How to read a Sanborn fire insurance map

Sanborn atlases were not made for historians. They were made so an underwriter in another city could price the risk of a building he would never see. That is why they are the most useful historical document about a piece of American urban land: everything on the sheet is there because it affected fire risk, and past use affected fire risk.

What the sheets are

From the 1860s to the 1970s the Sanborn Map Company surveyed some twelve thousand American towns and cities, block by block, and republished each atlas every few years. A sheet shows the footprint of every building, its construction material, its height in storeys, window and door openings, skylights, elevator shafts, boilers, stacks, tanks, and, written across or beside the footprint, what the building was used for. Around the edges you get street names, house numbers, water main diameters and hydrant positions.

The Library of Congress has digitised around twenty-five thousand editions. Editions published before 1931 carry no copyright restrictions and can be downloaded and reused by anyone, which is why this site can show them to you in full.

The colours

Colour is construction material, not use. On a hand-coloured sheet:

• Yellow — wood frame
• Pink or red — brick
• Blue — stone, concrete or hollow tile
• Grey — iron or other metal
• Brown — adobe

Some digitised editions are monochrome, and colour conventions vary between periods. Read the key printed on the atlas you are looking at; it is usually on the title or index sheet.

The lettering

This is the part that matters for a property purchase. Sanborn abbreviated aggressively to fit words inside a building footprint. AUTO REPG. is auto repairing. FILLING STA. is a gasoline filling station. DRY CL’G is dry cleaning. FDRY. is a foundry. BL. SM. is a blacksmith. PLANING MILL is woodworking. Tanks are drawn and their capacity written in gallons.

Our former-use decoder lists the abbreviations that come up most often in historical property research, what each one looks like on the plan, and why people look for it.

The index sheet

Every volume opens with an index or key sheet: a small map of the whole city divided into numbered rectangles. Find your block on the index, note the number, then go to that sheet. Large cities run to several volumes, each covering a section of the city, so check you are in the right volume first. In the Baltimore sheet browser the index sheet is one button away.

Reading a sequence, not a sheet

A single sheet is a single date. The value is in the sequence. Baltimore has editions from 1890 and 1901 in this release; put them side by side and a stable rowhouse block reads differently from a corner lot that was a blacksmith in 1890, a garage in 1901 and a filling station by the 1920s. That progression — horse to car — is the single most common reason a corner lot gets a second look during due diligence.

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