Sanborn Fire Insurance Maps: How to Read and Use Them

If you research old buildings for long, you will eventually hear the same advice from every archivist and preservationist: "Check the Sanborn maps." Sanborn fire insurance maps are the single richest visual record of American towns and cities between the Civil War and the mid-20th century. They show the footprint, construction, height, and use of nearly every structure on a block — block after block, town after town, updated every few years for almost a century. Once you learn to read their colors and abbreviations, you can watch a single lot change from a livery stable to an auto garage to a parking lot, and you can often date a building to within a few years. This guide explains what the maps are, how to decode them, and where to view thousands of them free online.

1. What Sanborn Maps Are — and Why They Exist

In the 19th century, fire was the great destroyer of American towns. Insurance companies needed to know, before writing a policy, whether a building was wood or brick, how close it stood to its neighbors, whether a bakery oven or blacksmith forge sat next door, and how good the local water supply was. Sending an inspector to every building in every town was impossible, so insurers bought maps instead.

Surveyor Daniel Alfred Sanborn began producing fire insurance maps in the 1860s, and the Sanborn Map Company he founded eventually dominated the industry, mapping roughly 12,000 American cities and towns. Field surveyors walked every street, measured every structure, and recorded its construction, height, roof type, and use. The resulting maps were sold to insurance underwriters by subscription and updated regularly — large cities might get a fully new edition every five to ten years, with paste-on correction slips issued in between. Production wound down after World War II as insurers changed how they rated risk, and regular updating had largely ended by the 1960s and 1970s.

The happy accident for historians is that a document created for actuaries turned out to be a near-photographic record of the built environment. No other source shows you, lot by lot, what stood on an American street in 1885 or 1912 or 1948.

2. What the Colors Mean

Sanborn maps use a standard color key for building construction, printed in the key at the front of each volume:

• Yellow (olive) — wood frame construction. Most houses and small-town commercial buildings are yellow.
• Pink / red — brick. Downtown commercial blocks are typically solid pink.
• Blue — stone or concrete block.
• Gray — iron or metal-clad structures, such as sheds and warehouses.
• Brown — adobe, mostly on maps of the Southwest.

Color tells you construction, not appearance. A "brick" building on a Sanborn map may have had a wooden storefront, and a frame building could be brick-veneered on later maps — the surveyors usually noted veneer with an abbreviation such as "Br. Ven." Early editions (before roughly 1890) and some very late ones were printed without color, using hatching and labels instead, so always check the key on the specific volume you are reading.

3. Decoding the Symbols and Abbreviations

The real information density of a Sanborn map is in its tiny annotations. The most useful ones:

• Numbers inside a footprint — the number of stories. "1½" means one story plus attic; "2-1" can indicate a two-story front section with a one-story rear addition.
• D — dwelling. F or "Flats" — a multi-unit residential building. S — store.
• Use labels — commercial and industrial buildings are labeled with their function: "Gro." (grocery), "Drugs," "Sal." (saloon), "Bak." (bakery), "Off." (offices), "Ware Ho." (warehouse), "Auto" or "Auto Rep." (garage), "M.E. Ch." (Methodist Episcopal church), and so on. Larger factories often carry a full name: "Acme Planing Mill."
• Dashed or broken outlines — porches, canopies, and other open or light structures attached to the main building.
• Small numbers along the street edge — house numbers. These are gold for address research, and they also reveal when a street was renumbered between editions.
• Fire-protection details — water mains with their diameters, fire hydrants, alarm boxes, fire walls (thick lines between attached buildings), windows and iron shutters, and notes on whether a building had a watchman, sprinklers, or "no exposure."

Every volume opens with a key page and a street index. Read the key first: abbreviations shifted slightly over the decades, and the key on that volume is the authoritative version. If you get stuck on an obscure mark, the Library of Congress publishes introductions to the collection describing the symbol conventions, and reference librarians who work with Sanborns can usually decode oddities on sight.

4. The Free Library of Congress Collection

The Library of Congress holds the largest Sanborn collection in existence — on the order of 700,000 sheets covering thousands of communities in all 50 states — and has digitized a huge share of it. The online collection at loc.gov/collections/sanborn-maps is free, requires no account, and offers high-resolution scans you can zoom, download, and print.

To find your town: use the collection search box, or filter by state and then county/city from the left-hand facets. Each result is a volume (or a set of sheets) for one town in one year, such as "Sanborn Fire Insurance Map from Springfield, Greene County, Missouri, 1910." Open the volume, use the index sheet at the front to find your street and block number, then jump to that sheet. Download the full-resolution image so you can zoom in on the annotations — the labels are small and often unreadable at screen resolution.

One caveat: the digitized set skews toward editions that are out of copyright, so online coverage is strongest for maps published before the mid-1950s. Later editions often exist but must be viewed on microfilm, in ProQuest's subscription database (below), or at the holding library.

5. State Libraries, Universities, and Other Sources

Many states digitized their own Sanborn holdings years before the Library of Congress project, and these collections sometimes include editions or correction-updated volumes the LOC set lacks. Worth checking:

• Your state library or state archives. Search the web for your state's name plus "Sanborn maps" — most state libraries have either a digital collection or a finding aid listing which towns and years they hold.
• State university libraries. Flagship university map libraries frequently host digitized in-state Sanborns and publish town-by-town indexes of what exists.
• ProQuest Digital Sanborn Maps (1867–1970). A subscription database of black-and-white scans, available free through many public and university libraries — ask whether your library card gives you remote access. Its post-1950 coverage fills gaps in the free collections.
• Local public libraries and historical societies. Many keep paper or microfilm Sanborns for their own town, including late correction-pasted volumes that show the town into the 1950s and 1960s. Your county historical society may also hold the related fire-insurance surveys and photographs. Find yours in our directory of historical societies by state.

6. How to Date a Building with Sanborn Maps

The classic Sanborn technique is bracketing. List every map edition published for your town — say 1888, 1893, 1899, 1906, 1913, and 1927. Find your lot on each one. If the lot is empty in 1899 and a two-story brick store stands there in 1906, the building went up between those dates. You have narrowed a construction date to a seven-year window using nothing but free maps, and you can usually tighten it further with a building permit or a newspaper item announcing construction — see our guide to free historical newspaper archives for how to run that search.

The same bracketing works in reverse for demolition dates, and for alterations: watch for a footprint that grows a rear addition, a porch that disappears, a frame (yellow) building replaced by a brick (pink) one on the same lot. If you are researching a house rather than a commercial building, Sanborns pair naturally with deeds and tax records — our walkthrough on researching your home's history shows how to combine them.

One critical warning about dates: Sanborn volumes were corrected by pasting slips over changed buildings, so a volume labeled "1913" may actually show the town as it looked in 1935 if it was updated in place. Look at the volume's title and publication notes — corrected volumes are often described with a date range such as "1913–1938" or noted as "republished" — and treat any paste-corrected sheet as evidence for the later date, not the cover date.

7. Tracking a Lot's Use Across Editions

Beyond dating construction, Sanborns let you reconstruct the working life of a property. Pull the same block from every available edition and read the use labels in sequence: a lot that reads "Livery" in 1893, "Auto Rep." in 1913, and "Filling Sta." in 1927 tells a complete story of American transportation in three words. This is the fastest way to answer the question what business used to be at an address when directories are missing — and when directories do exist, the two sources confirm each other. A Sanborn label gives you the type of business ("Gro."); an old city directory gives you the proprietor's name; a newspaper ad gives you the dates. Together they are close to airtight.

Watch the details as you compare editions: house numbers changing (street renumbered), a fire wall appearing (two buildings merged into one business), a "D" becoming an "S" (a house converted to a shop). Also note what surrounds your lot — a hotel, depot, or factory nearby explains why a business located there.

8. Limitations to Keep in Mind

• Not every place was mapped. Sanborn surveyed towns where insurers had business — generally incorporated communities of a few hundred people or more. Rural areas, farmsteads, and very small hamlets usually have no coverage at all.
• Small towns got few editions. A county seat might have editions in 1885, 1893, 1905, and 1926 and then nothing. Your bracketing window is only as tight as the edition gap.
• Coverage is mostly built-up areas. Even in mapped towns, outlying streets and new subdivisions were often omitted until later editions.
• Paste-up corrections blur dates (see section 6). Always verify which state of a volume you are viewing.
• Names are rare. Except for large industries, hotels, and institutions, Sanborns record what a building was used for, not who ran it. Pair them with directories and newspapers for names.
• The era ends around mid-century. For changes after the 1950s, you will need aerial photos, permits, phone books, and city records instead — our guide to finding old business records covers those sources.

Frequently Asked Questions

Are Sanborn maps free to view online?

Yes. The Library of Congress hosts tens of thousands of sheets free at loc.gov/collections/sanborn-maps, with no account required and high-resolution downloads. Many state libraries and universities host additional free scans of in-state maps, and most public libraries offer the ProQuest Digital Sanborn Maps database free with a library card.

What do the colors on a Sanborn map mean?

The colors indicate construction material: yellow means wood frame, pink or red means brick, blue means stone or concrete block, gray means iron or metal-clad, and brown means adobe. The key printed at the front of each volume explains the exact scheme used in that edition.

How do I date my house using Sanborn maps?

Find every Sanborn edition published for your town and locate your lot on each one. The building's construction date falls between the last edition where the lot is empty and the first edition where the building appears. Confirm with permits, deeds, or a newspaper construction notice, and remember that paste-corrected volumes may show a later date than their cover year.

Why isn't my town on the Sanborn maps?

The Sanborn Map Company only surveyed communities where fire insurers had enough business to justify a map — generally incorporated towns. Rural areas and very small hamlets were rarely covered. For unmapped places, try county atlases and plat maps, aerial photographs, and your local historical society.