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I am trying to install the following door threshold on a concrete slab. The door leads outside and the old threshold was rotted from water penetration. The following pictures show the concrete subfloor and the rotten threshold.
concrete slab

concrete slab

rotten threshold

The person that installed the old threshold first placed a bunch of caulk directly on the concrete slab, then placed a sheet of plastic over that, and then placed the threshold over the plastic and held it in place against the door frame with some staples.

I believe that the plastic might have contributed to the rotting by trapping moisture. Is the correct way to install the threshold to the concrete slab to:

  1. Lay down a bead of caulk (I have the DAP Alex Plus) at the very edge of the slab to prevent water from running under the threshold,

  2. Add more caulk to the rest of the underside of the threshold and put in in place on the slab,

  3. Fasten the threshold to the concrete with screws?

And should I leave the plastic sheet out of all of this?

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Placing a plastic sheet between a slab & wood which contacts it is a conventional practice. It is intended to prevent moisture from migrating upward from the concrete into the wood.

Given that the plastic was in place, the concrete itself was probably not the source of the moisture which rotted it out.

The problem may be that the slab which the threshold sits on is basically at grade level - i.e., its right at the same level as the outside surface. This would mean it is fairly likely to stay wet once it gets wet (i.e., rain) and will gradually rot.

Possible solutions:

  • Use a new threshold made of plastic or aluminum

  • Install with 100% silicone caulk. Silicone is much more long-lasting & water-repellent than other (e.g., acrylic) types as far as I am aware. This would be similar to a product used to seal a bathroom tub.

  • Improve outside drainage around the door area.

Given the proximity to grade level I would also be concerned about water flooding the interior.

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Even when swept clean I would not consider that a properly prepared surface for a threshold. Just the nature of the uneven surface of those filled concrete blocks would make water tend to collect underneath. You should use a concrete patch/resurfacer to create a smooth level (or slightly sloped away from the opening) surface, then attach the threshold per manufacturer's instructions. All the retrofit thresholds I have seen or installed need to be screwed down, yours included. You will need to mark and drill into the concrete and use an anchor to screw it down.

  • These are the installation instructions which are less than helpful at answering the questions I have regarding caulk and the plastic sheet. – derNincompoop Jul 27 '15 at 15:36

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