Building Diagram Number

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Building Diagram Numbers are used to classify a building’s foundation type on Elevation Certificates.

Diagram 1A: Slab-on-Grade

• Key Features:
  • Bottom floor directly on the ground, no additional fill used.
  • Often seen in non-coastal, flat areas.
• Example Uses: Standard residential construction in low-risk areas.

Diagram 1B: Slab-on-Stemwall

• Key Features:
  • Bottom floor elevated with fill but at ground level on one side.
  • Used to reduce flood exposure.
• Example Uses: Homes in areas with minor flood risks.

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.

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