Is the gap between the floor joist and hearth a structural requirement?

[fire_man]

I'm extending my brick Hearth 16" to meet my stove's front clearance requirements. There is a 1/8" gap (cardboard is inserted in gap in photo) between the floor joist and original concrete hearth extension which will get filled when I pour cement to fill the 2" gap between the subfloor and old brick hearth.

Is the 1/8" gap necessary for thermal expansion to isolate the masonry from the floor joist?

 

[Highbeam]

That crack was almost certainly not intentional. Both wood and concrete shrink as they dry.

 

[fossil]

And houses subside.