Tiling a fireplace

Hi Forum,
One of our New Years' resolutions is to finish our fireplace by adding marble tiles, a hearth & mantle. Another resolution was to try doing it ourselves.

After adding the hearth we were planning to tile the bottom section up to the bottom of the first step, then tile the mantle section so that the bottom of the tiles would sit over the first step. We'd then add a marble "moulding" to close the gap between the mantle tiles and fireplace tiles - in effect making one large mantle and eliminating the middle step.

However, the mantle face doesn't protrude an equal amount at both sides, the right side sits proud of the fireplace wall by about 40mm and gradually tapers down to the left side which protrudes only 25mm. This means the moulding section will have to taper and we cant find anyone who can cut such a section for us.

Can anyone suggest a way to build up the mantle face so that both sides protrude the same distance from the fireplace wall? We'd prefer not to cut away the proud side as the mantle & fireplace surface are a very hard cement-type render over bricks and the rest of the room is finished.

Any other suggestions would be most welcome!

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If you don't want to cut then your only alternative is to build up the short side. I thought of wood but the taper might be hard.

I thought of keying the face with concrete nails or scresa, building a form and ucing cement or builders bog. The trouble is that it would be messy and not sure how it would hold up.

I would use cement sheet with wood battens to pad it out using enough battens to ensure ther was no flex. You would have to notch the end of the sheet into the end unless you were prepared to increase the overhang to 46mm or so.