How to Read a Sanborn Fire Insurance Map

Sanborn maps pack an enormous amount of information into every sheet. Once you understand the system of colors, symbols, and abbreviations, you can read these maps like a book — learning not just where buildings stood, but what they were made of, how they were used, and how vulnerable they were to fire.

Color Coding: Construction Materials

The most immediately recognizable feature of Sanborn maps is their color coding system. Each color represents a different building material:

Understanding these colors was critical for insurance underwriters. A row of yellow (wood frame) buildings crowded together represented a much higher fire risk than spaced-out pink (brick) structures with fire walls between them.

Building Information

Within each building footprint, Sanborn maps include remarkably specific notations:

• Number of stories — Indicated by a numeral inside the building outline (e.g., “2” for a two-story building)
• Basement — Marked with a “B” or “Bsm't” if the building had a basement level
• Building use — Written inside the footprint: “D.” (dwelling), “S.” (store), “Off.” (office), or specific uses like “Planing Mill,” “Livery,” or “Opera House”
• Addresses — Street numbers are typically shown along the building frontage
• Fire walls — Heavy dark lines between buildings indicate fire-resistant party walls
• Window and door openings — Small marks along walls indicate window placements, important for assessing fire exposure from neighboring buildings

Common Abbreviations

Sanborn maps use a consistent set of abbreviations to convey information efficiently:

Street and Infrastructure Details

Beyond buildings, Sanborn maps document the urban infrastructure in detail:

• Street widths — The width of each street is noted in feet, helping assess whether fire equipment could navigate the area
• Water mains — The location and diameter of water mains are shown, indicating available water pressure for firefighting
• Fire hydrants — Marked with circles or specific symbols along streets
• Railroad tracks — Rail lines and spurs are carefully mapped, along with associated facilities
• Lot lines — Property boundaries are indicated with dashed lines

The Title Pages

While the interior sheets contain the detailed maps themselves, the title pages serve as the cover and index for each volume. These pages typically include:

• The city name in elaborate hand-lettered typography
• The county and state
• The date of the survey
• A small overview map showing how the detailed sheets fit together
• Population figures
• Information about the city's fire department, water supply, and other fire protection measures

It is these title pages — with their extraordinary Victorian-era typography and design — that form the core of our digital archive. Each one is a unique work of graphic design that captures a city's identity at a specific moment in time.

Tips for Researchers

If you're using Sanborn maps for research, keep these practical tips in mind:

• Check the date carefully. Maps were revised and updated over time. Pasted-on corrections may cover original information, and different sheets in the same volume may represent different survey dates.
• Compare across years. Sanborn mapped most cities multiple times. Comparing editions reveals how neighborhoods changed — buildings demolished, new construction, shifts in use from residential to commercial.
• Look at the key. While the color system remained largely consistent, Sanborn occasionally modified conventions. The legend on each volume clarifies the specific symbols used.
• Consider what's not shown. Sanborn maps focused on fire risk. They generally omit parks, vacant land, and areas outside the developed core of a city.