

The blocking consisted of two two-by-fours laid long side down, stacked on top of each other, and anchored into the roof joists.

This solution had to fit into our new roofing system. That is why we used two layers of 1 ½” polyiso boards, so that the insulation would be flush with the blocking for the solar posts and ease the installation of the roofing membrane.

I had marked the layout for the solar blocking right after the roof tear off and kept track of it during the insulation installation, including where to anchor it to the roof joists. I cut and removed insulation and laid out the two-by-fours. I made sure to stagger the joints between the bottom and top row. I also made sure that no joint was over a roof joist, or at an anchoring point for the posts.

After I had the two-by-fours anchored down I foamed around the edges to keep the insulation assembly sealed.

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About Marcus de la fleur

Marcus is a Registered Landscape Architect with a horticultural degree from the School of Horticulture at the Royal Botanic Gardens, Kew, and a Masters in Landscape Architecture from the University of Sheffield, UK. He developed a landscape based sustainable pilot project at 168 Elm Ave. in 2002, and has expanded his skill set to building science. Starting in 2009, Marcus applied the newly acquired expertise to the deep energy retrofit of his 100+ year old home in Chicago.

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