Hello,

This black stuff on the underside of the roof sheathing looked like mold to me.
Underside of roof sheathing

I marked out a small area that I then wiped down with bleach.
The bleach didn't affect the area at all--the dark spots stayed dark.
So, if it's not mold, what is it?
How can I find out for sure?
What can I do to fix it if it is mold?

Thanks in advance.

The black spots are caused by moisture and the moisture likely created a spot for mold. A bleach solution will kill the mold but will not return the underside of the decking to it's original color - it's stained and will remain that way.

I am wondering about attic venting. Noticed that there are two areas (one square, one round) that used to have attic vents. If those were replaced with continuous ridge vents then no problem. However, if there are no vents in the attic you are creating a mold environment. Also make sure that your soffit vents are open to provide proper attic air flow.

Thanks Jaybee. Yes, the attic vents were replaced by a ridge vent around 7 years ago when we had the shingles replaced. I think the spots were formed well before the ridge vent was installed. Looking at it now, I'm thinking we probably should have had the decking replaced too. I keep after the soffit vents we have, but I think we need more. I'm hoping to do a big project this spring/summer and part of that will be to replace all the soffits--maybe with the Hardi perforated soffit.
But, I'm thinking that, in a few years, when our kids are out of the house, my wife and I will want to sell. I'm worried that an inspection of the attic crawl space will make the home very difficult to sell at a fair price. Any thoughts on what I could or should do?

Thanks again.

The discoloration on the underside of the plywood decking is not a problem - as long as the wood has not deteriorated. The most common cause I've seen for the underside staining is a roof with old shingles that are past their life. This causes very small leaks around the multiple nails that are holding the shingles in place. There is no fix short of removing the roof and decking and replacing - to do this to a 7-year old roof would not make sense.

As to how it would affect the sale - Hard to say. Most home inspectors work from a checklist and have no real knowledge of house construction. As such, you could expect the roof decking to get flagged as a problem area - even though it isn't. Some potential buyers would run away or insist on a roof replacement or allowance, while some with real-world knowledge would not be bothered. You already have pictures that document what is there that I assume are dated. That should be more than enough to show that it's not a current problem - especially if you plan to sell several years down the road.

try wetting the area with straight, undiluted , liquid pool chlorine, you can get it a wal-mart. the pool chlorine is a 10% solution while household bleach is only 5%

Thanks again Jaybee.
I'm a little confused. I haven't hit the entire underside of the sheathing with bleach yet, so, it's possible that there is still live mold (i.e., not just staining).

Nona, thanks for the pool chlorine idea. I'd have a very large area to cover if I wanted to hit the entire underside of the sheathing. I'm leery of being in that confined space with such highly concentrated chlorine. Maybe I could do a little at a time. There are some areas though that are nearly impossible to reach.

It may not be mold at all, just discoloration. However, the discoloration is caused by moisture and the moisture makes a perfect environment for mold to grow.

I'd feel better about it being safe and killing any areas that could have mold. True, you will need to be careful as bleach in an enclosed space will take you out.

Here's another view of the extent of the discoloration:
Wider view of affected area

And that photo doesn't capture all of it. The house has an approximate 1400 sq ft footprint and I'd estimate that over half of the sheathing is discolored in this way.


Wiping that all down with a wet cloth may take years The cleaning rag gets torn up by all the nails and the rough surface of the wood. Plus, I can't even fit in near where the roof-line descends into the soffit. It's even worse on the other side of the house (shown in the, rather dark, photo I linked to below), because there are 10" heating ducts taking up much of the space.

East side of attic crawl space

I don't think spraying is an option as it will truly make breathing unsafe. Plus, I've read that using bleach in that way introduces more moisture into the space--ultimately worsening the situation.

Sorry, I don't mean to shoot down any ideas. I just know that it's going to be a tough job, no matter which approach I use, so I need my work to be as effective as possible.

Thanks again.

This message has been edited. Last edited by: SturdyNail,

Curious question from an amateur...

In the photo of the "wider view", there's something at the base of the attic. It's beige, looks like metal, has small cones pointing toward the roof. Looks like it comes in strips and fits underneath the roof joist (not sure if that's the proper term).

I've never seen anything like it (told you I'm an amateur!). Could you explain what it is and its purpose? I'm really curious.

Thanks a lot!

Garden Sprite - Those are foam baffles. They are installed in-between rafters or trusses anywhere that the insulation may get close to the underside of the roof decking. This keeps an air path open so that cool air from the soffits can exit out the ridge vents.

SturdyNail - I would spray it with a small bottle sprayer. But yes, you do need to be careful as bleach can do a number on you in an enclosed space. No need to wipe it, just spray. Also, don't worry about the 'moisture' form the spray - it's such a small amount that it will evaporate easily and since it's a bleach solution, no way is anything going to grow in it.

I see what you mean about access though, you are not going to be able to everywhere. Remember though, the discoloration itself is not a problem as long as there is no rot or softness.

if you're going to use chlorine to bleach out the mold / stain wear a painters respirator mask with a charcoal filter. thats one of those respirators that make you look like an outer space alien. They are for organic vapors. DO NOT USE A PAPER MASK..
They're not expensive

This message has been edited. Last edited by: nona,

OK Then. I think my mission has been defined.
Nona, I'll be buying pool chlorine and an alien mask.
Jaybee, I'll do my best to spray all of the discolored areas. I have a garden tank/pump sprayer that I think might do the job. I'll try to get that done before the Summer heat makes it totally unbearable up there.
I'll try to keep the insulation from getting we too.

Thanks for your good ideas.

It may or may not be a mold. Black mold is a dark greenish or black mold that grows in damp, poorly ventilated indoor environments, often occurring after flooding. While it can be eliminated, it often grows undetected for long periods, thus making its eradication more difficult.
Look beyond the mold you can see. Before you can eliminate mold, you need to find not only what is readily visible, but also what may be hidden. This involves getting down on your hands and knees, bellying into crawl spaces, pulling up rugs and maybe even pulling away old drywall.Bleaching is a great option but don't forget to use dehumidifier if necessary.