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The sandstone doorsill below the wooden threshold is eroding at the side door of my 1890 wood frame Victorian home. The stone face has begun to spall. I want to fill the gap to keep out water and critters, and retard further erosion.

Two parts to the question:

1. What should I use to fill the gap?
2. What should I do to preserve the sandstone?

The protocols I have found for repairing sandstone were written with brownstones and other historically significant structures in mind. They prescribe very time consuming and expensive procedures, involving drilling holes and cementing anchors in them, removing at least 1/2" of the surface (because the patching mix may fail if it's thinner), undercutting the surface with a thin diamond blade to hold the patch, drilling holes and cementing in anchor pins, and finally color matching the special sandstone cement mix. Portland cement and other commonly used patching compounds are eschewed because they are too hard or not permeable enough to allow moisture to escape, accelerating further deterioration.

The house has only the one sandstone sill; it's on the side of the house where it's not prominent. Historic preservation is not a requirement. It's not a major architectural feature. Still, I would like to keep it. Yet I'm balking at the amount of work required to do it as one would to keep an historic brownstone up to snuff. In short, I'm looking for a way to fill the gap to keep critters and water out, provide support for the threshold, and retard, if not prevent, further erosion.

Ideas I have considered: To fill the gap, foam caulk would seal well and be easy to apply, but provide little support for the threshold, which will get stepped on. Clear sealants (such as silicone, clear rubber coatings, and clear acrylic paint) would keep moisture out - but also in. Maybe coat the top and face, but leave the bottom open to breath?

Recommendations please!

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  • +1 Well explained, good photos. Hope you get a good answer. Commented May 27 at 2:29
  • Do you want to retain its present appearance? It looks like there used to be something adhered to the front—is that correct?
    – Huesmann
    Commented May 27 at 12:18
  • Not trying to restore a look. Nothing has been attached to front face in the 35 years I’ve had it. It does appear a sandstone “fillet” may have gone missing. I don’t know how or when or even if it detached, or whether it happened gradually or all at once. I’m ok with the current appearance.
    – MadMonty
    Commented May 28 at 17:54

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