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 proposed a dimensional control that would step back the fifth and sixth stories on the north side of a building by a combined 15 feet (a 10‑foot and a 5‑foot step). Roberts said the measurement would be taken in a north‑south direction rather than perpendicular to the lot line so the rule would specifically reduce north‑side shadowing. In a modeled 50‑by‑100‑foot lot, that envelope change reduced the buildable area of the top two floors by roughly 9% to 19%, which translated to approximately a 3% to 6% reduction in the building’s total floor area under the example used in the analysis.

Roberts and the consultant showed how shadow loss declines quickly as horizontal distance increases: example scenarios in the presentation showed about an 18% loss in annual solar radiation when a 74‑foot building is 10 feet from a 45‑foot roof, declining to about 8% at 20 feet. For a 30‑foot neighboring roof the modeled loss ranged higher (about 26% at 10 feet in one configuration), dropping toward the mid‑teens by 20 feet. Based on these results, staff said the proposed stepbacks are intended to “manufacture” roughly a 20‑foot effective separation that markedly reduces solar loss on nearby roofs.

City staff flagged tradeoffs. Roberts said modest stepbacks can affect building design and structural solutions and that in “bubble” cases the loss of the fifth and sixth stories could influence a developer’s decision whether to pursue an inclusionary (six‑story) project or a smaller four‑story building. He also noted that zoning must be applied uniformly across a district, so the stepback would apply even where no existing solar panels are present.

Presenters and public speakers emphasized housing and climate goals. Assistant City Manager Melissa Peters and Deputy Climate Chief Suzanne Rasmussen framed the proposal as an effort to balance housing production under the new multifamily zoning with the city’s solar and climate goals. Climate Chief Julie Wormser noted the city also pursues large‑scale contracts for new solar generation outside the city, saying those procurements affect the city’s overall carbon budget while the proposed stepbacks are intended to protect household and rooftop solar investments.

Representatives of Care Housing, a resident group that supports affordable housing and solar protection, urged stronger protections and raised questions about compensation for homeowners whose existing arrays lose production. Liz Byron of Care Housing said, “We know we can do both affordable housing and we can support the environment and protect solar.” Several public commenters urged compensation programs or community solar options for renters and low‑income households; Nancy Seidman, a resident and former Massachusetts DEP employee, said, “If my system or any existing system is affected, we should be compensated.” Other speakers urged the city to consider broader neighborhood‑scale scenarios (for example, taller corridor developments) rather than modeling only isolated building pairs.

Committee discussion focused on next steps and exemptions. Councilor Sabrina Wheeler asked whether inclusionary (affordable) projects might be exempted; staff said the principal sites that could be built to six stories would generally be subject to inclusionary requirements so a blanket exemption could undercut the very buildings intended to produce affordable units. Vice Mayor Mark McGovern and others asked staff to consult developers before finalizing language to understand financing and structural implications. Committee members also discussed timing and grandfathering rules: the city solicitor explained that projects with an issued building permit (or finalized special permit, if applicable) are protected from later zoning changes, while projects that have not yet received a permit would need to conform to an adopted amendment after advertisement.

On a motion by Councilor Wilson directing CDD to draft zoning language implementing the recommended north‑side stepback approach and return it to the council for referral to ordinance, the committee recorded four votes in favor and one absence; Councilor Azim was recorded absent. Committee members said the draft should consider (a) how the stepback affects inclusionary projects, (b) whether 100% affordable projects or other narrow categories should be treated differently, and (c) outreach to developers prior to final language.

Next steps: staff will draft a policy order and proposed zoning text for the council’s review and referral to the Ordinance and Planning Board process. Councilor Nolan noted staff previously suggested a possible timeline to bring a policy order forward to the full council by late September. The committee adjourned after taking the direction.