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Blackened Roof Sheathing Question


Mark P
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A section of the plywood sheathing is blackened only in one area approximately 32" wide. The rest of the sheathing is normal except where it is water damaged along the bottom edge from lack of a drip edge. There are 2 12x9 gable vents for this 800 sqft attic. The roof is 15-20 years old? My question is why is this section black? My 1st thought was a ventilation problem except the blackness is only in this one area. So, my next guess was it is related to the black staining on the outside of the shingles, which I always thought was the tar leaching from the aging shingles. Anyway, I’ve already told the buyer the roof will likely need replacement in the next 5 years and some of the sheathing replaced with it, but I wanted to give him an explanation (and I want to understand too) of why this section is black. You thoughts and opinions are always appreciated.

The last pic is just gee-wiz since I’m already posting this, it the dryer vent and years and years of lint build up in the attic, but not near the blackened area.

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Download Attachment: icon_photo.gif outside.jpg

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Download Attachment: icon_photo.gif Lint.jpg

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Mark,

Is the area in the attic above a bath? I'm guessing it is, and this is simply mildew on the underside of the roof deck, promoted by bath moisture naturally migrating up into the attic.

The black stuff on the shingles is algae, not tar. The algae feed off organics found in the shingles. When the new shingles are installed, nailing copper strips along the roof ridge will keep algae from growing on the new shingles; the ions in the copper will run down the roof and kill any algae growth.

For a good example of this, one that can be seen daily driving up and down the block, look for a roof with a lot of algae. Pay close attention to the shingles right under the metal flue chimney housing...not a speck of blackness (algae) under the chimney.

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Originally posted by Brian G.

What's the black-looking mess on the insulation below the affected area?

Yea, wondering the same thing myself. That area looks to be where the refrigerant lines penetrate the ceiling.

There might be a correlation in the temp of those lines and the amount of airflow 'leaking' through the ceiling during the cooling season that is causing the "mildew" growth on the attic sheathing.

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Originally posted by AHIS

A section of the plywood sheathing is blackened only in one area approximately 32" wide. The rest of the sheathing is normal except where it is water damaged along the bottom edge from lack of a drip edge. There are 2 12x9 gable vents for this 800 sqft attic. The roof is 15-20 years old? My question is why is this section black?

I don't know why it turned black, but the blackness is ancient history in this attic. It probably pre-dates this roof which you say is 15-20 years old. Look at the lower course of sheathing. It looks like it's been replaced. Also look at the sheathing to either side of the roofing nail tips. Where the splinters have come away from the underside of the plywood, the wood is bright. This blackening predates the roof shingles. What caused it is unimportant.

- Jim Katen, Oregon

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I think you've provided what you should if you just tell them....

1) The roof's ancient; replace it, the sooner the better.

2) Expect to replace some of the plywood as part of the job.

3) There's some black stuff in the attic; don't know what it is. If concerned, we can find out, but it's probably not important.

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Thanks all! I think Jerry nailed it, this is above the bathroom and that explains why it is concentrated in just this area.

The black mess on the insulation below the affected area is the insulation on the a/c refrigerant line, which was manufactured in 1982 and can you believe the furnace was 1978!

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Hi,

I often see discoloration like that above bathrooms and bedrooms. Lots of humidity in those areas. You'd be amazed at how much water we dump into the air while we're sleeping.

Mark, was there a ceiling fan under that insulation that's trying to duct air to the eaves instead of straight up to the roof or perhaps didn't even have a duct attached and was just venting against the underside of the insulation where that black spot on the insulation is? If not a fan, was there a ceiling light fixture of some sort with a leaky base that's allowing air passage? Jerry's right, the underside of the roof over those areas can discolor regardless, but any air leakage makes it worse.

OT - OF!!!

M.

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  • 10 months later...

It appears to be mold growth.. attic mold is normally evidence of the following:

1. Lack of proper attic ventilation

2. Roof leak in the past

3. Improper ventilation into the attic (bathrooms, kitchen, etc.)

(often more than 1 of the above)

Often we'll see it on the north facing section of the roof (or other area heavily blocked by trees) due to the cooler temperature and lack of direct sunlight. I would call what you saw out of apparent mold growth and pretty much report what I said above.

Maybe more attic ventilation is needed (most homes have too little), maybe the soffits are blocked by insulation, maybe a bathroom fan terminates right near this point, etc.

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