The need for secure footing between floor levels is the foundation of building code requirements for stairs. New Hampshire includes building code elements related to stairs and stairways in its state building code standards for both commercial and residential construction. These standards must be complied with during new construction as well as in remodeling projects.
State Building Code for Stairs
The codes regarding construction of stairs in New Hampshire is found in its state building code, which is based on the International Building Code of 2009. The International Residential Code (IRC) of 2009, contained in the overall state building code, relates to stairs and stairways in the home. New Hampshire adopts and amends the IRC, which itself is reviewed every three years, to satisfy the state's requirements for construction design and safety.
IRC Stair Code Basics
The current New Hampshire stair code contains updated elements of the 2009 IRC. Quoting their codes, the IRC defines a stair as "a change in elevation consisting of one or more risers." A stairway is defined as "one or more flights of stairs, either interior or exterior, with the necessary landings and platforms connecting them to form a continuous and uninterrupted passage from one level to another within or attached to a building, porch or deck." Stairs must have a minimum of 36 inches in width between handrails or between a wall and a handrail. Stair tread and riser measurements in the 2009 IRC are taken from the surface of the stairs, beneath any stair coverings, such as carpeting.
IRC Stair Code Measurements
Tread and riser requirements applicable in New Hampshire, based on the 2009 IRC, are consistent with previous versions of the IRC building code. IRC R318.104.22.168 requires a maximum of 8 1/4 inches of riser height, with no more than 3/8 inch difference between the greatest and least riser height in each stairway. R322.214.171.124 requires a minimum of 9 inches of tread depth, with the same 3/8 inch allowed variation between tread depths in a stairway.
Winding and Spiral Stairs
The New Hampshire State stair code also allows for stairways other than those that run in a straight vertical line from one level to another. Curved stairways, referred to as winder stairs, and spiral stairways take into consideration the usable walking surface of the treads, which narrow by design near the handrail. This surface, known as the walkline, is measured from the point at which a stair begins to be less than the required minimum depth of 9 inches and must provide at least 12 inches of walkline from that point to the tread edge opposite the narrowest point. Spiral stairs are allowed smaller tread dimensions in the 2009 IRC, consisting of a minimum of 7 3/4 inches of depth within the walkline.