How can we increase rentable square footage and building efficiency by eliminating redundancy in standard building egress?
On several of our feasibility studies for multifamily housing, we have employed an interlocking stair system to increase rentable square footage. Typically, two separate egress stairs are constructed in a multifamily building. By simplifying this and constructing two stairs in the same footprint, we can increase the rentable or sellable square footage and increase the building’s efficiency. In Massachusetts, the building code allows an Interlocking stair to act as two separate means of egress as long the building is not a high rise and the fire rating is maintained between the two stairs. The entry doors to the stair must also be separated by 1/3 the overall diagonal of the building, assuming the building is protected with automatic sprinklers (2015 IBC: 1107.1.1)
This system is often used in New York City on very tight infill sites for high rise construction. New York City building code is more generous in allowing the use of an interlocking stair. They are permitted in high rise construction of R-2 occupancies as long as the shaft and separation are constructed of 2 hour rated masonry, and the entry doors are separated by 15’ minimum (NYCBC 1015.2.1).
In terms of cost, from our experience in the Boston area a conventional stair costs approximately $400-500/ tread, while an interlocking stair costs approximately $350-$450/ tread. Landings for both systems cost approximately $1,000 dollars, and for an interlocking stair system, there are less landings. There is some extra cost in the interlocking stair for a 2 hour rated shaft wall separating the stairs. However, if steel and concrete construction is used and the landings are cantilevered, this could help to reduce the cost of a custom stair landing. We believe the cost of two typical switchback stairs versus one interlocking stair would be closely priced depending on the construction methods. Therefore, for a very similar construction cost, we can gain valuable square footage that can be rented or sold, increasing revenue and profit.