What a Yellow Building Told a Fire Insurer

A yellow building on a Sanborn fire insurance map is not being awarded a risk score. The color identifies frame construction. That is useful information for someone considering a fire policy, but it leaves much of the decision still to be made.

The same color can cover buildings with different uses, surroundings and arrangements. An insurer needed a way to separate those conditions, even when the properties lay far from the office where the map was being read. Sanborn’s compact language of colors, letters and lines helped turn a building into a description that could travel.

Today, the maps are often consulted for quite different reasons. Someone might be tracing a house, a factory or a neighborhood that has changed beyond recognition. Reading them well begins with the purpose that shaped what was put on the sheet. As the Library of Congress emphasizes in its guide to interpreting Sanborn maps, fire insurance dictated both the information selected and its portrayal.

Construction was one part of the description

Yellow supplied a construction category. Letters and numbers could then identify use, stories or a basement. A dwelling and a store did not become the same sort of property because their outlines received the same wash of color.

Nor did a single building necessarily have one uniform shape or height. A main portion might have two stories and a basement while an addition had only one story. Reading the marks together prevents a tidy outline from flattening those differences.

Walls and passages supplied another kind of information. Lines could distinguish a solid wall from an opening, and projections could indicate a firewall extending above a roof. These marks concern connections between spaces as well as the material from which a space was built. To understand the mapped property, a reader had to follow more than the color around its edge.

This was a prescribed system, but its interpretation was not timeless. Symbols were added and their application varied. The key belonging to the particular edition matters more than a remembered rule from a different map. A mark that looks familiar still needs to be read in the system that produced it.

The town belonged in the calculation

A building did not carry its firefighting resources inside its footprint. The maps’ introductory material could describe equipment, firefighters and water supplies, including mains, pressure and hydrants. Some editions included a separate water-system map.

Construction describes the property, while the water report describes resources around it. Both could matter to an insurer, and neither can stand in for the other.

A colored plan therefore offers ingredients for a judgment rather than a complete judgment already made. It does not, on its own, tell us what premium a customer paid, whether a policy was issued or how a later fire behaved. Those questions need evidence of decisions and events beyond the key’s construction categories.

The same care applies to gaps in the map. In her guide to finding the collection, Library of Congress cartographer Amelia Raines explains that the company generally concentrated on denser populations. Rural areas and town edges could be left outside the coverage, while some small places appeared under a neighboring town’s name.

An uncolored area beyond the mapped streets cannot simply be read as empty ground. Before interpreting an absence, the researcher needs to know whether the survey was meant to reach that place. Coverage is part of the record’s design. In John Snow’s water-company inquiry, a street address likewise left something crucial unresolved. Neighbouring houses could receive different water supplies.

Which year does the color describe

Even after the symbols are understood, the date can cause trouble. Some Sanborn volumes were kept current through correction slips pasted onto the printed map. The original date could remain visible beneath a much later account of the city.

The Library’s interpretation guide gives a Chicago example catalogued as 1912 to May 1950. The earlier year belongs to the original edition. The later date belongs to its last revision. Reading the whole sheet as an untouched view of 1912 would assign later information to the wrong moment.

That does not mean each pasted change can be dated precisely. Successive corrections accumulated, and the date of every individual slip cannot always be recovered. A corrected atlas may establish the state of the map at its latest revision without supplying a complete sequence of when every feature changed.

A footprint visible on a corrected sheet might be evidence that the structure appeared in the revised record. The base publication date alone cannot establish that it already had that form when the first sheet was printed.

The color question has now become a reading procedure. Identify the edition and its corrections. Consult its own key. Put construction beside use, arrangement and the relevant town reports. Then ask whether the conclusion concerns something actually described on the map or a decision that would have required another record.