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filled Sunken den - now rotten- help!

Eighteen months ago we filled in a sunken den on a slab floor with a wooden decking, then covered with laminate flooring. a couple months ago, we noticed soft spots. When we pulled the floor up, all the wood was rotten! No evidence of standing water, don't think there was an kind of leak. Could the fact that the floor had no kind of air flow cause condensation to build up and rot the wood? Help! We live in south Louisiana.

Re: filled Sunken den - now rotten- help!

Possible but more likely because you didn't install a moisture barrier over the concrete. Did you use PT wood?


Re: filled Sunken den - now rotten- help!

Adding a moisture barrier isn't a wonderfully effective way of stopping the water from passing through the slab. Hydrostatic pressure usually wins. By capping the wood deck with a nearly air tight cap, you trapped the moisture below the flooring, rotting out the wood.

I'd recommend filling in the area with packed sand to a depth 3" below the finished height, then finishing the top smooth with concrete or "deck mud" made of five parts sand to one part portland cement, which gets re-enforced with 2" welded wire mesh. With either method an expansion gap around the perimeter is needed and recommended in case anyone wants to 'restore' the house to original.

Re: filled Sunken den - now rotten- help!

Talking to a few people, we now realize that by sealing this almost airtight, this caused the problem. However, the one thing that puzzles me is that one area is really bad (25% of the floor), completely rotted, while the rest of the floor is fine. We did not use PT wood for the deck, and we did first install tar paper as a moisture barrier.

Re: filled Sunken den - now rotten- help!

There are waterproof mopping products available in the shower section of you hardware store. These are for waterproofing a mortar bed of a shower and are applied with a mop. I would suggest using this on the concrete. If you or someone else decides to return to the sunken deck, the concrete would get covered by something, carpet, tiles, wood, etc.

Then use PT lumber for the joists, or if you are concerned about the chemicals in PT lumber, then use borate treated "blue wood" or treat the joists with a sodium borate solution like Boracare or Timbor. Sodium Borates are used in cellulose insulation to keep it from molding, rotting and to make it fireproof. It also resists termites and other insects.

Then cover the joists with a heavy duty plastic vapor barrier, not tar paper. Tar paper is a water barrier intended to shed water from vertical surfaces, it is not a vapor barrier. Then put down the plywood deck and the final floor finish.

If you have your HVAC ducts in the sunken floor, still waterproof the concrete, but use regular lumber, skip the vapor barrier and do not use any ducts in the sunken floor area. Have the ducts blow into the cavity and then just cut the vents in teh new floor. Your HVAC system will then keep the cavity dry.

Re: filled Sunken den - now rotten- help!


I also live in south Louisiana, and had this exact same problem happen. Previous owner installed this raised floor and now 3 years later the wood is rotting underneath. How did you wind up resolving this? I’m wondering if this is a sign of slab moisture problems. 

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