Committee directs city staff to draft zoning language to limit rooftop shading with north‑facing stepbacks

The Cambridge City Health and Environment Committee voted 4–0 to ask city staff to draft zoning language that would require upper‑story stepbacks oriented toward the north on taller multifamily buildings to reduce shading of existing rooftop solar installations.

Councilor Patty Nolan, chair of the Health and Environment Committee, opened the meeting by describing the charge: the committee asked staff to analyze “how we think about protecting solar systems because people have made investments in this.” After presentations by planning and sustainability staff and public comment, members moved and approved a request that Community Development Department (CDD) staff prepare zoning language to enact the recommended dimensional limits and return it to the council for consideration.

The zoning division, working with the Office of Sustainability and a consultant who modeled solar radiation, presented the technical findings. Jeff Roberts, director of zoning and development, described the modeling by Professor Mehdi Harris, which calculates annual solar radiation at rooftop level across the year and then compares scenarios. Roberts said the most substantial shading impacts from a 74‑foot building on lower neighboring roofs are localized: “the most substantial impacts occur ... within about 20 feet of that building.” The model compared a 74‑foot structure (allowed under the new multifamily zoning in some cases) with surrounding roofs at 45 feet and 30 feet and reported the resulting percent reductions in annual solar radiation.

To mitigate those localized effects, staff…