Building Diagram Number

Building Diagram Numbers are a field on the Elevation Certificate that is used to classify a building’s foundation type. They are used by surveyors, architects, and engineers to document and communicate a building’s structural setup.

They can be easily misrepresented for a number of reasons and should be seen as an indicator of a foundation type that must be confirmed by the measurements shown on the elevation certificate.

Number	Foundation Type	Key Features	Image
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 Diagram Number 7.
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).

Measurement Rules

Five Foot Rule

To be considered a basement or an enclosure, a measurement between the top of the basement or enclosure floor and the top of the floor above it must be more than five feet. If it is five feet or less, then the basement or enclosure may be considered a crawlspace.

Example

An elevated building is raised off the ground by concrete posts. The entire area underneath the main floor is enclosed by solid walls that extend to the ground, which is flat. The main floor sits 58 inches off the ground. The enclosure would be considered a crawlspace.

Two Foot/Five Foot Rule

To be considered a subgrade crawlspace, two measurements are required.

• Five Foot Rule: (as above) the distance from the crawlspace floor to the top of the next higher floor must be five feet or less.
• Two Foot Rule: the crawlspace floor must not be more than two feet below the Lowest Adjacent Grade on all sides.

If the subgrade crawlspace does not satisfy both rules in the Two Foot/Five Foot Rule, then it is considered a basement.

Example

The elevation certificate for a building shows a Diagram Number 9, a subgrade crawlspace. The crawlspace sits 20 inches below the surrounding grade, which is flat. The main floor of the building sits 62 inches above the crawlspace floor. The foundation would be considered a Diagram Number 2, a basement.

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