Moisture readings skewed by minerals in plywood

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Forum topic by EricinNorCal posted 03-16-2012 12:22 AM 1290 views 0 times favorited 5 replies Add to Favorites Watch
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2 posts in 2436 days

03-16-2012 12:22 AM

Topic tags/keywords: moisture meter false readings plywood mineral

Hello – I’m new here. I’ve been a professional restoration contractor for many years and have come across instances of plywood that is dried for extended periods of time through conventional means using dehumidification and air movement yet still tests “wet”. I’ve come to know that slow leaks can allow mineral from tap water to build up in the wood and that those minerals cause the moisture meter to show a wet reading due to conductivity of the minerals. Does anyone have more info on this subject? I’m working with a client with a plywood subfloor that is heavily stained from a long-term dishwasher leak, but there is no delamination or rot present. The plywood surface is cracked and brittle and the material appears and feels very dry (after 2 weeks of dehumidification), but the floor installer insists that it’s still wet because his meter says so. If anyone can point me in the direction of more in-depth professional information about this subject I’d really appreciate it.


Eric Stockwell

5 replies so far

View cabmaker's profile


1740 posts in 2982 days

#1 posted 03-16-2012 01:03 AM

I would say that if you had sustained leaking for a period of time that you would certainly want to replace the affectted area. The cracking ply veneer tells me that it has been wet and delaminating allowing inner llayers to expand and contract. Dont need a moisture meter for that. I well understand that as a restoration pro. you see lots of this. Whether its wet or dry presently, just do the right thing, thats why the property owners have insurance.

View Dusty56's profile


11822 posts in 3861 days

#2 posted 03-16-2012 01:06 AM

Is it possible to take a “core” sample using a hole saw ?

-- I'm absolutely positive that I couldn't be more uncertain!

View Jorge G.'s profile

Jorge G.

1537 posts in 2648 days

#3 posted 03-16-2012 01:15 AM

It is possible although I doubt it. When you read percent moisture you are actually reading the resistance to an electrical current (mhos). Your meter converts this resistance to % readings. You would have to have very hard water, and I mean really hard to deposit such an amount of salts on the wood so that is would read as wet.

That said, have you calibrated your meter? Have you zero it out? If you have not done any of this, then probably the meter is at fault and not the wood/plywood sample.

On the other hand if the plywood is on the floor, although it might appear dry on the surface it is possible it still has moisture trapped in the layers. Which sounds to me a more probable scenario. I would listen to your floor guy, or try and remove at least one sheet of plywood and see if it still has moisture on surface in contact with the floor.

-- To surrender a dream leaves life as it is — and not as it could be.

View EricinNorCal's profile


2 posts in 2436 days

#4 posted 03-16-2012 04:21 PM

Thank you for the responses!

As for replacing the plywood subfloor, it runs below cabinets, and I was hired to dry it. The surface has very minor cracking, but there is no delamination of the plywood. Insurance doesn’t cover long term leaks. The plywood is definitely in good enough shape to put a new floor down on.

Core sample is too much for this little area.

Zero out the meter – the process of relative testing entails finding a “dry standard” – this is easy when most of the plywood subfloor is unaffected and unstained. The dry standard of 13-16% (average for our area) was established, of course.

View Jorge G.'s profile

Jorge G.

1537 posts in 2648 days

#5 posted 03-17-2012 02:18 AM

Ok, here is the thing, how much responsibility you want to take? If it was me I would unscrew one of the cabinets, cut a small piece of plywood and see if it is really dry. Since the plywood has cabinets on top of it of course you would not see any delamination, it has weight on top plus it is secured to the floor.

Looks to me you have made up your mind it is dry and you are looking for a way to prove it, the only way is to cut a small piece. Other than that I don’t know what to tell you. Good luck.

-- To surrender a dream leaves life as it is — and not as it could be.

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