A gutter system that collects every drop from the roof and then puts it against the foundation has not solved a problem, it has relocated one. The half of the drainage system below the downspout elbow is the half nobody inspects, and in Seattle it is where roof water turns into a basement problem.
An inch of rain on 1,000 square feet of roof is roughly 600 gallons. A Seattle wet season delivers that repeatedly across eight months.
All of it arrives at a small number of discharge points. If those points are at the base of your foundation wall, that is the volume being introduced there, over and over, into soil that in much of this region drains slowly.
Wet foundation soil produces the familiar consequences: hydrostatic pressure against basement walls, damp or seeping basements and crawlspaces, efflorescence on concrete, and over long periods differential settlement as soil moisture content varies around the building.
Water needs to leave meaningfully away from the structure. Several feet is a working minimum, and more is better on slow-draining clay soils.
A splash block sitting at the base of a downspout is better than nothing — it prevents erosion and spreads the impact — but it does not move water anywhere. It deposits the same 600 gallons within a foot of the wall. Splash blocks are widely treated as a drainage solution and they are not one.
Downspout extensions, whether rigid or hinged, are the simple fix and they work provided they are actually in place. The common failure is an extension removed for mowing and never replaced.
The ground should fall away from the house in every direction. A great many Seattle homes sit on sloped lots where that is impossible on at least one side, because one side of the house is uphill.
On those lots, a downspout discharging on the uphill side is sending its water toward the foundation no matter how long the extension is — gravity returns it. Downspout placement on a sloped lot is a design decision rather than a default, and where water cannot be discharged downhill by surface flow, it needs to be carried there.
Worth checking too: over years, landscaping, mulch top-ups, and settlement can reverse a grade that was originally correct. A bed built up against the house is a common and avoidable cause of foundation moisture.
Buried drain lines or tightlines. Solid pipe connected to the downspout, running underground to a discharge point well away from the structure. Effective and tidy. The critical detail is that it must be maintainable — buried lines clog with sediment and debris over years, and a blocked tightline can back water up against the foundation invisibly, which is worse than no line at all. Cleanouts at accessible points are what make the difference between an asset and a hidden liability.
Dispersion into vegetated areas. Directing flow onto a suitably sized planted area where it can infiltrate. Works where soil and space allow, and it is generally favoured from a stormwater management perspective.
Infiltration systems. Drywells or similar, sized for the volume and appropriate to the soil. Requires soil that actually accepts water — in areas with heavy clay or high groundwater, infiltration may not be viable.
Rain gardens. A designed depression with appropriate soil and planting that receives and infiltrates roof runoff. Genuinely effective and increasingly common in this region.
Seattle regulates stormwater, and how roof runoff is managed on a property can be subject to code — particularly for new construction, significant remodels, and anywhere a connection to the public drainage system is involved.
Two practical cautions. First, requirements vary by project type and location and they change, so drainage work beyond simple extensions should be checked against current city requirements rather than assumed. Second, and more specifically: roof drains should never be improvised into a side sewer. Sanitary sewer connections are regulated for good reason, and an unpermitted connection is both a code problem and a liability at sale.
Where a property already has an existing storm drainage connection, confirming that it is intact and unblocked is worth doing — older connections fail silently.
During the next sustained rain, put on a coat and walk the perimeter. It takes ten minutes and it is far more informative than any inspection done in dry weather.
Where is each downspout actually discharging? Is water running away from the house or pooling within a few feet of the wall? Are extensions in place and connected? Is any downspout discharging onto a paved surface that directs the water back toward the foundation? Is water backing up out of a buried line, which indicates a blockage? Are there low spots along the foundation holding standing water?
Then go into the basement or crawlspace during the same storm and look at the corresponding interior locations. The correlation between an exterior discharge point and an interior damp spot is usually unmistakable, and it is the cheapest diagnosis available in home maintenance.
This post is part of our in-depth coverage of moss removal & gutter care in Seattle.
Gutters, Downspouts & Drainage → Moss Removal & Gutter Care →
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