Attic Mold on Roof Sheathing, Causes and Fixes

What Attic Mold Looks Like

Attic mold usually shows up as gray, black or dark green staining on the underside of the roof sheathing, the plywood or OSB boards the shingles are nailed to. It is often heaviest on the north-facing side of the roof, which stays colder and gets less sun. You may also see it around nails, along rafters, near the peak and around bathroom fan ducts.

Other clues point to a moisture problem even without mold.

• Rusty nail tips poking through the sheathing, or rust stains below them.
• Frost on nails and sheathing in winter, or drips on the insulation when it melts.
• Water stains or compressed, matted spots on the insulation.
• Wet or dark insulation directly under a fan duct.
• A musty smell when you open the attic hatch.

Before you panic, check for look-alikes. Dark marks on new wood can be stains from the mill or lumberyard. Soot from an old fire looks black but wipes off greasy. Water stains from a roof leak are brown rings, not fuzzy growth. A white, crystalline dusting on wood can be dried sap.

The pattern tells you the cause. Mold spread evenly over one side of the roof suggests humid house air and poor ventilation. Mold in a spot under a fan duct points to that fan. A stain that ends at a roof vent, chimney or valley points to a roof leak.

The Four Usual Causes

Almost all attic mold comes from one or more of these.

1. Fans venting into the attic. Bathroom fans, kitchen range hoods and dryer vents must end at a cap on the roof or an outside wall. Many are just aimed at a soffit or left open in the attic. A single bathroom fan can dump a large amount of moisture into the attic every day. This is the most common cause and the easiest to fix.

2. Air leaks from the house. Warm indoor air carries moisture. It rises through gaps around recessed lights, plumbing and wiring holes, the attic hatch, chimney chases and the tops of interior walls. When it hits cold roof boards in winter, the moisture condenses. That is how you get frost and dripping nails.

3. Blocked or missing ventilation. Most attics are designed to pull outside air in at the soffits and out near the ridge. If insulation is packed against the soffit vents, or there are no low vents at all, damp air sits in the attic. Building codes commonly call for about 1 square foot of net free vent area for every 150 square feet of attic floor, or 1 per 300 when vents are balanced between high and low.

4. Roof leaks. A failed flashing, a cracked vent boot or damaged shingles can soak the sheathing in one area. This mold stays local and lines up with the leak.

High indoor humidity makes causes 1 and 2 much worse. Aim to keep the house between 30 and 50 percent.

How to Inspect Your Attic Safely

Plan a visit on a cool morning. Wear a respirator, goggles, gloves and long sleeves. Bring a headlamp, a moisture meter and your phone.

Once inside, stay on the joists or a plank laid across them. The ceiling drywall between joists will not hold your weight.

Work through this checklist.

• Trace every duct. Find where the bathroom, kitchen and dryer ducts go. Each should connect firmly to a roof or wall cap, with no gaps and no sag where water can pool.
• Look at the soffits. From the attic side, you should see daylight at the eaves or foam or cardboard baffles holding insulation back from the vents.
• Check the insulation. Look for matted, stained or thin spots. Dark, dirty insulation often marks an air leak from below.
• Test the wood. Take moisture readings on the sheathing in stained and clean areas. Wood is generally considered dry below about 15 percent, and readings near 20 percent or higher support mold growth. Push a screwdriver into stained areas. Solid wood resists. Soft, spongy wood is a structural concern.
• Look at roof penetrations. Check around vents, chimneys and skylights for stains that line up with a leak.
• Measure the air. Leave a hygrometer for 20 minutes and compare it to outdoor humidity.

Take photos of everything, including labels on fans and the size of any vents.

Fix the Cause First

Cleaning attic mold without fixing the cause is wasted money. It will be back by the next winter.

Reroute fan ducts. Connect each bathroom and kitchen duct to a proper roof or wall cap with a backdraft damper. Use rigid or semi-rigid duct where you can, keep runs short, and slope them slightly toward the outside so condensation drains out rather than back into the fan. Insulate bath fan ducts in cold attics to limit condensation. Dryer vents should be rigid metal duct and never end in the attic.

Seal air leaks from below. Seal gaps around plumbing, wiring and duct holes with spray foam or caulk. Weatherstrip and insulate the attic hatch. Air sealing around chimneys and flues requires fire-rated materials and clearances, so leave those to a pro. Many recessed lights are not rated for contact with insulation or for air sealing, so check the label before covering them.

Open up ventilation. Install baffles at each rafter bay over the soffit vents so insulation cannot block them. Make sure there is an exhaust path near the ridge. Do not add a powered attic fan without advice, since it can pull conditioned air, and even combustion gases, out of the house.

Fix roof leaks. Replace failed flashing and vent boots, and repair damaged shingles.

Lower indoor humidity. Run bath and kitchen fans every time, and use a dehumidifier in damp seasons.

Cosmetic Stain or Real Damage

Once the cause is fixed and the wood has dried, many attic mold stains are dormant. The question is whether you need to remove them and whether the roof is still sound.

Some homeowners leave dormant, dry staining in place in an attic that is rarely used and not connected to living space. Others remove it because of a home sale, because ducts or the air handler sit in the attic, or because someone in the home is sensitive to mold.

If ductwork or a furnace sits in the attic, or if the attic is part of the air path to the house, removing the mold is the better choice. Leaky ducts can pull attic air into the home.

How Professionals Clean Attic Mold

Most attic mold jobs are larger than 10 square feet, so a specialist is usually the right call. Knowing the methods helps you compare bids.

• HEPA vacuuming and wiping. For light growth, loose mold and dust are vacuumed with HEPA equipment and the wood is wiped or scrubbed with a cleaning solution.
• Sanding or wire brushing. Growth on the surface is mechanically removed, with HEPA vacuums capturing the dust.
• Media blasting. Some companies blast the wood with dry ice or baking soda to remove growth and staining quickly. It is fast but creates a lot of residue that must be cleaned up.
• Sheathing replacement. Soft or rotted boards are replaced, usually along with a reroof or partial roof repair.

Many contractors apply a clear or white encapsulating coating after cleaning. A coating is fine as a final step, not as a substitute for removal or for fixing the cause. Be cautious of any bid that only sprays a product with no cleaning.

Moldy insulation is often removed and replaced, especially if it is wet or matted. Ask how the contractor will protect the living space below, how they will handle any old insulation that could contain asbestos, and what they will do about the cause. A bid that does not mention the cause is not a complete bid.

Step by Step

1. Plan a Safe Attic Visit

   Go on a cool morning with a respirator, goggles, gloves and a headlamp. Tell someone you are going up and stay on the joists.

2. Photograph the Pattern

   Take wide and close photos of the staining, including which side of the roof it is on and where it is heaviest.

3. Trace Every Fan Duct

   Follow each bathroom, kitchen and dryer duct to its end. Note any that end in the attic, are disconnected or aim at a soffit.

4. Check the Soffit Vents

   Look for daylight or baffles at the eaves. Note any bays where insulation is packed against the vents.

5. Test the Sheathing

   Take moisture readings in stained and clean spots and probe stained areas with a screwdriver. Soft wood needs a roofer.

6. Look for Leaks and Air Paths

   Check around vents, chimneys and skylights for leak stains, and look for dark, dirty insulation that marks air leaks from below.

7. Fix the Causes

   Connect fan ducts to exterior caps, install vent baffles, seal plumbing and wiring holes, and repair roof leaks. Leave chimney and flue sealing to a pro.

8. Let the Wood Dry and Recheck

   Give the attic a few weeks and recheck moisture. Sheathing should read below about 15 percent before any cleaning or coating.

9. Decide on Cleaning

   If growth is larger than about 10 square feet, active, or in an attic with ducts or a furnace, get bids from mold specialists and compare how each handles the cause.

Mistakes That Make It Worse

• Paying for attic mold removal without fixing the bathroom fan that vents into the attic. It returns by next winter.
• Spraying bleach on roof sheathing. It adds water, does not remove the mold and can corrode nails and metal plates.
• Adding a powered attic fan without checking air sealing. It can pull humid and conditioned air, and in some homes combustion gases, out of the house.
• Stuffing insulation into the eaves and blocking the soffit vents.
• Stepping between joists and putting a foot through the ceiling.
• Disturbing old vermiculite insulation, which can contain asbestos, while cleaning.

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