Rotten Window Frames: Repair, Patch, or Replace?

Finding rot in a window frame is unsettling. The soft, discolored wood tells you that moisture has been working against your home, possibly for years. The question is: how bad is it, and what is the right response?

The answer depends entirely on severity. Minor surface rot can be repaired for under $50 with epoxy products and an hour of work. Deep structural rot requires removing the window entirely and replacing it. This guide helps you assess exactly where your situation falls and choose the right approach.

How to Assess Rot Severity

Before deciding on a course of action, you need to understand how far the rot has progressed. Appearances can be deceiving: rot that looks minor on the surface may extend deep into the wood, and rot that looks dramatic (peeling paint, surface discoloration) may actually be superficial.

The Screwdriver Test

This is the standard method used by home inspectors, contractors, and insurance adjusters. It takes 30 seconds per window.

How to Do It

1. Get a flat-head screwdriver with a standard-width blade (not a small precision screwdriver).
2. Press the tip firmly into the suspect area of the wood.
3. Apply moderate pressure, about the same force you would use to pry open a paint can lid.

Interpreting the Results

• The screwdriver does not penetrate: The wood is sound. What you are seeing may be surface staining, mildew, or paint failure rather than actual rot. Clean the area, apply a wood preservative, and repaint.
• Penetration of 1/8 inch or less: Very early-stage rot. The surface fibers are softened but the structural wood beneath is intact. This is the ideal stage for epoxy repair.
• Penetration of 1/4 inch: Moderate rot. The decay has moved past the surface and into the body of the wood. Epoxy repair is still possible but becomes less reliable. Partial wood replacement may be the better option.
• Penetration of 1/2 inch or more: Structural rot. The wood has lost its structural integrity in this area. The fibers are soft and spongy, possibly crumbling. Repair is not advisable. Replacement is the appropriate response.

Where to Test

Focus your testing on the areas where rot is most common:

• Window sill (the flat horizontal surface at the bottom): This is the number one location for rot because water pools here. In Utah, snow piling on exterior sills during winter is a major contributor.
• Lower corners of the frame: Where the sill meets the vertical jambs, water can collect and wick into end grain.
• Exterior casing (the trim around the outside of the window): Exposed to rain, snow, and sprinkler overspray.
• Any area where paint is peeling or bubbling: Paint failure often indicates moisture underneath, which may be feeding rot.

Understanding What Causes Window Frame Rot

Rot is not caused by age alone. It is caused by a specific combination of conditions:

1. Persistent moisture: Wood must maintain a moisture content above 20% for fungal organisms to grow. Brief exposure to water (a single rainstorm) does not cause rot. Persistent or repeated wetting does.
2. Moderate temperatures: Rot fungi are most active between 40-100 degrees Fahrenheit. Utah's climate provides these temperatures roughly eight months of the year.
3. Oxygen: Rot fungi are aerobic organisms. Submerged wood does not rot (which is why ancient wooden pilings survive underwater for centuries).

In Utah, the most common moisture sources that lead to window rot are:

• Snow accumulation on sills: Packed snow sits against the wood for weeks or months, keeping it wet.
• Sprinkler overspray: Landscape sprinklers that hit the side of the house repeatedly wet the window frames.
• Failed caulk: Gaps between the window frame and the siding allow rainwater behind the trim.
• Interior condensation: Moisture running down the glass pools on the sill, especially in winter. This problem is covered in depth in our condensation and mold guide.

Option 1: Epoxy Repair for Minor Rot

Epoxy repair is the go-to solution for early-stage rot where the structural integrity of the frame is not compromised.

When Epoxy Works

• Rot is less than 1/4 inch deep
• The affected area is smaller than a credit card (roughly 3 by 5 inches)
• The rot is on the surface of the sill or casing, not at a structural joint
• This is the only or one of very few areas of rot on the window

How to Do It

1. Remove the rotted wood. Use a chisel, screwdriver, or rotary tool to dig out all soft, decayed material. Go slightly beyond the visible rot into solid wood. The repair only works if you reach sound material.

2. Apply wood hardener. Products like Minwax High Performance Wood Hardener penetrate the remaining porous wood fibers and solidify them. Apply liberally and let it dry for 2-4 hours. In Utah's dry climate, curing is typically faster than the label suggests.

3. Fill with epoxy filler. Mix two-part epoxy wood filler (such as Bondo or Abatron WoodEpox) and press it firmly into the cavity. Overfill slightly, as the epoxy can be sanded flush once cured. Shape it roughly to match the existing profile.

4. Sand and prime. After the epoxy cures (1-2 hours in warm, dry conditions), sand it smooth with 80-grit followed by 120-grit sandpaper. Apply a quality exterior primer.

5. Paint. Two coats of exterior acrylic latex paint. Pay attention to full coverage at the edges where the epoxy meets the original wood, as this joint is the most vulnerable to future moisture penetration.

Cost

• Wood hardener: $10-15
• Epoxy filler: $15-25
• Sandpaper, primer, paint: $10-20 (using supplies from a partial can)
• Total: $35-60 per window (DIY)

Option 2: Partial Wood Replacement

When the rot is too extensive for epoxy but the overall frame is still serviceable, you can cut out the damaged section and splice in new wood. This is sometimes called a "Dutchman" repair.

When Partial Replacement Works

• Rot extends 1/4 to 1/2 inch deep over a larger area
• The damage is confined to the sill or a single section of casing
• The rest of the frame is solid and the window still operates properly
• The window is a standard size where matching lumber is readily available

How to Do It

1. Cut out the damaged section. Use an oscillating multi-tool or hand saw to cut clean, square edges around the rotted area. Remove all damaged wood plus 1-2 inches of sound wood on each side for a clean bonding surface.

2. Shape the replacement piece. Cut a new section of matching lumber (same species, same dimensions) to fit the cavity. For sills, use a rot-resistant species like cedar or pressure-treated wood if available in the right dimensions.

3. Install with epoxy adhesive. Apply two-part epoxy adhesive to both mating surfaces. Press the new piece firmly into place and secure with stainless steel screws or galvanized finishing nails.

4. Seal all joints. Fill any remaining gaps with epoxy filler. Once cured, sand smooth.

5. Prime and paint. Apply primer to all surfaces, including end grain, before painting.

Cost

• Lumber: $10-30
• Epoxy adhesive and filler: $20-30
• Hardware and finishing supplies: $10-20
• Total: $40-80 per window (DIY) or $150-350 (professional carpenter)

Option 3: Full Window Replacement

When rot has progressed beyond what repair can reliably address, replacement is the right call.

When Replacement Is Necessary

• Rot extends more than 1/2 inch into the frame at structural joints
• Multiple areas of the same frame are affected
• The sill is rotted through to the rough framing
• The window no longer operates properly due to frame distortion from rot
• Rot is present on multiple windows throughout the home, suggesting a systemic moisture problem
• You discover mold or moisture damage in the wall cavity behind the rotted frame

What Replacement Involves

A full-frame replacement removes the entire window unit, including the frame, from the rough opening. The installer then:

1. Inspects the rough opening and surrounding framing for moisture damage or rot
2. Repairs any damaged framing (adds cost but is critical for a sound installation)
3. Applies new flashing and waterproof membrane to protect the rough opening
4. Installs the new window with proper shimming for square and level
5. Insulates the gap between the window frame and rough framing with low-expansion foam
6. Applies exterior trim and sealant
7. Finishes interior trim

Cost

• Vinyl replacement windows: $400-700 per window installed
• Fiberglass replacement windows: $600-900 per window installed
• Wood-clad replacement windows: $800-1,200 per window installed
• Additional framing repair (if rot reached the rough opening): $100-300 per window

For a more detailed look at replacement costs across materials and window types, see our window replacement cost guide.

Why Replacement Can Actually Be the Economical Choice

When rot is advanced, the math often favors replacement even though the upfront cost is higher. Consider a scenario where you spend $250-350 on professional epoxy repair of a moderately rotted window. If the repair lasts 3-5 years before the rot returns (common in Utah's climate), you face another repair cycle. Two repair cycles at $300 each totals $600, which equals the installed cost of a new vinyl window that comes with a 25-year warranty and modern energy performance.

Preventing Future Rot

Whether you repair or replace, preventing moisture from reaching the wood is the long-term solution.

Exterior Maintenance

• Caulk inspection: Check exterior caulk annually, especially where the window meets siding. Recaulk any gaps or shrinkage cracks.
• Paint maintenance: Repaint exterior frames every 5-7 years, or at the first sign of peeling. Paint is the primary moisture barrier for wood frames.
• Sprinkler adjustment: Ensure no sprinkler heads direct water at the house. This is one of the most common and most preventable causes of window rot in Utah.
• Snow removal: After heavy snowfall, brush accumulated snow off exterior window sills, especially on the north side of the home where snow persists longest.

Drainage and Grading

• Ensure the ground slopes away from the foundation so rainwater and snowmelt drain away from the house, not toward it.
• Clean gutters regularly. Overflowing gutters dump water directly onto window frames below.

Choosing Rot-Resistant Materials at Replacement

If you are replacing due to rot, consider materials that eliminate the problem entirely:

• Vinyl frames: Cannot rot. Period. This is the primary reason vinyl is the most popular frame material in Utah new construction.
• Fiberglass frames: Also impervious to rot, with superior structural strength.
• Aluminum-clad wood: The exterior is protected by aluminum cladding while the interior retains the warmth of wood. The wood interior can still be affected by interior moisture if not maintained.

For most Utah homeowners who are replacing rotted wood windows, vinyl or fiberglass is the practical choice. You get modern energy performance, zero-maintenance frames, and you never deal with rot again.

When to Act

Rot does not get better on its own. It only spreads. If you have identified soft, spongy, or discolored wood on any of your window frames, assess the severity with the screwdriver test and take action based on what you find. Early-stage rot caught and repaired today is a $50 fix. That same rot ignored for two years could become a $700 window replacement with an additional $300 in framing repair.

For a complete assessment of all the signs your windows may be showing, use the diagnostic checklist in our pillar guide on 10 warning signs you need new windows.