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Repairing A Severely Rotted Subfloor

Construction How-To, Flooring Installation, Floors March 15, 2016 Sonia








By Larry Walton

Stepping through the floor was not the first hint that it had rotted out, but it sure moved the floor repair up on the priority list for a house with plenty of remodel projects to do. Other clues about the floor damage included discolored flooring, mold along the wall and lots of opportunity for water leaks with washer, toilet and water heater all in the vicinity.

If you want to make a distinction between dry rot and wet rot, this was quite wet, which means that the fungi that caused this wood damage were the types that prefer a wet environment. Dry rot involves less water, but the destructive fungi that cause dry rot still need some moisture to do their wood fiber-eating thing.

The homeowner had recently completed some plumbing upgrades, which took care of the water source for the rot, but the floor damage was already done. The demolition stage of repairing a rotted section of subfloor is like cancer surgery. Once you get started, you must keep following the path of the water-born destruction until you reach the full extent of the damage. You don’t always know where the surgery will take you. Sometimes it extends along subfloor framing for some distance. It can also damage the wall framing around it.

This project has all of the above, which makes it ideal for demonstrating subfloor repairs. The damaged area was large, it involved subfloor framing, it extended into two rooms and there was damage to the partition wall, which happened to be full of plumbing. We dealt with most of the worst-case components you might run into in a subfloor repair.

Subfloor repairs can seem very radical, but keep in mind that the original construction process was just putting boards together. Not going deep enough to address the full extent of the problem may seem easier at the moment, but covering it up will come back and bite you. The wood destroying fungi will continue to grow if not removed, and anything the subfloor system supports will be vulnerable. So do the radical surgery and put it back together better than new. Here’s how we did a recent subfloor repair.


Step by Step

The worst of the subfloor damage was found with a foot going through the floor. It didn’t take much to dig up some of the flooring, plywood underlayment and the 3/4-in. shiplap that served as the subfloor.

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Pulling back more of the underlayment revealed that the water-fueled damage spread more than eight feet along the wall. Further exploration uncovered additional water damage in the adjacent room.

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The damage to the subfloor extended under the wall and up into the bottom plate on this plumbing wall. To properly eradicate the damage, the repair would need to include the floor under the wall and at least the wall plate.