• Sonny Rivers

Rooftop Solar

Most residential solar panels are installed on rooftops, often an ideal location for maximum sunlight exposure. Rooftop systems also install faster and at a lower cost compared to a ground-based system.

A typical home may have a rooftop "solar capacity" of 3 kW to 6 kW for a median-sized home. Suitable locations where the roof orientation is to the south must have no shading, so the usable roof space does vary significantly from home to home. Rooftops that get full sunshine every day can have enough capacity to cover a majority of electricity use.

Not all homes have suitable roofs. One reason is simply trees casting shadows over the roof. Some roofs can be very complex (especially hipped roofs), limiting the usable space for installing solar panels. In some cases, there are chimneys, pipes, or nearby utility poles that cast significant shading over the roof space. These situations can result in significant loss of performance, throwing into question the viability. In these cases, a ground-based system may be a suitable alternative.

The ideal orientation of the roof is essentially anywhere between southeast and southwest. Direct south will offer the most hours of direct sun exposure, but all the way to direct east or direct west can be acceptable in some cases.

The optimal tilt or pitch for a direct-south roof is roughly equal to the latitude, or about 40 degrees (10:12), all things considered, in the Black Hills area. As the roof faces more to east or west, lower angles of the roof pitch become slightly more favorable. But a roof that is less than 4:12 (18 degrees) will have a much greater performance loss due to snow. Though it is not necessary to have a very high-pitched roof either, as it is generally still very good to have a pitch of around 6:12 (25 degrees), which is more common.

Rooftop solar arrays are attached to the roof structure with the appropriate hardware designed for the specific roofing type (i.e. asphalt shingle, metal, tile, etc.). The hardware design is engineered according to the type of attachment needed, the slope of the roof, and local wind/snow loads.

Solar panels must have a setback from the roof edge, usually equal to the overhang (rake) of the roof or about two feet. The setback ensures accessibility and keeps the edge of the panels away from the strongest wind zone. The setback from the roof peak is at least one foot, and the setback from the bottom edge (eave) is one to three feet depending on the pitch of the roof (steeper roof = larger setback).

The distributed weight of a solar array is relatively low (only about 2.5 psf), so structural modifications are not required. Most roofs are engineered to carry much more than the maximum weight of the snow load, which is many times heavier than solar panels (generally about 20-40 psf snow load in the Black Hills).

Roof trusses/rafters provide strong anchoring points, or in the case of metal standing seam roofs, the attachments are made by clamping hardware directly to the metal seam (without penetrations or sealant). On average about one to two attachments are needed per solar panel.

When properly installed, all types of roof attachments are waterproof for the life of the roof and are guaranteed to not leak or result in structural damage. In fact, the attachment location will become even more waterproof than it was before.

solar roof
Asphalt shingle rail system attachment with metal flashing.
  • Asphalt flashing roofs attachments may use a metal flashing that slides beneath the shingles, guaranteeing a dry penetration even in the event of shingle failure. Adhesive-flashing systems are also widely used, which provide a permanent seal by bonding to the shingle.

solar roof
Rail-less attachment system by Tesla.