# Base Flood: Building Code Definition > How the IBC 2009 define base flood, where the wording differs, and which chapters depend on the term. - Canonical: https://harth.build/resources/glossary/base-flood/ - Last updated: 2026-09-07 - Data source: Definitions quoted from the 2009 International Codes as transcribed in this site's building-codes collection, with the source chapter linked for each quotation. Usage counts are computed over the chapters hosted here, one copy per code. The [ICC code library](https://codes.iccsafe.org/codes) publishes the current model text and the [ICC adoption maps](https://www.iccsafe.org/advocacy/code-adoption-maps/) show which edition a jurisdiction has adopted; those sources control. ## Key facts | Fact | Value | | --- | --- | | Defined in | IBC 2009 (2009 editions transcribed in this corpus) | | Primary source | IBC 2009 Structural Design (model text, not a jurisdiction's adopted text) | | Chapters using the term | 4 (counted across the codes hosted here) | | Wording identical across codes | Single code (compare before citing one code's wording under another) | ## Direct answer Base flood is a defined term in the International Building Code (IBC) 2009, in Structural Design. The code's own words are: “The flood having a I-percent chance of being equaled or exceeded in any given year.” No other I-Code in this corpus defines it, so the IBC 2009 definition is the one to work from. The term is used in 4 chapters of this corpus, most often in Building Planning (IRC 2009), Structural Design (IBC 2009), Definitions (IBC 2009). These pages transcribe the 2009 model text; verify the definition against the edition and amendments your authority having jurisdiction has adopted. Record the jurisdiction, the adopted edition, the application date and any local amendment that rewrites the definition before you use it on a live project, because a term redefined by amendment changes every requirement that depends on it. *Content and cited inputs reviewed by [Scott Barrington](https://nz.linkedin.com/in/barrington), Harth founder and architectural technology specialist · Published 2026-09-07 · Updated 2026-09-07.* ## What does this term mean? **Base Flood** — IBC 2009, Structural Design: > The flood having a I-percent chance of being equaled or exceeded in any given year. ![Labeled code-term definition diagram: Base Flood: Building Code Definition](https://efwwel0vyt9vy5so.public.blob.vercel-storage.com/images/glossary/base-flood/diagram-1344.webp) *How the IBC 2009 define base flood, where the wording differs, and which chapters depend on the term.* ## What must be verified before using this guidance? **Base Flood** is defined once in this corpus, in the [Structural Design](https://harth.build/resources/building-codes/us-states/maryland/ibc-2009/structural-design.md) chapter of the International Building Code (IBC) 2009: > The flood having a I-percent chance of being equaled or exceeded in any given year. ## Where base flood is used in the code The term appears in 4 chapters of the 2009 I-Codes hosted here. The chapters that lean on it hardest are the ones to read next. | Chapter | Code | Mentions | | --- | --- | --- | | [Building Planning](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/building-planning.md) | IRC 2009 | 9 | | [Structural Design](https://harth.build/resources/building-codes/us-states/maryland/ibc-2009/structural-design.md) | IBC 2009 | 5 | | [Definitions](https://harth.build/resources/building-codes/us-states/maryland/ibc-2009/definitions.md) | IBC 2009 | 2 | | [Definitions](https://harth.build/resources/building-codes/us-states/new-hampshire/ipc-2009/definitions.md) | IPC 2009 | 1 | ### The term in use How the code actually writes about base flood, quoted from the 2009 text hosted here: > Located at the base flood elevation plus 1 foot (305 mm), or the design flood elevation, whichever is higher, if the lowest horizontal structural member is oriented perpendicular to the direction of wave approach, where perpendicular shall mean greater than 20 degrees (0.35 rad) from the direction of approach. — [IRC 2009, Building Planning](https://harth.build/resources/building-codes/us-states/maryland/irc-2009/building-planning.md) ## Base Flood: what to open, and in what order Every row is a document this term depends on. The [ICC code library](https://codes.iccsafe.org/codes) and the [ICC adoption maps](https://www.iccsafe.org/advocacy/code-adoption-maps/) cover the middle rows. | Check | Where | Confirmed | | --- | --- | --- | | Read the definition in the adopted IBC | Structural Design | | | Re-check the requirement that uses it | Building Planning (IRC 2009) | | | Re-check the requirement that uses it | Structural Design (IBC 2009) | | | Re-check the requirement that uses it | Definitions (IBC 2009) | | | Re-check the requirement that uses it | Definitions (IPC 2009) | | | Local amendment to any of the above | Jurisdiction's amendment list | | **A research aid, not a code determination.** The authority having jurisdiction decides which edition and which amendments apply. ## Where can you explore related guidance? - [Explore all glossary guidance](https://harth.build/resources/glossary.md) - [Design Flood Elevation: Building Code Definition](https://harth.build/resources/glossary/design-flood-elevation.md) - [Loads: Building Code Definition](https://harth.build/resources/glossary/loads.md) - [Seismic Design Category: Building Code Definition](https://harth.build/resources/glossary/seismic-design-category.md) - [Design Flood: Building Code Definition](https://harth.build/resources/glossary/design-flood.md) ## Sources and method Definitions quoted from the 2009 International Codes as transcribed in this site's building-codes collection, with the source chapter linked for each quotation. Usage counts are computed over the chapters hosted here, one copy per code. The [ICC code library](https://codes.iccsafe.org/codes) publishes the current model text and the [ICC adoption maps](https://www.iccsafe.org/advocacy/code-adoption-maps/) show which edition a jurisdiction has adopted; those sources control. ### Source links - [codes.iccsafe.org](https://codes.iccsafe.org/codes) - [iccsafe.org](https://www.iccsafe.org/advocacy/code-adoption-maps/) ## Frequently asked questions ### What is the code definition of base flood? The International Building Code (IBC) 2009 defines it in Structural Design as: “The flood having a I-percent chance of being equaled or exceeded in any given year.” The 2009 model text is reproduced here; the definition that binds your project is the one in the edition your jurisdiction has adopted, as amended locally. ### Which parts of the code depend on base flood? In this corpus the term is used across 4 chapters, led by Building Planning in the IRC 2009. Those are the sections whose scope moves if the definition is amended locally, so read them together with Chapter 2 rather than on their own. ### What is considered a base flood? In the 2009 I-Codes hosted here it is a defined term rather than a description. The IBC 2009 states, in Structural Design: “The flood having a I-percent chance of being equaled or exceeded in any given year.” It is defined in IBC 2009. Because it is a defined term, whatever the adopted edition says it means is what every section using it means — so check Chapter 2 of that edition and any local amendment before relying on this wording. --- - Continue in Harth (open this topic in the Harth agent): https://app.harth.build/agent?context=%2Fresources%2Fglossary%2Fbase-flood%2F - View this page in a browser: https://harth.build/resources/glossary/base-flood/ - All resources for agents: https://harth.build/llms.txt