hearthstone on timber joists

[BigClick]

Is it ok to lay a slate hearthstone directly on to the floor joists?

The wood burner is approved for lower temperature below grate.

Was thinking of just making the joists level and then sitting the slab on that without any mortar or adhesive?

The joists would be 100 x 50 section and about 850mm long supported about halfway along by a strange wall across the grate (see pic).

I was going to use one at each end and three in the middle. Its 1400 wide

Thanks

 

[geraldthehamster]

When I did something similar I built up in blocks from the oversite under the floor.

 

[BigClick]

When I did something similar I built up in blocks from the oversite under the floor.

Thanks,
When you say oversite do you mean you filled in the hole with blocks?