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Mold on roof sheathing: what dark growth on the deck tells you

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Dark patches on the plywood above your head are a sign of damp, not a verdict. The useful question is where the moisture comes from.

Dark spots or fuzzy growth on the underside of the roof deck mean the wood has been damp for a while. That is the part you can act on. What the growth actually is cannot be settled from a photo or a glance, so treat it as a moisture clue and work on the source.

What you are looking at

Check from the hatch with a flashlight and keep your feet on a board you trust. Do not walk the joists and do not go onto the roof. You are looking for the pattern of the growth. Patches along the north slope, near the eaves, or around bathroom fan ducts tell a different story from a stain under a chimney.

Growth spread across a whole slope is most often condensation: warm indoor air rising into a cold attic and meeting the cold underside of the deck. Our guide to attic condensation explains the mechanism. A single patch with a tide line, or growth under a flashing, points to a leak from outside.

Find the source before you touch the growth

Cleaning growth off wet wood without fixing the cause only buys a few weeks. Work through the likely sources in order. Bathroom and kitchen fans that vent into the attic instead of outside are a common culprit, and so are ducts venting into the attic. Gaps around the hatch and recessed lights let warm air in. Blocked soffit vents starve the deck of air.

If the wetness lines up with a chimney, vent pipe or valley, the entry is outside. Use the find the leak tool to narrow it down, and then bring in a roofer.

What not to do

Do not climb up to inspect or spray the deck. Do not scrub or sand the wood without protection. Do not cover growth with paint or a sealer to hide it. And do not call it mold on your own say-so. Confirming what it is takes sampling by someone qualified, and you do not need a result to start fixing the moisture.

When to call a pro

Bring in an attic or building-envelope contractor if growth covers a large area, if the wood feels soft, or if it returns after a season. Soft or sagging sheathing may need a structural look, which is a job for a licensed professional. For the wider picture of growth in the attic, see attic mold.

Keeping the deck dry afterwards

Once the source is fixed, the aim is a dry, ventilated deck. Air seal the ceiling below, vent fans outdoors, and keep the soffit intake clear. Check again after the next cold snap, since winter is when condensation shows up.

Frequently asked questions

What causes mold-like growth on roof sheathing?

Persistent moisture. Usually warm indoor air condensing on the cold deck, a fan venting into the attic, or a slow roof leak.

Is dark staining on plywood in the attic mold?

You cannot tell from a photo. Treat it as proof of dampness and have a qualified person sample it if you need to confirm.

Can I clean growth off roof sheathing myself?

Cleaning without fixing the moisture source only helps briefly. Fix the source first and call a pro for large areas.

Should I go on the roof to check the sheathing?

No. Inspect from the attic hatch with a flashlight. A roof is a dangerous place to stand and the evidence is inside.

Sources

What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.

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