Building Diagram Number

This page is a rough draft and will need more work before it's finalized.

Building Diagram Numbers are a field on an x that is used to classify a building’s foundation type on Elevation Certificates.

Why do they still matter?

Foundation Typing: Essential for documenting and communicating a building’s structural setup.
Floodplain Management: Helps identify mitigation needs like elevation, venting, or floodproofing.
Policy Documentation: Useful for underwriters and agents when verifying compliance with NFIP requirements.


Number	Foundation Type	Key Features
1A	Slab on Grade	Bottom floor directly on the ground, no additional fill used. Often seen in flat areas.
1B	Slab on Stemwall OR Slab on Fill	Concrete slab poured over a stem wall foundation, often with earthen fill mounded inside the stem wall.
2A	Basement	The bottom floor is below ground level (grade) on all sides. The basement level measures more than five feet to the floor above it.
2B	Basement with sub-grade egress	The bottom floor is below ground level on all sides, and the area of egress is also below grade on all sides. This also applies to buildings with negative flow.
3	Split-level Slab	The building layout has an intermediate level between an upper and lower floor. The lowest floor sits at or above ground level.
4	Split-level Basement	The building layout has an intermediate level between an upper and lower floor. The lowest floor sits below ground level on all sides.
5	Elevated without Enclosure	The area below the elevated floor is open, with no obstruction to the flow of floodwaters. Open lattice and insect screening is permissible. "Pier and beam" foundations.
6	Elevated on Piers, Posts, or Piles with Enclosure	The elevated floor is raised by piers, posts, or piles. An area below the elevated floor is enclosed, either partially (a portion of the building footprint) or fully (encloses the entire building footprint). The enclosure measures more than five feet to the floor above it.
7	Elevated on Solid Perimeter Walls	The elevated floor is raised by solid perimeter, load-bearing walls, which also enclose the area below the elevated floor. The enclosure measures more than five feet to the floor above it. Walk-out basements qualify as this diagram number (see below).
8	Above-grade Crawlspace	The main floor is raised above ground by any method, so long as the crawlspace (enclosure) floor sits at or above the surrounding grade. The crawlspace measures five feet or less to the floor above it.
9	Subgrade Crawlspace	The floor above the crawlspace is raised above ground by any method, but the crawlspace floor sits below the surrounding grade on all sides, like a basement. Subgrade crawlspaces must conform to the 2-foot/5-foot rule (see below).

This page contains information about the NFIP. Find more NFIP Resources.