Proudly American
Name everything the proposed bench must carry
An installer needs to know whether a projecting slab is a finish ledge or a seat someone will actually use. Give its intended use, extent, thickness and proposed support points to the person designing its support. In the VWDV70 manual, for example, the manufacturer calls for suitable structure to carry a mantel at certain attachment locations. A facing panel alone is not a general-purpose structural support. Consult the actual model and obtain suitable structural advice for the proposed bench.
Draw the junction in section
One line should show the finished top of the stone. Another should show the bottom of the removable fireplace front and any air openings. Add supporting framing, a finish joint and the space required to reach controls. A design that supports the stone can still trap the front; a design that clears the front can still transfer a load into an inappropriate part of the appliance.
Agree on sequence and future removal
Ask the fireplace installer to approve the interface and the stone trade to confirm how the slab is supported independently where required. If the bench projects into the space normally used for service, ask how that appointment will work after installation. Resolve those details before a full slab is fabricated, and keep the final section drawing with the template.