I have a 60 year old house on long island with a back door at grade level that exits onto a concrete patio. The door has a wood sill plate that seems to be the cause of some recent water and mold damage. I know that having a door at grade level is not ideal (and not code in many areas) but i don’t have a good way to change that as the wall height right there is less than 7 ft due to some plumbing lines in the ceiling. My question is, when i replace the sill, what can I use to better waterproof that door sill to avoid future problems?
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You can, of course. replace the wood threshold with metal. In addition to not absorbing water and not itself being subject to rot, the metal threshold will expand/contract less and be less apt to tear itself loose from the concrete at the caulked joint.
Another thing to do is to get the concrete surface perfectly flat, so that the sill seals to it well, with no gaps. I've used thinset to fill in gaps under a threshold, to good effect. And with a metal sill you can actually create a small ridge in the thinset under the sill (as there is inevitably a lengthwise slot in bottom of the metal sill), so that gravity helps to seal things. (Could probably do this with wood too, if there is a decent size slot in the bottom.)
In any event, you need to pay careful attention to sealing the ends.
jb
Is this a wall plate or part of the door (threshold)?
If a plate that merely fills in the space the door sits on, you could certainly change it over to a pvc material (Azek for example).
I would first install a pan (either prebuilt or one made up of a flexible peel and stick flashing). You want to go up the sides of the opening as well as create a damn at the rear. This will keep the water out and allow it to drain to the exterior.
Then, hopefully you have a metal threshold that runs along the bottom of your door unit, so there's no soaking up of moisture. Install the unit, taking care to weatherproof up the sides and across the head.
An awning or roof overhang above will help immensely in limiting the amount of water introduced around the door.