Updated 2026-09-07

Building Code Glossary

Subsoil Drain: Building Code Definition

How the IRC 2009, IPC 2009 define subsoil drain, where the wording differs, and which chapters depend on the term.

On this page

Last updated September 7, 2026 · 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.

Add to Harth agentView as markdown

Direct answer

Subsoil drain is a defined term in the International Residential Code (IRC) 2009, in Definitions. The code's own words are: “A drain that collects subsurface water or seepage water and conveys such water to a place of disposal.” The IPC 2009 carries the same definition word for word, so the term means the same thing in each. The term is used in 5 chapters of this corpus, most often in Storm Drainage (IRC 2009), Storm Drainage (IPC 2009), Soils and Foundations (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, Harth founder and architectural technology specialist · Published 2026-09-07 · Updated 2026-09-07.

What does this term mean?

Subsoil Drain — IRC 2009, Definitions:

A drain that collects subsurface water or seepage water and conveys such water to a place of disposal.

Labeled code-term definition diagram: Subsoil Drain: Building Code Definition — How the IRC 2009, IPC 2009 define subsoil drain, where the wording differs, and which chapters depend on the term.

What must be verified before using this guidance?

Two of the I-Codes in this corpus define Subsoil Drain, in identical words.

Code	Where it is defined	Definition
IRC 2009	Definitions	A drain that collects subsurface water or seepage water and conveys such water to a place of disposal.
IPC 2009	Definitions	A drain that collects subsurface water or seepage water and conveys such water to a place of disposal.

Where subsoil drain is used in the code

The term appears in 5 chapters of the 2009 I-Codes hosted here. The chapters that lean on it hardest are the ones to read next.

Chapter	Code	Mentions
Storm Drainage	IRC 2009	10
Storm Drainage	IPC 2009	7
Soils and Foundations	IBC 2009	2
Definitions	IPC 2009	1
Definitions	IRC 2009	1

The term in use

How the code actually writes about subsoil drain, quoted from the 2009 text hosted here:

Where the building is subject to backwater, the subsoil drain shall be protected by an accessibly located backwater valve.

— IRC 2009 §P3302.1

The terms subsoil drain is built out of

The definition above leans on 1 other defined term. Each carries its own code definition, and a local amendment to any of them changes what subsoil drain covers:

Drain — IRC 2009: Any pipe that carries soil and water-borne wastes in a building drainage system.

Subsoil Drain: what to open, and in what order

Every row is a document this term depends on. The ICC code library and the ICC adoption maps cover the middle rows.

Check	Where	Confirmed
Read the definition in the adopted IRC	Definitions
Read the definition in the adopted IPC	Definitions
Re-check the requirement that uses it	Storm Drainage (IRC 2009)
Re-check the requirement that uses it	Storm Drainage (IPC 2009)
Re-check the requirement that uses it	Soils and Foundations (IBC 2009)
Re-check the requirement that uses it	Definitions (IPC 2009)
Confirm the dependent definition	Drain
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.

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 publishes the current model text and the ICC adoption maps show which edition a jurisdiction has adopted; those sources control.

Source links

Frequently asked questions

What is the code definition of subsoil drain?

The International Residential Code (IRC) 2009 defines it in Definitions as: “A drain that collects subsurface water or seepage water and conveys such water to a place of disposal.” 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 subsoil drain?

In this corpus the term is used across 5 chapters, led by Storm Drainage 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 the code for drain slope?

Slope is a separate defined term. The IRC 2009 defines it as: “The fall (pitch) of a line of pipe in reference to a horizontal plane. In drainage, the slope is expressed as the fall in units vertical per units horizontal (percent) for a length of pipe.” That is not the same as subsoil drain, which the IRC 2009 defines as “A drain that collects subsurface water or seepage water and conveys such water to a place of disposal.” — read both before applying either, and confirm the wording in the edition your jurisdiction has adopted.

Related pages

The AI-powered workspacefor architects.

Get started for free Continue in Harth