Sanborn Fire Maps: The OG Blueprints of America

In the mid-19th century, as American cities rapidly expanded, the risk of urban fires became a pressing concern. Enter Daniel Alfred Sanborn, a civil engineer and surveyor.

In 1866, he was commissioned by the Aetna Insurance Company to produce detailed maps of Boston and several cities in Tennessee to assess fire risks. Recognizing the broader potential, Sanborn established the D.A. Sanborn National Insurance Diagram Bureau in 1867, which later evolved into the Sanborn Map Company.

These maps weren’t your average city layouts. They were intricate, large-scale depictions (typically at a scale of 1:600) detailing building footprints, construction materials, street widths, property boundaries, and even the location of windows and doors. Their primary purpose? To help fire insurance companies assess the risk associated with insuring properties without having to conduct on-site inspections.

Rise of the Sanborn Fire Map

By the early 20th century, the Sanborn Map Company had become the preeminent provider of fire insurance maps in the United States. Their meticulous detail and standardized format made them indispensable tools for insurance underwriters. At its peak, the company employed hundreds of surveyors and cartographers, producing maps for over 12,000 cities and towns across the U.S., Canada, and Mexico.

These maps were updated regularly. “Pasters” or “correctors” would visit subscribers to paste updates onto existing maps, ensuring they reflected the latest urban developments. This system allowed insurance companies to maintain up-to-date records without the need for entirely new maps.

Decline and Transition

The mid-20th century brought significant changes. Advancements in building codes, fire prevention methods, and the consolidation of insurance companies reduced the reliance on detailed fire insurance maps. By the 1960s, many insurers had shifted to alternative risk assessment methods, leading to a decline in demand for Sanborn maps.

The last Sanborn fire insurance maps were published on microfilm in 1977. However, recognizing their historical value, institutions like the Library of Congress began efforts to preserve and digitize these maps for public access.

While their original purpose has faded, Sanborn maps have found new life in various fields, especially in historic preservation. Here’s how modern professionals and enthusiasts utilize these detailed documents:

Historic Property Research

Personally one of my favorite things to do it to look through our local Sanborn Fire Maps here in Orlando. They were redone every 3-5 years so I can watch as my neighborhood developed and see which houses were first, which houses were torn down and when and what the neighborhood used to look like once upon a time.

Sanborn maps provide a snapshot of urban development at specific points in time. Preservationists use them to:

• Verifying a Building’s Age: A lot of old records were lost over the years but these maps can clearly show when a building was built which can cause serious disagreements.
• Determine Original Building Footprints: Understand the original size and shape of structures.
• Identify Construction Materials: Colors and symbols indicate whether a building was made of brick, wood, stone, and details like roofing materials are noted too which is incredibly helpful.
• Track Structural Changes: By comparing maps from different years, one can identify additions, demolitions, or other alterations.

For properties seeking historic designation or undergoing restoration:

• Certificate of Appropriateness: Sanborn fire maps can provide evidence of a building’s original features, aiding in obtaining necessary approvals for modifications .
• Tax Incentives and Grants: Detailed historical data supports applications for preservation funding and tax relief programs.

Genealogists and social historians delve into Sanborn maps to:

• Locate Ancestors’ Residences: Pinpoint exact addresses and understand the surrounding neighborhood context.
• Study Community Evolution: Analyze how neighborhoods changed over time, including shifts in demographics, infrastructure, and land use.

Urban planners and environmental researchers utilize these maps to:

• Assess Historical Land Use: Identify previous industrial sites, gas stations, or dry cleaners that might have contributed to soil contamination.
• Plan Redevelopment Projects: Understand past urban layouts to inform future development that respects historical contexts.

Decoding Sanborn Maps

Understanding Sanborn maps requires familiarity with their unique symbols and color codes some of which are completely clear and others that really can leave you scratching your head. Here is a detailed list of the keys, symbols and markings common on Sanborn Fire Maps.

These symbols changed over the decades occasionally so creating a comprehensive list is almost impossible over their 100+ year history, but this list should cover almost everything you’ll encounter on these amazing old maps.

Color Codes (Building Construction Types)

• Yellow – Wood-frame (Frame, F)
• Pink / Red – Brick (B)
• Blue – Stone (S)
• Gray – Iron or steel (IR or ST)
• Brown – Adobe
• Green – Fireproof (e.g., concrete block, asbestos cement)
• Black / Uncolored – Mixed or undefined construction

Building Use Abbreviations

• D – Dwelling
• F – Flat or apartment
• S – Store
• H – Hotel
• O – Office
• CH – Club House
• G – Garage
• Mfg – Manufacturing
• W.H. – Warehouse
• Sal. – Saloon
• Bak. – Bakery
• Stab. – Stable
• Sch. – School
• Ch. – Church
• Res. – Restaurant
• A – Auto repair
• L – Laundry
• T – Theater
• Bnk. – Bank

Structural Features

• X through building – Stable or barn
• Dashed lines – Property boundaries or fences
• Solid black lines – Building walls
• Thin lines – Porches, open balconies
• STA – Stairs
• ELE or Elev. – Elevator shaft
• D (within a building) – Dumbwaiter
• Open space inside walls – Courtyard or atrium
• Skylight – Marked as “Skylt”
• Shaft – Air or freight shaft
• Stairs outside – Fire escapes
• Hatched areas – Roof outlines

Roof Material Types

(Sometimes noted with abbreviations or marked next to the building footprint.)

• Tin (TIN) – Common in industrial or commercial buildings
• Slate (SLT) – Fire-resistant and common on higher-end buildings
• Metal (MTL) – Similar to tin, often corrugated
• Shingle (SH or SHNG) – Typically wood shingle roofs
• Tile (TILE) – Clay or concrete tile roofing
• Composition (COMP) – Asphalt or felt roofing
• Gravel (GRAV or GRVL) – Gravel-coated flat roofs
• Tar & Gravel (T&G) – Common on flat commercial roofs
• Asbestos (ASB) – Found post-1920s on fire-resistant structures
• Rubber (RUB) – Rare but may appear on late 20th-century maps

Roof Form Abbreviations

• G.R. – Gable Roof
• F.R. – Flat Roof
• H.R. – Hip Roof
• M.R. – Mansard Roof
• S.R. – Shed Roof

Fire Protection & Hazards

• FP – Fireproof construction
• SPKR – Sprinkler system
• Stand. P – Standpipe
• Hose – Hose cart or rack
• No Sprinklers – Often noted for risk
• Comb. – Combustible materials
• OIL / GAS / TANK – Presence of fuel sources

Floors & Elevations

• 1 STY, 2 STY, 3 STY – Number of stories
• 1½ STY – Story-and-a-half
• Bas. – Basement
• Sub-Bas. – Sub-basement
• Mezz. – Mezzanine level
• U.B. – Unfinished basement

Property Notes

• Vac. – Vacant
• Condemned – Unsafe structure
• Under Const. – Construction in progress
• To Be Rzd. – To be razed (demolished)