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Community and Q&A

Soaked subfloor: Advice on what to do?

Tommy87 | Posted in General Questions on


A huge storm rolled in over the weekend while I was out of town for the weekend(several inches of rain and hail). I actually have 90% of the roof sheathing done so majority of the house floor is fine but the end that was exposed got soaked. When I got back home, much of the section that is wet has black spots all over it.

I’m not sure if this is mold or just dirt/mud/some other substance. The likelihood of mold growing in 3 days seems low but there could of been some organic materiel on top of the wood to encourage growth. What do you guys think?

Anyways, what do you guys think my plan of action should be? Should I let it air dry out or (because it’s such a small section) use a skill-saw to cut out the exposed areas of plywood and replace with new pieces?

I have a few concerns
1) Will the subfloor dry out adequately under the bottom plate (since they are sandwiched together)?
2) Do I need to worry about my insulation underneath (Rockwool)?

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  1. Expert Member
    Malcolm Taylor | | #1

    Here in the PNW if we didn't frame in the rain we almost couldn't build at all. Wet framing isn't much of a problem as long as you ensure you wait until it is dry enough before insulating.

    That said, I've never seen black marks like that on my subfloors.

  2. Tommy87 | | #2

    Hi Malcolm,

    That's the truth!

    Yea, I'm not sure if it's mold or mud. What would you do if you were in my shoes?

    Would cutting out the affected plywood be ok? I would hate to start off the build with mold on the subfloor.. I suppose I could spray mold cleaner and sand it off as well.

  3. Expert Member
    Malcolm Taylor | | #3

    Unless the sheet delaminates or goes mushy I'd leave it alone. Once it's dry, if you are still concerned you can remove the stain with bleach and encapsulate it with Concrobium.

  4. Tommy87 | | #4

    Thanks Malcolm. I'll keep an eye on it over the next week. Hoping for some dry weather.

    So should I not be concerned with the rockwool insulation underneath?

  5. GBA Editor
    Martin Holladay | | #5

    During a construction project, wet subflooring is normal, as Malcolm correctly points out.

    Insulating your home before you have a roof is not.

  6. Brian Knight | | #6

    Metal shavings or pulled nails could be causing the stains.

  7. Tommy87 | | #7

    Brian you may be right.

    Martin - I know this was not normal. The roof was almost finished and I was not expecting rain.

    Given that standing water sat on the plywood for probably 36 hours and I DO have my rockwool insulation underneath, I'm curious what you guys would do?

    It's clear that you would just let it dry out if no insulation was underneath, but how about given my situation?

  8. GBA Editor
    Martin Holladay | | #8

    Mineral wool insulation isn't usually harmed by water, but we really can't answer your question unless you describe all of the layers in this floor assembly that have already been installed.

  9. Tommy87 | | #9

    Hey Martin. There is a vapor retarder membrane underneath the plywood (acts as vapor barrier but opens up to 15 perms when there is a lot of water). Then it's the mineral wool.

  10. GBA Editor
    Martin Holladay | | #10

    Care to be more specific about the "vapor retarder membrane"? Is it asphalt felt? A fancy membrane from Europe? Do you know the brand name?

  11. Tommy87 | | #11

    Hi Martin,

    Sorry about that.

    The product is called CertainTeed MemBrain

    It states: "In summer, when humidity is high, it increases permeability so that moisture can escape, as air tightness is maintained. In winter, when humidity is low, it continues to block moisture."

    Was thinking about cutting a small circle out of the subfloor to see if the bottom is damp or not.

  12. Steve Knapp CZ 3A Georgia | | #12

    You could rent a couple of fans from the big box store to accelerate the drying process and possibly prevent mold and mildew issues.

  13. GBA Editor
    Martin Holladay | | #13

    We need more information.

    OK. You have joists that form joist bays. The joist bays are insulated with mineral wool. Right?

    Above the mineral wool there are two layers: MemBrain and plywood. Right?

    What's under the mineral wool? A basement? A crawl space? More plywood? OSB? Drywall?

    I have never heard of anyone installing MemBrain under a plywood subfloor. It's very odd. Plywood is already a smart vapor retarder.

  14. Tommy87 | | #14

    So I ended up cutting put the Subfloor today...and whadda ya know, the bottom was moist and there was black mold on the bottom end. This is because the end was sitting on top of epdm rubber gasket that was hanging outside the bottom plate a bit.

    Good to see no mold underneath majority of the subfloor (above the insulation) but I have to think the bottom of the plywood under the bottom plate also has mold because it's sitting on epdm.

    Removing the Subfloor out from underneath the bottom plate is out side my skillset. Any tips?

    Malcolm- do you think if I just sprayed concrobium around the entire bottom plate, and that it would take care of any potential mold spores under the bottom plate /subfloor?

  15. Expert Member
    Malcolm Taylor | | #15


    I've had a lot of luck encapsulating with something like Concrobium, but I'd let things dry a bit first.

    One of the reasons I was reluctant to suggest removing the plywood is that it creates the problem you now have. The plywood under your bottom plates was part of a structural diaphragm tying the floor together and any joints that don't fall on joists below or aren't t&g need to be blocked. That's sort of a reno solution, not something you like to see in new construction.

  16. Tommy87 | | #16

    Good to know. I'll let things dry first. I have a few fans pointed at the bottom plate. Right now it's reading over 70% moisture so it's soaked. Warmer weather all next week so hopefully it will dry in a week or two.

    Agreed- definitely not ideal, but it was something I had to do. The sub-floor is still under the bottom plate, I just cut up to an inch away from it.

    Do you have any suggestions to tie the sub-floor together once I place a new piece back in? Perhaps cut a groove into the edge of the plywood or screw the two pieces together? At the very least, I will use some sort of glue between the two pieces.

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