Castro District Entryway, January 2026

This entryway in a Castro District duplex was a little problematic from start to finish. The tile is a marble herringbone mosaic, which has to be installed very meticulously. Its joints are smaller than a sixteenth of an inch, so there’s almost no room to hide an error.

The surprise came at demo. When the old tile came up, the underlayment underneath was wet. The owner planned to sell this two-story duplex, and a water problem could have been very serious: a hidden leak under an entry floor can mean rot, mold, and a failed inspection. We started looking for where the water was coming from. We set up fans, let the subfloor dry, and watched to see if the moisture came back.

A couple of days in, the owner remembered that he’d sprayed down the entryway with water to flush out a spilled gallon of paint. Mystery solved. After a few more days of drying, we were back in business. It’s a good reminder to always get to the bottom of wet framing before covering it up. This time the answer was simple, but you don’t know that until you look.

With the subfloor dry, I installed backer over it and began setting the stone. Herringbone mosaic is all about alignment. The zigzag pattern exaggerates any drift, so each sheet has to be set and checked so the lines run true across the whole floor.

The grout was the next problem. Polished stone doesn’t strictly need a sealer, but for some reason the black grout stuck to the surface of the marble. When I tried to buff it off, it left the surface uneven, with light spots and dark spots across the floor. It was very problematic. In the end, the only fix was to abrade the entire floor down to the same level so the finish was uniform everywhere.

That was very time-consuming. But the result is a consistent, even surface that will last forever and looks very high-end, exactly what you want to greet buyers at the front door.

Lesson learned: with light stone and dark grout, a sealer or grout release applied before grouting is cheap insurance against a much longer job afterward.