hearth for gas fire design?

Hi, I'm making a tile covered cement hearth (back and front hearths). The wooden floor (over a cellar) level needs to be raised 3 inches to match the other floor levels. The wooden floor goes right up to the fire place opening (no constructional hearth).
I'm raising the floor level by battens then chip board flooring.
1. do I raise the floor then make the hearth - approx 2" high plus tile.
2. make the hearth first - would be approx. 5" high plus tile thickness.
3. is it ok to put some plastic sheeting etc. under the cement hearth to protect the wooden floor?
Thank you, Neil
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If you get the floor level first, then making a shuttering to the shape you want the new hearth to be, it should be easy enough to pour your concrete mix into this and make your hearth. Remember to line the inside of the shuttering with a polythene membrane before pouring your concrete mix.
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