Here is scenario:

  • House has cement foundation/slab
  • An asphalt mastic was put over foundation/slab as a vapor barrier
  • 5/8 inch plywood put over asphalt mastic and nailed into foundation
  • Heavy plastic put over plywood
  • Hardwood flooring nailed into plywood -- Plywood was completely soaked due to slab leak over a period of time.

We pulled up hardwood flooring (nothing wrong with hardwood floors) and most of the plywood had deteriorated to point of falling apart/mushy.


  1. How long could this plywood stay wet between the two moisture barriers?
  2. How long does it typically take for plywood to deteriorate to the point of being mushy?
  • 1
    Plain internal plywood would probably not take long to be mushy, maybe a month or two. Between two vapour barriers, will take a long time to dry, but it is ruin now replacement only choice. Plywood usually has layers of real wood veneer and fake/particle wood.
    – crip659
    Feb 2 at 1:04

1 Answer 1


Question 1: - more or less forever, which is why you use ONE such barrier, not two. Trapping moisture between vapor barriers leads to rot...

Question 2: - depends on the quality of the plywood, the temperature, etc. can happen in months under favorable (to the rot) conditions. Could take years.

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