Victorian fireplace

Hello,

I am renovating the fireplace in the front room of my Victorian terrace.

I have dug out the old hearth which wasn't in a good state, and dug down a couple of foot of what was pure dust. The timbers are all nice and dry. I am now looking to fill it with cement and tile on top. To lay a solid base ive filled in the space with three layers of paving bricks I had going spare, around 10cm per brick, 30cm in total. I plant to fill in the remaining 4cm with cement. Would this constitute 125cm constructional hearth which I believe you need to have, or should i aim to do this with concrete.

Should I use a DPM under concrete or at all?

Thought I should add, in the long term I plan on tiling the hearth and having a log burner installed.

I'm not a builder so someone may correct me here if I'm wrong, but from my understanding a constructual hearth needs to be 125cm of non combustible material capable of absorbing the heat of the fire.

If you're going with a burner in future, you can get burners certified not to heat the hearth below it to more than 100c, meaning you dont need a constructual hearth 125cm deep and can instead get away with much shallower hearths.

I guess the answer is "it depends"