Riven Slate Hearth Support

[GrantMcscott]

I am putting my hearth in and to is 1800mm x 600mm x 20mm  it is raise of the ground by 45mm so there is a space below it.  The company that supplied it have stated that I need to bed it on cement but I thought it would be strong enough with out doing this and just setting it in.  There is a support underneath where the pink line is in pic

 

 

 

 

 

 

 

[BotusBuild]

If you really mean there will be a 25mm (45-20) or 45mm airgap underneath, I think you are asking for trouble. A heavy object falling on that and I think it might crack. If you are seeking to see a "gap" at the edges, I would create a wooden form that is easy to remove after pouring a cement base on which this lovely piece of stone will sit.