How to deal with mold on underside of roof deck

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  #1  
Old 01-23-21, 08:12 PM
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How to deal with mold on underside of roof deck



A pest control guy was inspecting my attic crawlspace for termites. Fortunately, he didn't find any, but he did notice a small section of the underside of the roof deck, out at the edge, that had mold on it. It also seems to have spread to the lower part of a rafter. He suggested painting it with bleach, but since it's at the edge of the roof, it's hard to get to in the attic. Also, it could come back, and it would still involve finding and repairing a leak that's letting the water in.

I'm planning to get a roofer to repair it, but I just want to confirm that it will probably involve removing the shingles, cutting out the moldy section of roof deck (about 2' x 3') and probably the rafter, then splicing in a new piece of rafter, replacing the plywood section, and reshingling. Is there anything I need to watch out for, and does anyone have an idea what I should expect to pay?

Thanks.
 
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Old 01-23-21, 09:06 PM
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Small areas like that aren't much of a surprise, but its good you found it. There is not really much point putting bleach on it. It looks like you have had a roof leak in the past... or it could be an active leak. Or possibly due to ice dams in the winter. A photo of the outside might give us a clue as to why its leaking, but being so close to the edge my guess is ice dam. If its soft when you poke at it it should be replaced eventually. If the roof is getting old and you plan to do it in the next 5 yrs or so, it can probably wait that long.

Its also possible that you have a large amount of interior conditioned air leaking into the attic, so you could move the insulation aside and inspect whats directly below. Humid air (such as above a shower) leaking into the attic so close to the roof deck would cause a large amount of frost on it during winter months.
 
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Old 01-23-21, 09:15 PM
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Thanks. I live in Florida, so it's unlikely to be an ice dam. I looked at it from the outside but don't see anything amiss. The shingles are rated for 30 years, and they've got 15 on them, so I'm not planning to reroof anytime soon, unless the home insurance company makes me. If I'm going to fix it, I want to do it sooner rather than later, so the mold doesn't spread. Just curious if there's any other way to do it besides cutting out the moldy part and replacing it.
 
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Old 01-23-21, 10:32 PM
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Its also possible that you have a large amount of interior conditioned air leaking into the attic, so you could move the insulation aside and inspect whats directly below. Humid air (such as above a shower) leaking into the attic so close to the roof deck would cause a large amount of frost on it during winter months.
I don't think that's it. The bathrooms are on the other side of the house, and the ceiling ducts aren't anywhere near this area. Also, since I'm in south Florida, there's no frost--it doesn't get down to freezing here. But I'm still curious if there's any other way to repair it besides cutting out the moldy part and replacing it.
 
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Old 01-24-21, 03:54 AM
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Unless you have a magic wand that you bought up in Orlando, I dont think so.

A leaking A/C duct would do the same thing in the humid south. This is why you would poke it with a screwdriver. A leak from rain will be worse on the shingle side and would likely rot the sheathing through and through. While condensation on the inside might only be on the interior surface, and not very deep.

I suppose its possible it could be one time damage from a near miss with a hurricane... wind blown rain driven up... in which case it may not be soft at all... it would just look bad. In that case, remediation would be possible since replacement is not needed.
 
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Old 01-24-21, 04:35 AM
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When I look at the picture I see a moldy area centered on a rafter. It looks like there is also mold to the left of the rafter, and, water marks on the joist supporting the rafter. I think getting the area repaired by a roofer will fix your problem.
I would lift the insulation and get a good look at the area as xsleeper suggested. This would not be easy to do from the inside because you will need something to lay on and there is romex routed above the joists. The best way to inspect will be when the roofer has the area open.
To reduce the mold, RMR 86 has worked well for me. There is some rafter and joist wood that needs treating. Vinegar and hydrogen peroxide are effective also. Bleach will kill the surface mold, but will not penetrate the wood and kill the roots.
 
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Old 01-24-21, 06:48 AM
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I live on the coast near Myrtle Beach and found a similar condition when we bought a 20 year old home. The cause of my issue was water working up under the edge of the roof because the builder did not install a metal drip edge and mounted the gutter so it touched the bottom side of the shingles. There were covers on the gutters and pine needles on top of the covers. Rain coming off the roof backed up when it hit the pine needle dam and soaked up under the shingles.

You should look at the edge of the roof and see if you might have a situation where water can back up under the roof edge and wick up under the shingles. Or possibly blown up under the shingles during storms.
 
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Old 01-24-21, 11:26 AM
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Thanks, guys--very helpful.

XSleeper, the nearest AC duct is about 6 feet away, so there should also be mold closer to it if that were the problem. Hard to imagine it would be one-time damage from a hurricane, since it would probably dry out before it could cause a problem, or if not, it should affect more of that side of the house than just that small section.

jrsick, good point about being centered on the rafter, and I hadn't noticed the joist. Is it practical to cut out and replace the moldy rafter and joist sections, or will applying a fungicide like RMR-86 do it?

I hadn't heard of RMR-86 before, but according to the SDS, it's 4-6% sodium hypochlorite, which is bleach. It also contains a surfactant, so maybe that keeps it in place and makes it more effective.

Reviews on Amazon are mixed--some people say it's the greatest thing since sliced bread, and others say it's just expensive bleach and not worth the extra cost. One of the answers to customer questions says, "bleach degrades and weakens over time and is typically too weak to sanitize around the house about 90 days after it is bought off the store shelf per EPA standards."

The RMR mfr's site says, "Once the stain has been removed to your satisfaction, we recommend using RMR-141 RTU Mold Killer to treat all surfaces after RMR-86 has been applied." Has anyone used RMR-141? It contains n-Alkyl dimethyl benzyl ammonium chloride and N-alkyl Dimethyl Ethyl Benzyl Ammonium Chloride, so I don't know if those would be more effective.

clancy, there is no gutter on that part of the house. Rain just runs off the roof and onto the mulch bed below. From the ground, I don't see anything unusual about that section of the roof.
 
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Old 01-24-21, 12:46 PM
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I don't think you will need to replace the rafter, but you will know better when you get the roof off. I recommend getting some mold killer in between the rafter and roof. If it turns out to have some rot you could apply some wood hardener to the area.
I have had excellent results with RMR 86. It's different than any of the other mold killers I've used because it completely removes the mold stain. The wood looks new. It has a strong chlorine odor and you will smell it in the house after you use it.
It can't hurt to try it on your stain. You can return it if it doesn't work.
I use the 141 also, but it is difficult to tell if it is working or not.
 
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Old 01-24-21, 06:21 PM
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Thanks, jrsick. I'll give it a shot.
 
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Old 02-14-21, 12:32 PM
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So here's the update:

I got three bids. First one, guy goes up on the roof, walks around, says the section above the obviously rotted part is also rotted, because it bounces under his foot. Says it all needs to be replaced. Sent me a bid for $5400, with replacement of any additional plywood at $265 a sheet.

Second guy goes up, I ask about the rot further up, he says he thinks the wood is just "weak"--thin, warped, whatever. Doesn't think it needs to be replaced. Says it would be $800-900 and sends me a bid for $1100.

At this point I wasn't going to bother with the third guy, but since he was already scheduled, I went with it.

Third guy goes up, doesn't think anything but the rotted area needs replacment. Says his guy will go into the attic crawlspace and knock on that area to confirm to the outside guy where it is. Sends me a bid for $450, plus $50 to caulk the nails on the ridge vent, which were bare.

Now THIS was what I had in mind. You can guess which one I took. Below is a photo of the removed plywood--the guy with the high bid was wrong.

About RMR-86: I called the company and asked how it was different from straight bleach. Rep said it contains a proprietary ingredient, the surfactant. I bought some, and the roofer sprayed it on the moldy rafter and joist, but I'm not sure it's any better than bleach. I wouldn't buy it again.

Also: I have 30-year shingles that are 17 years old, but the second guy said they don't last that long in the Florida sun. I asked him and the third guy how much a new roof would be. Both gave me a price of $14k, which seemed kind of high, or maybe I just don't keep up with roofing prices. The third guy said prices have gone up because of covid--the shingle factories can't get enough people to work because of the virus, so they raised prices to keep their revenue consistent. Don't know if that's true or not, but if so, try to hold off on a roofing job until the pandemic has passed and factories are back to normal production. He also said I could get a low-end metal roof for that price.


 
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Old 02-15-21, 06:23 AM
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So did you get an answer to why the wet roof deck in that location? How do you know it will not reoccur?
 
  #13  
Old 02-15-21, 10:19 AM
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Turned out that area had no roofing paper on it, from there to the edge.
 
 

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