The Sanborn fire-insurance maps: a hidden treasure for house history

If you research one source about your house, make it the Sanborn maps. They were never meant to be history — they were a business tool for fire insurers — but in doing that job they recorded American towns and cities building by building across the better part of a century. For the house historian, there's nothing else quite like them.

What they are

From 1867 into the mid-20th century, the Sanborn Map Company surveyed American communities to help insurance underwriters assess fire risk. Their surveyors walked every block and drew it to scale: the footprint of every building, its number of stories, its construction material, its use (dwelling, store, stable, factory), the location of windows and doors, and even details like whether a porch was open or enclosed. Big cities were re-surveyed every few years and the sheets updated, so for many places you get a series — the same block in 1894, 1904, 1916, 1929.

The result is an astonishingly fine-grained portrait of the built environment, made for thousands of towns most other map series ignored.

How to read them: the colors

Sanborn sheets are color-coded by construction material, and once you know the key you can read a whole block at a glance:

• Pink / red — brick or masonry.
• Yellow — wood frame (the most flammable, hence the insurer's interest).
• Blue — stone or concrete.
• Gray — iron or metal-clad.

Numbers inside each footprint give the stories (and sometimes the street number). Letters and symbols flag use and hazards — "D" for dwelling, "S" for store, markers for stables, kilns, and tanks. Set two editions side by side and you can watch a frame cottage get a brick addition, a stable become a garage, a row of houses replace an orchard.

What they reveal about your house

A Sanborn sheet can tell you, often more reliably than the assessor's record:

• When your building first appears — bracketing its construction date between two editions.
• What it was built of — and whether that changed.
• Its original footprint and height — and any additions, extensions, or demolitions since.
• What stood there before — the earlier sheet might show a different building, or open land.
• What the block was for — purely residential, or mixed with shops, workshops, and stables.

For our own flagship block in Brooklyn, the Sanborn sheets confirmed the story the deeds suggested: a row built whole, brick on one side and frame on the other, recorded house by house exactly as the insurers found it.

Where to find them free

The single best free source is the Library of Congress, which has digitized its enormous Sanborn collection and put it online — searchable by state, county, and city, free to view and download. Many state and university libraries hold Sanborn sets for their own regions, sometimes with editions the LOC lacks, and some offer the Sanborn digital edition through library subscriptions (free with a library card). For the years and places the LOC hasn't digitized, a local research library is the next stop.

A few cautions

The maps were drawn for insurers, not historians, so read them with care. Surveyors occasionally erred; updates were sometimes pasted over older sheets, so a "1904" sheet may contain patches from later years. They show buildings, not people — pair them with the census for residents and the deeds for owners. And coverage is uneven: not every town in every year was mapped.

How Plot uses the maps

Plot treats historical maps and the documentary record as one layer of an address's story, alongside the deeds, the census, and the press — assembling them into a single written history. The Sanborn maps are exactly the kind of source that turns a list of facts into a picture of a place; reading them yourself, with Plot's assembled history open beside you, is one of the most satisfying ways to get to know a house.

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