How to Identify Rot in Wood Windows (Before It’s Too Late)

Written By Jon Hussey

Rot is like termites with a college degree—it’s sneaky, destructive, and knows exactly where to hide. And when it comes to historic wood windows, a little rot left untreated can spiral into a full-blown restoration headache. The good news? Most rot gives away clues long before it eats your sash alive. In this post, we’ll show you how to spot wood rot early, how to test for it, and what to do when you find it (hint: don’t panic).

What Is Wood Rot?

Wood rot is the breakdown of wood fibers caused by fungi, typically in areas with prolonged moisture exposure. Historic windows—especially those painted shut or poorly maintained—are prime real estate for rot colonies.

The Two Main Culprits:

  • Brown Rot – Common in windows, makes wood crumbly and dark

  • White Rot – Slower, but leaves wood spongy and pale

Both thrive in warm, damp conditions—which is why windowsills, bottom rails, and meeting rails are often the first places we check.

Signs of Rot You Can See (and Smell)

Rot doesn’t always shout. Sometimes, it whispers. Here’s what to look (and sniff) for:

  • Peeling paint or bubbles near joints

  • Discoloration—often darker or grayer than surrounding wood

  • Mushy or soft spots when gently prodded with a screwdriver

  • Musty odor—especially after a rainstorm

  • Visible fungal growth (e.g., mildew or dark streaks)

  • Separated joinery (mortise and tenon gaps may indicate internal failure)

Pro tip: Use a skewer or ice pick to test wood in suspect areas. If it sinks easily, rot is present.

Common Rot Zones in Historic Windows

  • Sash bottom rails (especially meeting the sill)

  • Windowsills and sub-sills

  • Jamb bottoms

  • Glazing putty edges (moisture seeps behind and settles into rail corners)

  • Muntin intersections

What to Do When You Find Rot

For Minor Surface Rot:

  • Dry it out thoroughly (use fans or sunlight)

  • Treat it with borate preservative (like BoraCare)

  • Consolidate soft fibers using an epoxy consolidant

  • Rebuild missing sections with an epoxy wood filler (e.g., Abatron or System Three)

  • Prime, glaze, and paint like new

For Structural Rot:

  • Remove and replicate damaged components

  • Rebuild sashes using Accoya or Red Grandis for unmatched durability

  • Use traditional mortise and tenon joinery with glued and pegged construction

  • Apply borate and epoxy treatments during reassembly to prevent future decay

At Austin Historical, we only replace parts beyond reasonable repair—and always with historically appropriate profiles and species.

Prevention Is Better Than Rebuilds

To keep rot from coming back:

  • Paint regularly (especially exposed ends and drip edges)

  • Use breathable paints (like linseed oil-based or mineral paints)

  • Keep gutters and flashing in working order

  • Ensure water can escape from sill slopes and storm windows

  • Install spring bronze or wool pile weatherstripping to minimize moisture entry

Tools We Use in the Field

  • Moisture meter (reads wood moisture content)

  • Lead-safe HEPA vac and PPE

  • 5-in-1 painter’s tool for putty and probing

  • Borate treatments like BoraCare

  • Epoxy products from Abatron or Smith & Co.

  • Hand planes, saws, and routers for replacement work

Final Word

Rot doesn’t have to be a death sentence for your historic windows. In fact, catching it early is part of responsible stewardship. With regular inspections, a bit of detective work, and the right restoration mindset, your windows can stay strong and beautiful for another hundred years.

Jon Hussey
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Weatherstripping That Works – Modern Comfort, Historic Integrity