Kai Mallari's Notes'

Driveway Pitched Toward the Garage Floods the Slab and Foundation (Home Systems)

3 min read

Driveway Pitched Toward the Garage Floods the Slab and Foundation

idea

Claim: A driveway or apron that slopes toward the garage (rather than away to the street or a catch basin) is not a cosmetic defect — it is a structural drainage failure that drives water against the garage slab, under the slab edge, and toward the foundation wall. The BC Building Code (Section 9.14) requires finished grade to slope away from any structure at not less than 2% over the first 3 metres.

Mechanism

The driveway is part of the site drainage system. Every square metre of driveway surface is impervious — rain that falls on it cannot soak into the ground. The slope of the driveway determines where that water goes.

When the driveway pitches toward the garage:

  • Rainwater runs against the bottom of the garage door and through any gap
  • Water pools at the slab edge, where the concrete-to-asphalt junction creates a natural low point
  • Over months and years, water works under the slab edge through capillary action and freeze-thaw action
  • The slab edge erodes, the soil under it saturates, and the slab settles or cracks
  • The foundation wall is exposed to constant wetting at the base, accelerating efflorescence, moisture intrusion, and in severe cases, foundation wall degradation

The BC Building Code minimum slope away from any structure is 2% — that is 6 cm of fall over the first 3 metres from the building.1 This applies to all grades adjacent to the foundation, including driveways.

Conditions

  • Worst in winter and spring when rainfall is sustained and the ground is saturated — the driveway has nowhere to shed water
  • Exacerbated by apron settlement (the section immediately adjacent to the garage door often settles independently of the driveway, creating a bowl that catches water even if the main driveway is correctly sloped)
  • Frost heave can also shift a correctly-sloped driveway over time, gradually redirecting water toward the structure

Scope

This failure mode applies to the driveway-to-garage interface. It is distinct from:

The correction requires a contractor — correcting drainage direction means re-grading the driveway apron or installing a trench drain at the garage threshold. This is not a DIY project.

Sources

Idea Compass

North: Where this comes from

  • driveway-walkway (Home Systems) — the parent component note where this failure mode is the load-bearing concern
  • BC Building Code Section 9.14 — the drainage requirement this driveway orientation violates

East: Tensions / failure

  • foundation-drainage-waterproofing (Home Systems) — what the misdirected water does to the foundation over time
  • Settlement of the driveway apron — frost heave or base failure can redirect a correctly-graded driveway over time, meaning this failure can develop even in a home that started correctly graded

South: Where this leads

West: What’s similar

  • Window well drainage failure — same pattern: an impervious surface that should shed water away from the structure instead collects it against it
  • Negative grade around a foundation (soil that slopes toward the house) — the same BC Building Code 2% rule applies there too

Footnotes

  1. BC Housing, Builder’s Guide to Site and Foundation Drainage — BC Building Code s. 9.14.6; ground must drain away from structure at minimum 2% slope — https://www.bchousing.org/publications/Builder-Guide-to-Site-and-Foundation-Drainage.pdf ↩

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