A three-storey apartment building is often called a triple-decker or three-decker in the USA. These buildings are typically of light-framed, wood construction, where each floor usually consists of a single apartment, although two apartments per floor is not uncommon.
During the late 19th and early 20th centuries, tens of thousands of triple-deckers were constructed, mostly in the New England region of the United States, as an economical means of housing the thousands of newly arrived immigrant workers who filled the factories of the area. The economics of the triple-decker are simple: the cost of the land, basement and roof are spread among three or six apartments, which typically have identical floor plans. The triple-decker apartment house was seen as an alternative to the row-housing built in other Northeastern cities of United States during this period, such as in New York, Philadelphia, Baltimore and Washington, D.C.
Three-deckers were most commonly built in the emerging industrial cities of the New England region of the United States between 1870 and 1920. There are large concentrations in Massachusetts and Rhode Island. They can also be found in the former industrial cities of New Hampshire, Maine, and Connecticut, as well as the New York City area (particularly in northern New Jersey). They were primarily housing for the working-class and middle-class families, often in multiple rows on narrow lots in the areas surrounding the factories. They were regarded as more livable than their brick and stone tenement and row house counterparts in other Northeastern cities, as they allowed for airflow and light on all four sides of each building.
It is estimated that by 1920, the city of Boston had over 15,000 triple decker houses. Areas such as Dorchester, Roxbury and Jamaica Plain were popular with the emerging middle class, and became "streetcar suburbs" as transportation systems expanded from the older, core sections of the city. Typically, the affordable triple decker homes attracted live-in landlords who would collect rent from the other two apartments.
In Worcester, Massachusetts sewer connection charges were based on street frontage, so builders favored houses with as little frontage as possible, This is one reason why three-deckers are often situated on narrow lots and are in rectangular shape, with the smaller sides at the front and the rear.
In the textile mill city of Fall River, Massachusetts, thousands of wood-framed multi-family tenements were built by the mill owners during the boom years of the 1870s to house their workers. Many more were built by private individuals who rented their apartments to the mill workers and their families. This style of housing differed greatly from the well-spaced boardinghouses of the early 19th century built in Lowell and Lawrence, Massachusetts or the cottages of Rhode Island.
The triple-decker style apartment house is also prolific in urban, working class neighborhoods in northern New Jersey (particularly in and around Newark, Jersey City and Paterson). They are sometimes locally referred to as "Bayonne Boxes".
Similar brick apartment buildings were built in Chicago in the 1910s and 1920s. They are locally referred to as "Three Flats".
Triple-deckers are usually defined by the style of their roofs, being either gable, hip or flat-roofed. While typically lacking the ornamentation found on other homes of the Victorian period, they sometimes were built with certain decorative details, such as porch railings and posts. A typical feature of the triple-decker is a vertical three-sided column of bay windows, usually facing the street side of the house.
It is not uncommon for triple-deckers to feature two apartments per floor, with the units sharing a common wall (with each unit enjoying three sides of external light and air flow).
Each apartment typically has a front and/or back porch for each apartment, and, because the buildings are usually freestanding, windows on all four sides. Some triple deckers feature a single front door, which accesses all three units; others feature two entrances (one accessing the first floor unit externally, with the other leading to a stairwell to units two and three).
Triple-deckers were built in huge numbers, in some areas, comprising entire neighborhoods, but by the 1950s, a number of them had been abandoned or razed, due to suburban growth and urban renewal. Starting in the early 1980s, however, they became desirable again as older streetcar suburbs began to gentrify, often by buyers looking for homes where they could live in one unit and rent the other two, thus helping them pay their mortgage. As condominiums became more common, many were converted into individually owned units.
Recently, a new wave of triple decker apartment houses has been built in areas of Boston, as an alternative to the townhouse style condominium or apartment buildings more typically associated with suburban areas. Boston's zoning regulations allow new three-family houses to be constructed in areas with existing triple-deckers. However, building codes for the new buildings are far more stringent today, with requirements for fire sprinkler systems and handicap access.