A two-alarm fire at a three-family house on Roberts Road on Thursday offers Cambridge a sobering reason to examine an issue that received surprisingly little attention during the city’s debate over multifamily zoning: What happens to fire safety when buildings get larger and the spaces between them get smaller?

Nine people and two dogs were displaced by the fire. When firefighters arrived, flames were coming from the third floor of the three-family residence. Fortunately, no injuries were reported, and firefighters brought the blaze under control in a little over an hour. The cause remains under investigation.

The fire should nevertheless remind us how quickly conditions can become dangerous in Cambridge’s densely built residential neighborhoods – and how little fire cares about the legal boundary between one parcel and another.

This is not an argument that upzoning causes fires. It does not.

It is an argument that Cambridge, via the citywide multifamily zoning adopted in February 2025,  has changed the physical relationship permitted between future buildings without, to my knowledge, publishing a comprehensive analysis of what those changes mean for structure-to-structure fire exposure, firefighter access and the older houses that will remain beside new development.

It allowed multifamily housing throughout nearly the entire city, eliminated general residential limits on the number of units and floor-area ratio, reduced setbacks and increased permitted residential heights. The new minimum side or rear setback for residential uses is generally five feet. But five feet does not necessarily mean that every architectural element must remain five feet from the property line. Projecting eaves, bay windows, balconies, and similar projections sometimes extend as much as 3½ feet, leaving  only 1½ feet between that projection and the property line.

Of course,  zoning is not the only law governing construction. Massachusetts’ state building code separately regulates such matters as fire separation, exterior walls, openings and projections. But Cambridge should not force residents to piece together its zoning ordinances, state building codes and fire regulations; it should publish its own comprehensive analysis. 

The importance of distance between buildings is not theoretical.

In full-scale experiments, the National Institute of Standards and Technology studied fire spread between simulated residential structures separated by six feet. In one experiment involving combustible exterior construction, the neighboring structure ignited less than 80 seconds after flames emerged from the burning structure. NIST emphasized that structure-to-structure fire spread depends on several factors, including construction type, separation distance, windows and weather conditions.

That does not mean a Cambridge building six feet from another will behave like the structures in the NIST experiment. It does mean that separation distance is a legitimate fire-safety variable worthy of serious analysis.

Modern fire protection is of course of enormous import; The National Fire Protection Association’s analysis of fires from 2017 through 2021 found that sprinklers operated effectively in 93 percent of home fires large enough to activate them. In homes with sprinklers, fires were confined to the object or room of origin 96 percent of the time, compared with 72 percent in homes without an automatic extinguishing system. 

But a sprinkler system in a new building does not install sprinklers in the older house next door.

Cambridge has painful experience with fires spreading through a densely built neighborhood. In December 2016, a 10-alarm fire in Wellington-Harrington damaged 18 properties and affected 80 families. City records describe multiple residential buildings being damaged and more than 100 residents displaced.

That catastrophe demonstrated a basic fact of urban neighborhoods: Once a fire escapes the building where it began, the characteristics and proximity of surrounding structures matter.

None of this means Cambridge should retreat from building housing. The city needs housing, and greater density is a legitimate public-policy objective.

But housing policy and fire safety are not competing values. Good planning requires us to pursue both.

During the zoning debate, Cambridge devoted enormous attention to height, density, affordability, historic preservation, shadows, trees, open space, parking and transportation. Fire safety deserves the same seriousness.

That analysis should involve the Cambridge Fire Department and qualified fire-protection professionals and examine at least three questions: structure-to-structure fire exposure under the development patterns now permitted; the practical working space firefighters need for ladders, hoses, rescues and protection of neighboring structures; and the vulnerability of existing older buildings when larger new buildings are constructed close beside them. How Cambridge zoning interacts with Massachusetts building and fire codes should also be addressed.

If so, residents deserve to see the evidence.

Perhaps it will identify modest changes that could reduce risk without sacrificing meaningful housing production. We should want to know that, too. The Roberts Road fire wasn’t caused by zoning. But it does offer a cautionary tale for what can happen with closely spaced buildings. The city should therefore commission and publish a fire-safety analysis of the new residential zoning.

Zoning determines where the property line is.

Fire doesn’t stop there.

A stronger

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