French Drain Installation in Tacoma, WA
The trench of rock and pipe that gives water trapped on Tacoma's hardpan a way out, and the four versions of it a Pierce County lot might need.
What a French Drain Does on Till Soil
Most of Tacoma sits on glacial till, and till decides how a French drain has to be built here.
A French drain is a trench of drain rock and perforated pipe that collects groundwater and carries it to an outlet. On Tacoma's till, where hardpan 20 to 40 inches down holds winter water near the surface, it is the standard fix for a saturated yard or a wet foundation.
Water takes the easiest path, and open gravel with a perforated pipe is far easier than compacted loam over hardpan, so water near the trench migrates into it and runs along the pipe. The trench has to slope, the pipe has to end somewhere lower, and in Tacoma's fine glacial silt the rock has to be wrapped in filter fabric or the whole thing silts shut in a few winters.
Footing drain
Laid outside the foundation at footing depth, this is the drain most Tacoma houses were meant to have from the start. On the pre-1930 houses of the North End and Stadium District it was clay tile, and a replacement in modern pipe is the usual answer when those tiles fail.
Crawl space drain
Because so much of Tacoma's post-war housing sits on crawl spaces, a shallow perimeter drain inside the crawl space, run to a sump, is one of the most common installations in the county.
Yard drain
A shallower run through a lawn or planting bed that gathers water from a low spot or a seep and carries it away. The answer for the back corner of a till lot in Fircrest or University Place that ponds from November to April.
Curtain drain
Cut across a slope above the house, it intercepts water before it reaches the foundation. On the bluff lots around Old Town, Ruston, Browns Point, and Gig Harbor it often does more than any digging at the house itself.
Where the Water Ends Up in Tacoma
A drain without an outlet is a buried bathtub. The outlet is the first thing a contractor establishes and the largest single factor in cost.
- Daylight. On a lot that falls away, the pipe simply exits the ground somewhere lower. Cheapest and most reliable; common on hillside and bluff lots.
- A dry well. On the outwash gravel of South Tacoma, Parkland, Spanaway, and much of Lakewood, a rock-filled pit will soak water into the ground faster than the rain delivers it. On till it fills and stays full, so it only works where the soil test says so.
- The City storm system. Inside Tacoma city limits, connecting a private drain to the public storm system or doing anything at the curb requires a Stormwater Construction or Repair Permit from Environmental Services. Sending drainage to the sanitary sewer is prohibited under Tacoma Municipal Code 12.08.030.
- A sump. Flat lots in the Puyallup valley and any basement below the street have no downhill, so the drain ends in a basin and a sump pump lifts the water out.
A quote that does not name which of these four the water reaches is an unfinished design.
How the Job Goes
- Find the water and the outlet. Where it comes from, where it pools, how deep the hardpan sits, and where a pipe can end. Utilities get located before anyone digs.
- Trench. Footing depth around a foundation, shallower for yard and crawl space work. Hardpan slows the digging and is one reason Tacoma trenches cost more per foot than sandy ground.
- Line the trench and lay the pipe. Filter fabric lines the trench, clean drain rock goes under and over a perforated pipe set to a real slope, and the fabric folds over the top so silt stays out.
- Connect the outlet. Daylight, dry well, permitted storm tie-in, or sump, with cleanouts so the line can be jetted later.
- Backfill and restore. Compact, then put back lawn, gravel, or hardscape.
A short yard drain on open ground is a day; a full footing drain around a North End basement with alley-only access can be a week. Look at the slope and the outlet before backfill, the one moment you can inspect the part that matters.