Planning a Solar-Ready Roof

by Jonathan McGaha | August 3, 2014 12:00 am

Beginning this year, the California Energy Commission’s new energy-efficient standards require solar ready roofs for all newly built residential and commercial structures. While home and building owners will not be required to install photovoltaic (PV) panels at the time of construction, buildings will need to be equipped for future PV installation.

The best time to think about a solar roof plan is during the design stage. Pre-planning a solar-ready design is a must for an installation during build or in the future. This entails developing a solar-ready review, which focuses on optimizing the design and power production while minimizing the initial and long-term costs.

Components of a solar ready review include:

Orientation

To maximize power production of a PV system, building orientation is a very important factor. When steeper slopes are involved, south-facing roof surfaces are best. If a south orientation is not possible, southwest or southeast can also be viable options. Orientation is not as important for low-slope roofs 5 degrees or less.

Roof Pitch

PV modules are normally installed planar to the roof surface on steep roofs, and planar or very slightly tilted on low-slope applications. Aggressive tilting is seldom done anymore due to economic considerations and adverse wind effects. Tilted systems are still used in very northern geographies or on some roofs not oriented to the south. It is a delicate balance of increased cost versus increased power production.

In theory, the best pitch is the latitude of the job site, but in practice this is seldom done.A lower pitch than optimum is not as critical as orientation; the difference in power production is nominal. The increase in power production is not usually worth the premium cost of unusually steep pitches, unless the steeper slope is also an intentional design element.

[1]Shading

The best design for a solar system is an unobstructed roof area with no shading. Building components such as plumbing stacks, skylights, chimneys and parapets can create shadows on the solar system. Consideration should also be given to any trees or future buildings that could cast a shadow on the system.

Attention to all the details in designing a solar system is crucial to achieve the most power production and to run an accurate return on investment (ROI) for the system.

With a basic ROI, the system designer and owner can accurately evaluate whether PVs makes financial sense. (Figure 1)

Example case ROI analysis:

• Net system cost after tax credit: $236,250
• What year do you go cash positive? Year six
• Positive cash flow for balance of service life: ROI
• At the end of 25 years (warranted period for solar modules) produced a ROI of $1,230,377
• If the system goes 35 years (still producing power) produced a ROI of $1,940,377

Was this a good ROI? Yes!

Roofing Material vs. Service Life

Generally, the warranted life of solar modules is 25 years, but actual service life may reach 35-plus years. The roof’s service life should exceed the service life of the solar system. Otherwise the roof must be replaced, necessitating the de-commission, removal and re-assembly of the solar system.

It is vital to understand the relative costs of the roof asset
(the mounting platform) and the solar asset (the revenue generator). Let’s consider an example of a new TPO roof of significant size that is covered with PV. The initial cost of the roof is approximately $3/sq. ft. Calculating the value of the solar array at about 12 watts per square foot and a typical cost of
$2.50/watt, the cost of the array is $30/sq. ft. The aggregated cost is $33/sq. ft. But the TPO roof is a 15-year roof and will have to be replaced during the life of the solar system.

When re-roofing is required after 15 years, the cost of reroofing is not only the cost of the new roof, but also the cost of dismantling and reassembling the solar array, in addition to the loss of power production during the process. Estimating the re-roof cost at $2/sq. ft. and the cost of removing and reinstalling the solar array at $13/sq. ft., the total is $15/sq. ft. for the complete re-roof.

The aggregated cost of this total system over time is now
$48/sq. ft. for a total 30-year system.

Compare this to the cost of using standing seam metal roofing. Instead of expensive ballasted or penetrating racking systems required for the TPO roof, standing seam roof clamps can be used to mount the system without penetration. This provides as much as 15 percent lower costs than mounting costs on membrane roofs.

The cost of a mid-sized, low-slope commercial standing seam roof is about $4/sq. ft. But the metal standing seam roof outlives the solar, avoiding all costs of roof replacement. And the savings in mounting costs more than offsets the premium cost of the roof.

Consequently the aggregated initial cost with the standing seam metal roof is approximately $31/sq. ft., making the aggregated cost of ownership over the life of the solar system $31/sq. ft. for the metal roof vs. $48/sq. ft. for the TPO.

So when you look at the roof (the mounting platform) and the solar system (the revenue generator) as a single asset, the initial costs are lower when using a standing seam metal roof versus any other roof type. And the long-term costs are much, much lower.

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Harry J. Lubitz, CSI, is the architectural and national accounts manager for S-5! Metal Roof Innovations Ltd. of Colorado Springs, Colo. For more information, visit www.s-5.com[2].

Source URL: https://www.metalarchitecture.com/articles/planning-a-solar-ready-roof/

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