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Dark patches on attic sheathing: what they are and what comes next

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We can tell you where to look and what the pattern means. We cannot tell you what it is, and neither can anyone else from a photograph.

Somebody goes into the attic — usually for an unrelated reason — and finds dark staining on the underside of the roof boards. It is a genuinely unsettling thing to find, and there is a great deal of alarming material written about it. This page tries to be useful and honest instead, which means being clear about where our competence stops.

What we will not do

We will not tell you that a dark patch is mold. Not from a photograph, not from a description, not from where it is. Discolouration on attic sheathing can be several things, and the only way to know is sampling and analysis by someone qualified to do it.

We also do not write about health effects, at all. That is a medical question, it depends on the individual, and a roofing site has no business having a view. If you have health concerns, those go to a doctor rather than to a website about roofs.

What we can be useful about is the moisture, because whatever is growing is growing for one reason: something up there is wet.

Where it appears, and what that tells you

The pattern is diagnostic, and it is worth reading before anyone is called.

Spread broadly across the north-facing slope is the classic condensation pattern. The north slope gets the least sun, stays coldest, and therefore condenses first and dries last. Growth that follows the cold surface rather than following a water trail is telling you about air and temperature, not about a hole.

Concentrated directly above a bathroom, in a fan-shaped plume, points at a specific moisture source below — usually an extractor duct discharging into the attic rather than through it.

Following a defined line or a stripe down a rafter is the exception: that is a water trail, and that is a leak rather than a ventilation problem.

Worst at the eaves, in a band along the bottom, often indicates that the soffit vents are blocked — typically buried under blown insulation — so the coldest, least ventilated part of the attic is also the part that never dries.

Around the roof penetrations and nowhere else is worth looking at closely: either something is leaking at those points or they are thermal bridges that condense first.

Why the attic is such a good place for it

Three conditions have to coincide, and a badly ventilated attic delivers all three. There is organic material — plywood, OSB, the paper facing on insulation, dust. There is moisture, arriving from the house below. And there is still air, because the ventilation that should be carrying that moisture away is not working.

Only one of those three can practically be removed, and it is the moisture. That is why the answer to this problem is almost never a treatment applied to the wood; it is stopping the water arriving and making the attic able to dry. Cleaning or coating sheathing while the conditions that caused it are untouched buys a couple of years at best.

What to do, in order

1. Do not disturb it. Scrubbing, brushing or blowing at it puts whatever is there into the air in a confined space. Leave it alone until you know what you are dealing with.

2. Photograph and map it. Which slope, how far it extends, whether it is above a particular room, whether it follows a line or covers an area. Note the date. This is the information an assessor will want and it costs you nothing.

3. Find the moisture. Check where the extractor ducts terminate. Look for frost or damp on the deck on a cold morning. Check whether the soffit vents are buried. This is the part that actually changes the outcome.

4. Get it identified if you need to know. Assessment and sampling is its own trade — and it is worth being aware that an assessor who also sells the remediation has an obvious conflict of interest. Independent assessment first, quotes second, is a sound order.

5. Fix the cause before anyone treats the symptom. A remediation done while an unvented bathroom fan is still pumping steam into the attic is a scheduled repeat.

One practical note

If you are going up there to look, an attic with a moisture problem has poor air movement by definition and can be an unpleasant environment. Keep the visit short, and if there is significant visible growth, the sensible thing is to look from the hatch and take photographs rather than to crawl around in it.

Frequently asked questions

Is the dark staining on my attic sheathing mold?

We cannot tell you that, and nobody can from a photograph or a description. Discolouration up there can be several things and identification requires sampling by someone qualified. What is certain either way is that something is wet, and that is the part worth chasing.

Why is it worse on one side of the roof?

Usually because that side is the north slope: least sun, coldest surface, condenses first and dries last. Growth that follows the cold surface rather than a water trail is pointing at ventilation rather than at a leak.

Can I clean it off myself?

Disturbing it puts whatever is there into the air in a confined space, so leave it alone until you know what it is. And cleaning treats the symptom: with the moisture source untouched it comes back.

Do I need a specialist?

If you want it identified, yes, and that is a separate trade from remediation. An assessor who also sells the remediation has an obvious conflict of interest, so independent assessment first and quotes afterwards is the sounder order.

What about the health effects?

We do not write about that here. It is a medical question, it varies by individual, and a site about roofs is not the right source. If you have health concerns, take them to a doctor.

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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