Building Diagram Number: Difference between revisions

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


==== 1. Purpose of Building Diagram Numbers ====
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.


* '''Foundation Identification''': Used to classify a building’s foundation type for documentation and floodplain management purposes.
{| class="wikitable"
* '''Historical Relevance''': Previously essential for flood insurance rating but still vital for understanding elevation and flood risk.
|+
!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.
|[[File:Img-nfip-bdn-1a.png|240px|link=]]
|-
|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.
|[[File:Img-nfip-bdn-1b.png|240px|link=]]
|-
|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.
|[[File:Img-nfip-bdn-2a.png|240px|link=]]
|-
|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.
|[[File:Img-nfip-bdn-2b.png|240px|link=]]
|-
|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.
|[[File:Img-nfip-bdn-3.png|240px|link=]]
|-
|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.
|[[File:Img-nfip-bdn-4.png|240px|link=]]
|-
|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.
|[[File:Img-nfip-bdn-5.png|240px|link=]]
|-
|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.
|[[File:Img-nfip-bdn-6.png|240px|link=]]
|-
|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. [[NFIP Foundations#Walkout Basements|Walk-out basements qualify as Diagram Number 7]].
|[[File:Img-nfip-bdn-7.png|240px|link=]]
|-
|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.
|[[File:Img-nfip-bdn-8.png|240px|link=]]
|-
|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).
|[[File:Img-nfip-bdn-9.png|240px|link=]]
|}


----
== Measurement Rules ==


==== 2. Building Diagram Numbers and Descriptions ====
=== 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.


===== Diagram 1A: Slab-on-Grade Without Fill =====
{{box-summ
| title = Example
| color = yellow
| text = 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.
}}


* '''Key Features''':
=== Two Foot/Five Foot Rule ===
** Bottom floor directly on the ground, no additional fill used.
To be considered a subgrade crawlspace, two measurements are required.  
** Often seen in non-coastal, flat areas.
* 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.
* '''Example Uses''': Standard residential construction in low-risk areas.
* Two Foot Rule: the crawlspace floor must not be more than two feet below the Lowest Adjacent Grade on all sides.


===== Diagram 1B: Slab-on-Grade With Fill =====
If the subgrade crawlspace does not satisfy both rules in the Two Foot/Five Foot Rule, then it is considered a basement.


* '''Key Features''':
{{box-summ
** Bottom floor elevated with fill but at ground level on one side.
| title = Example
** Used to reduce flood exposure.
| color = yellow
* '''Example Uses''': Homes in areas with minor flood risks.
| text = 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.
 
}}
===== Diagram 2A: Full Basement =====
 
* '''Key Features''':
** Fully below ground level (subgrade) on all sides.
** High flood exposure due to water intrusion risks.
* '''Example Uses''': Common in older residential structures in flood zones.
 
===== Diagram 3: Split-Level Slab-on-Grade =====
 
* '''Key Features''':
** Multi-level design with no part below grade.
** Lower levels directly on the ground.
* '''Example Uses''': Residential homes with uneven interior levels.
 
===== Diagram 4: Split-Level Basement =====
 
* '''Key Features''':
** One portion of the structure includes a subgrade basement.
** Other levels are at or above ground.
* '''Example Uses''': Homes built on sloped lots.
 
===== Diagram 5: Elevated on Piers or Posts (Open) =====
 
* '''Key Features''':
** Elevated structure on piers, posts, or pilings with no enclosed area underneath.
** Designed for high flood zones or coastal regions.
* '''Example Uses''': Beach houses or properties in velocity flood zones.
 
===== Diagram 6: Elevated on Piers or Posts (Enclosed) =====
 
* '''Key Features''':
** Elevated on piers with a partially or fully enclosed area below.
** Requires proper venting to equalize hydrostatic pressure.
* '''Example Uses''': Homes in coastal or flood-prone inland areas.
 
===== Diagram 7: Elevated with Walkout Basement =====
 
* '''Key Features''':
** Basement partially below grade on some sides, with direct outdoor access via a fully above-grade wall.
** The ground slopes away from the building.
* '''Example Uses''': Structures on hilly terrain.
 
===== Diagram 8: Non-Elevated with Attached Garage =====
 
* '''Key Features''':
** Ground-level structure with a garage attached that could be below the main floor.
** Susceptible to flooding in garage and adjacent spaces.
* '''Example Uses''': Suburban homes with drive-under garages.
 
----
 
==== 3. Why Diagram Numbers 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.




{{nfip}}
{{nfip}}

Latest revision as of 07:16, 10 March 2025

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.

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

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