Basement / cellar drainage

Short definition

Basement drainage is the family of systems that keep water out of below-grade space and move it out once it gets in. In wet Puget Sound homes, that usually means an exterior footing (perimeter) drain at the foundation, a sump pump and pit, an interior floor drain, and — in homes connected to a combined sewer — a backwater valve. Older WA basements often have only some of these, which is why they leak.

What it is

Three layered components work together:

1. Exterior perimeter drain (footing drain). Perforated pipe in gravel along the foundation footing, surrounded by filter fabric. It collects groundwater before it reaches the foundation wall and either daylights to a lower spot on the lot or feeds a sump.
2. Interior drainage. A floor drain (usually trapped) and sometimes a French drain along the perimeter inside, both feeding the same sump.
3. Sump pump system. A sump pit collects water from the perimeter and interior drains; the pump moves it up and out to a discharge line that ends well away from the foundation.

When a basement floor sits below the elevation of the public sewer, gravity drainage is impossible and a sewage ejector pump becomes necessary for any sewage fixture (toilet, shower, bathroom sink) installed below the sewer line.

Why it matters to a homeowner

A flooded basement is one of the more expensive water events a homeowner will deal with — drywall replacement, flooring, possessions, mold remediation. Most WA basement floods come from one of three causes:

• Footing drain clogged with iron-ochre or sediment after 20+ years; wet basement returns gradually.
• Sump pump fails during the storm that tested it — pump life is roughly 7–10 years.
• Backwater event on a CSO main pushes sewage up through the basement floor drain.
• Cracked basement wall lets water in directly through wall + floor joint.
• Downspouts dumping at the foundation rather than away — saturates the footing drain.

WA basement drainage standards have improved over time:

• Pre-1970 Seattle/Tacoma craftsman/bungalow basements — poured concrete walls, sometimes drainage tile around the footing, often a sump in a corner pit. Older systems often had no perimeter drain at all.
• 1970s–90s WA basements — footing drain + sump pump + interior floor drain.
• Post-2000 WA basements — full waterproofing membrane on the exterior, interior + exterior drainage tiles, dual-pump sumps.
• Daylight basements — walk out at one end; the upper end is fully buried with drainage tile.

When you’ll encounter this term

• “My basement floods every couple of years” — usually a footing-drain or sump-pump issue, sometimes CSO surcharge in older Seattle neighborhoods.
• Pre-purchase inspection in Seattle/Tacoma: ask whether the basement is dry in winter and what the sump-pump runtime is during a typical winter month.
• Re-doing a basement: PNW homeowners commonly add a perimeter interior French drain and dual-pump sump to a 1970s-era system.

Washington note

Seattle, Tacoma, Bellingham, Olympia, and Spokane all combine high water tables and heavy winter precipitation, making basement drainage a perennial concern. Puget Sound winter rainfall averages 35–40 inches a year; Spokane gets less rain but adds snowmelt. RainWise rebates (Seattle/SPU + King County) help homeowners disconnect downspouts from the combined sewer, which also reduces footing-drain load.

King County requires perimeter drainage tile and waterproofing on new below-grade construction.