Exterior Foundation Waterproofing in Seattle, WA

What Washington Code and Seattle Rules Ask

Washington builds houses to the 2021 International Residential Code as the state has amended it, and that code sets the grading, drainage and waterproofing rules for a new foundation. Whether a repair or alteration to an existing foundation triggers any of them is decided by the building department of the city the house is in. Inside Seattle, four further sets of rules bear directly on exterior work.

• Where the footing drain goes. Seattle’s Stormwater Code allows approved footing drains and foundation drains to discharge to the drainage system. The city’s side sewer rule says water from subsurface drains passes through a catch basin before it reaches a public system, never goes to a sanitary sewer unless specifically allowed, and never discharges onto a public sidewalk. Seattle Public Utilities counts the solid pipe from a perforated footing drain to the drainage system as side sewer, which needs its permit, and says footing drains cannot be daylighted to the right of way.
• Peat. Seattle designates peat settlement-prone areas from the city’s mapped bogs, and SDCI warns that peat-rich soils settle when the groundwater table is lowered — which is what a footing drain is designed to do around a foundation. Check the parcel on SDCI’s critical areas map before a drain is designed.
• Slopes. Seattle’s landslide-prone areas include areas with springs or groundwater seepage and slopes of 40 percent or more over at least 10 feet of rise, and the city identifies 8.4% of its surface as slide prone. Its stormwater manual says dispersing water on the surface is not typically permitted in landslide-prone areas, which limits where a drain can let water out on a hillside lot.
• The building permit. Seattle’s residential code requires a permit to construct, alter or repair a building, with exemptions for listed minor work. Ask the bid which permits it includes.

Most of SDCI’s critical areas maps are advisory, and a property can contain a critical area the map does not show. Every other city in the area has its own stormwater and critical areas rules, which this site has not read; the building department that covers the address is the one to ask.

The Exterior Assembly, Layer by Layer

Read a bid against this sequence. A step missing from the written scope is usually a step missing from the job.

1. Excavation to the footing, along every wall in the scope, with shoring where depth or soil calls for it.
2. Wall preparation: cleaning, patching voids and repairing cracks, because a membrane over a dirty or broken surface fails at the defect.
3. The membrane, named by product and method, and carried from finished grade down to the footing.
4. A drainage board over the membrane, giving water against the wall a free path down instead of pressing on the membrane.
5. A footing drain: perforated pipe in gravel, wrapped in filter fabric so silt does not clog it, sloped to an approved discharge point.
6. Inspection before backfill, while every layer can still be seen.
7. Backfill and grade: free-draining material against the wall, native soil above it, and a finished grade that falls away from the house.

This site’s modelled range for the full sequence is $15,825 to $40,050 before sales tax. What it includes and leaves out is on the cost guide, alongside every other crawl space and foundation service.

Before Anyone Digs: Utilities, Permits and Engineering

• Utility locate. Washington requires an excavator to mark the area and notify facility operators through the one-number locator service at least two full business days, and not more than ten, before digging. The exemption covers only an owner or occupier digging less than 12 inches deep on private non-commercial property, so a trench to the footing is not exempt, whoever holds the shovel.
• Permits. Inside the City of Seattle, the Seattle Department of Construction and Inspections issues the building permit and Seattle Public Utilities the side sewer permit for a new drainage connection. Elsewhere in the area, ask the city or county that covers the address. Get the permit status on the written bid.
• Engineering. Where an excavation or a repair falls outside the code’s prescriptive rules — deep cuts, shoring, a hillside, a critical area — the building official decides whether engineered drawings are needed.
• Registration. Anyone contracting for the work in Washington must be registered with L&I, bonded and insured. Checking a registration takes a couple of minutes.