Should I add plywood to a flexing subfloor or replace it?
Adding a second layer of 1/2-inch plywood over a flexing 3/4-inch subfloor can reinforce the wood and pull up valleys, but it raises concerns about building on top of a subpar structure rather than fully replacing it. This leaves you weighing whether to lay down an extra sheet with a primer layer or tear out the original 40-year-old flooring entirely.
What people tried
Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.
- 1Tearing out the flexible wood and replacing it entirely
- 2Screwing down the existing floor before gluing and screwing a new plywood layer
In their words
Unedited, most upvoted first, each linked to the thread it came from.
“Recently purchased a house in SW FL that has 24" OC joists and the 3/4" T&G subfloor flexes a fair bit (over 40 years old). My plan is to sand the floor to remove any ridges and then add a second layer of 1/2" plywood to reinforce and pull up any valleys before adding the final flooring (~1.25" final thickness).”source ↗
“Is there any sort of primer or layer I should add to the original subfloor before I add a second sheet of plywood?”source ↗
“why wouldn’t you just replace the 3/4 subfloor this is flexing? Adding on top of a sub par structural floor seems off to me.”source ↗
Where this came up
People with this problem also raised
- 7Do I need a structural engineer for a sagging floor beam?
- 2Why is my subfloor wavy and dropping?
- 21How to tile over an uneven subfloor without tearing it up
- 2How to fix an H-pattern in installed flooring without tearing it all up
- 5How to fix squeaky flooring joints along the wall
- 2Sudden split and missing chunks in bathroom subfloor