Hi,

I'm new to Houdini so this might be a very silly question.

I'm playing with Voronoi Fracturing.

Got it simulating with a small cube but when I replace that cube with a simple stair.obj model, it seems there is some gaps.

On the static object under Collisions > Bullet Data, there's a ‘Show Guide Geo’ toggle that you can use to visualize the collider.
It's probably doing what you're showing because it's represented as a convex hull. Changing it to concave should help.