This connects directly to our gutter cleaning service — see our full Portland gutter cleaning guide for complete pricing, scope, and scheduling.

That Brown Streaking Below Your Portland Windows? Your Gutters Caused It.

Most Portland homeowners who notice brown streaking below window frames assume the window cleaner didn’t do a thorough job. In nearly every case I investigate, the real cause is overflowing gutters directly above those windows — depositing tannin-rich water across the glass and frames repeatedly through the wet season. The window clean was fine. The gutters re-stained the glass within the first autumn rain event.

Gutter overflow and window staining happen months apart, which is why the connection is so rarely made. Gutters fill in October. They overflow through winter. The staining accumulates on glass gradually. By spring when it’s finally visible in direct sunlight, nobody is thinking about the gutters that caused it.

The Tannin Staining Cycle — Exactly How It Works

• Autumn — leaves, fir needles, and moss fill gutters and begin decomposing in contact with rainwater. Decomposition releases tannins — the same compounds that stain tea cups and red wine glasses.
• Winter rain events — clogged gutters overflow. Tannin-rich water goes over the gutter edge and down the exterior wall, directly across window glass and frames below.
• Repeated cycles — each rain event deposits another layer of dissolved tannin onto glass and frames. The film is invisible when wet; it becomes progressively more visible as layers accumulate.
• Spring discovery — Portland homeowners see the brown streaking for the first time in strong spring sunlight. By this point, 3–5 months of overflow events have deposited multiple tannin layers that require professional treatment to remove.

Why Standard Window Cleaning Doesn’t Remove Tannin Staining

Tannin staining bonds chemically to glass — the same way tannin stains a coffee mug. Standard window washing (soap or deionized water and a squeegee) removes surface contamination but does not break the tannin-to-glass chemical bond. Removing established tannin staining requires a mild acid treatment compound applied with appropriate dwell time before the final window wash.

Cleaning windows without fixing the gutters means tannin staining returns with the first autumn rain event. The fix must address both the staining already on the glass and the overflow source creating it.

The Correct Fix — In the Right Order

• 1. Clean gutters first — removing the source of tannin-rich overflow is the essential first step. Any other sequence returns the problem within one rainy season.
• 2. Apply tannin treatment to affected glass and frames — professional compound dissolves the tannin-to-glass bond. Most tannin staining 1–2 seasons old responds fully to a single treatment pass.
• 3. Full deionized water window wash — removes treatment residue and leaves glass clear.
• 4. Schedule annual autumn gutter cleaning — prevents the cycle from repeating. October–November gutter cleaning before the rainy season is the single most effective upstream prevention for window tannin staining.