Black Stains on Roof Sheathing: Mold or Just Old Wood?

Black stains on roof sheathing are mold more often than not, but not always. Rusting nails, stain that was in the lumber before the house was built, soot and plain weathering all leave dark marks too. The pattern, the texture and a moisture reading will usually tell you which one you are looking at.

What mold on roof sheathing looks like

Attic mold on plywood or OSB tends to show as clusters of dark spots that merge into blotchy patches. Up close it often looks speckled, like pepper shaken on the wood, and heavier growth can look sooty or slightly fuzzy. Colours range from grey and dark green to black. On plywood it often follows the grain in streaks. On OSB it can outline the individual wood flakes.

Location matters as much as appearance. Mold goes where the moisture goes. Expect it on the north slope, on the lower few feet near the eaves, above bathroom fans and around the attic hatch. It rarely respects the edges of a single board or sheet, and it usually fades out gradually instead of stopping at a hard line.

Other things that cause black stains on roof sheathing

• Nail rust. Roofing nails that come through the deck collect frost and condensation. They rust and leave a dark halo or a drip streak below each one. Neat rows of dark dots spaced along the framing are nails, although widespread rust also means the attic has been damp.
• Lumber-yard stain. Some lumber picks up blue-grey or black staining from fungi while it is still green, before it is dried and sold. The stain is in the wood, has a hard edge where one board meets the next, and does not spread once the wood is dry. It is common on trusses and rafters.
• Old water marks. A leak that was fixed years ago leaves brown or black tide lines with a clear outer ring. If the wood is dry and hard now, it is a stain and nothing more.
• Soot and dust. Staining near a chimney, or dark lines where air has been filtering through gaps in the deck for decades, is often dirt.
• Age. Plank decks in older homes darken with heat and time. Even, uniform darkening on every board on both slopes is usually just old wood.

How to tell mold from staining yourself

You can sort most cases from the hatch with a good flashlight. Work through these checks in order.

1. Compare slopes. If the north side is clearly darker than the south, moisture is involved. Even colour on both sides points to age or lumber stain.
2. Look at the edges. Mold blotches cross from one sheet to the next. Lumber stain stops dead at the end of a board.
3. Look at the framing. Stain on one truss member and not its neighbours came from the lumber yard. Growth on sheathing and framing together in one zone is mold.
4. Shine the light along the surface at a low angle. Mold often shows a faint raised or powdery texture. Stain is flat and in the grain.
5. Check for a cause nearby. A fan duct, a pot light, a buried soffit or a plumbing stack right below the dark area makes mold much more likely.

Do not rub or scrape a suspect area with your bare hand or a dry brush to test it. That puts whatever it is into the air right at face level.

Does the colour tell you what kind of mold it is?

No. Colour does not identify the species, and black growth in an attic is not automatically the black mold people read about online. Several common types that grow on damp wood are dark. Health Canada's position is that indoor mold should be removed whatever the species, so the name matters less than people expect. What matters is whether it is active growth, how much area it covers and why the wood got wet.

When to test and when to skip it

Testing is worth doing when the answer changes a decision. In a real estate deal, a lab result from a tape or swab sample settles an argument between buyer and seller quickly. Samples go to an accredited lab and come back in a few days. It is also useful when staining is faint and the attic is otherwise dry, and you want to know whether there is anything to remove.

Testing is usually a waste when the picture is obvious. If there are blotchy black patches on the north slope above a bathroom fan that vents into the insulation, a lab report will only confirm what you can already see. Put that money toward the fix.

A moisture meter adds more than a lab report in many cases. Dry sheathing with hard-edged staining is probably old. Wet sheathing has a current problem whatever the stains are. We check with a moisture meter and thermal camera as part of an inspection, which runs $249 to $549.

What to do next

Take three photos: a wide shot of each slope and a close-up of the darkest area with something for scale. Note what sits under the stained section. That is enough for someone experienced to tell you whether it looks like growth, old staining or something that needs a sample, and it costs you nothing to ask before the next step.

Common questions