Sounds like a mildew trap to me not to mention it won't look good. Can you not just replace that tile and regrout? It doesn't have to be with one piece. Is there a photo of your niche?

corian

You could use an impervious through body porcelain and scribe fit it, and silicone it in place with some composite shims to pitch.. I would first remediate moisture and clean things . Blowdryers work great for heating up and mitigating the existing moisture after you clean for mildew.

A patch repair that would buy you time. Not the proper way...... yet a cost effective solution

I agree that a solid surface piece will be the most effective. You can use a small grinder to take the original tile out (if water sat and got behind it, it will be even easier to remove then). You can use a piece of kerdi band to ensure that its water proofed at that point. You could probably get an off cut for a fairly reasonable price if you're not terribly picky on material/colour.

There are a lot of different thicknesses available in solid surface 3/8" is the smallest I've seen. Depending on the width of your niche you might be able to use a large scale tile instead. I would call a good quality smaller tile or counter company. they are more likely to have a better solution.

Yes, Corian works. That's what we used for our shower threshold and it's a great choice.

joobemelon:

Silicone waterproof plastic shims in the back thick enough to make the solid surface sill pitch water forward. Having fitted the sill for a 1/8" perimeter gap, place a dab of hot melt on top of each shim, place a bead of silicone across the front, and place the sill with the gap. Over fill the gap with silicone slightly, spritz it with Windex, spit on your finger, and remove the excess silicone. Wipe up the Windex without touching the fresh silicone.

@home depot , eco shims and GE brand 100% silicone II, maybe $10 total.- best

@joobemelon

yes silicone will stick to nearly all types of properly cleaned and dry tile stone and solid surface.

Again with the proper cleaning and drying first, you can also add a good rubdown on the tile with denatured alchohol or acetone to remove any surface oils. before siliconing.

"Do solid surfaces come with a rough edge like tile? If so how could I either chamfer or bullnose it?"

Yes and yes. You can have any edge you want on solid surface. Gloss to chiseled.