Hi there,
Try experimenting with different height offset values. Looks like you using code similar to this thread: https://groups.google.com/forum/#!searchin/cesium-dev/height$203d$20tiles|sort:relevance/cesium-dev/CqYeu6L6bhU/P0WywsBhBQAJ , in which our resident 3D tiles expert Sean explains how to position tilesets.
Hope that helps,
- Rachel