Discovering the History of Your Home with Sanborn Fire Insurance Maps

Written By William Merritt


March 29, 2025

If you’ve ever been curious about the history of your house—when it was built, how it changed over time, or what used to exist nearby—there’s a powerful (and often overlooked) resource that can help: Sanborn Fire Insurance Maps.

These highly detailed maps were created by the Sanborn Map Company between 1867 and 1977 to help fire insurance companies assess risk. Today, they’re an invaluable tool for historians, preservationists, genealogists, and homeowners alike.

 What Are Sanborn Maps?

Originally designed for the insurance industry, Sanborn maps documented the built environment in over 12,000 cities and towns across the U.S., Canada, and Mexico. They include remarkable detail about:

  • Building materials (wood, brick, stone, metal)

  • Building use (residential, commercial, industrial)

  • Number of stories

  • Roof materials and shape

  • Windows, doors, porches, chimneys

  • Water sources (hydrants, cisterns)

  • Fire stations and available fire-fighting equipment

This information helped insurers calculate premiums—but today it helps us unlock the stories of the past.

What Can Sanborn Maps Tell You About Your House?

If you're researching your home or neighborhood, Sanborn maps can help you:

  • Estimate construction date: If your house appears on a later map but not on an earlier one, it was likely built in between.

  • Identify changes: Additions, demolitions, or alterations show up as changes from one map edition to the next.

  • Learn original materials: Maps show whether a home was originally made of wood (yellow), brick (red/pink), stone (brown), or metal/concrete (blue/green).

  • Understand your neighborhood’s evolution: See when businesses, schools, or industrial sites appeared or disappeared near your home.

  • Spot environmental or fire hazards: Maps often show proximity to railroads, factories, or lumber yards—factors that may have influenced how the area developed.

How to Read Sanborn Maps

Sanborn maps use a specific system of colors, symbols, and codes to pack in information. Here are some basics:

Colors

  • Yellow = Wood (frame)

  • Red/Pink = Brick

  • Brown = Stone

  • Blue/Green = Metal or concrete

Symbols

  • Numbers = Number of stories (e.g., “2” means two stories)

  • D = Dwelling (residential house)

  • S = Store or shop

  • F = Factory

  • FP = Fireproof building

Lines and Patterns

  • Solid lines = Permanent structure

  • Dotted lines = Wooden porches, sheds, or awnings

  • Cross-hatching = Roof materials (slate, shingles, etc.)

Fire Safety Indicators

  • Blue circles = Hydrants

  • Black dots = Cisterns

Each map includes a legend or key to help decode these symbols.

Where to Find Sanborn Maps

Many of these maps are now digitized and available online, often for free:

  • Library of Congress Sanborn Map Collection - Jersey Coast – Free and comprehensive resource for many cities.

  • Local historical societies and libraries – Especially helpful for smaller towns not covered online.

  • University library archives – May have regional Sanborn collections.

  • ProQuest Sanborn Maps – A subscription-based resource (often available through universities or public libraries).

How to Estimate the Age of Your House Using Sanborn Maps

  1. Find your property’s location in a known edition of the Sanborn maps (using street names or landmarks).

  2. Check earlier editions of the map to see when your house first appears.

  3. Compare features across editions to note any additions, changes in use, or demolition.

  4. Cross-reference with building material and footprint to understand how the structure may have evolved.

For example, if your home appears in the 1915 map but not the 1909 version, it was likely built between those years. Look for small details like added porches or second stories, too.

Now have some fun!

William Merritt