How to Slant the Wings of a Masonry Fireplace Chimney

Build wings on the fireplace if wings are not already present. Use the most cost-effective nonflammable material; the facade will cover an ugly or unevenly constructed substrate. The cinder blocks sold at builder’s yards and home improvement warehouses are ideal; you'll need only one or two bags of ready-mix mortar, likely.

Remove any floor covering that exists under the spot where you'll build the wings. Mark the size and shape of the wings on the floor; the project will be easier if you can use full blocks without having to cut them. Spread a layer of mortar within the shape to approximately 1/2 inch of thickness, then bed the first course of blocks. Repeat the process until the wings reach the desired height -- it is usual for the tops of the wings to be at least a little higher than the opening where the fire burns. Repeat the process to build the second wing, then allow the mortar to dry.

Cut two panels of cement board to the sizes of the horizontal tops of the wings -- this will be your first step if your fireplace already had flat-top wings. Decide on the angle you want the slopes to be, and mark their uppermost extent on the sides of the chimney breast. Cut two more panels of cement board to the size and shape of the rectangles the tops of the wings form, the front and back of the chimney breast’s sides, and the newly drawn line. Cut two further panels, sized to bridge the space between the outside of the horizontal panels and the tops of the vertical panels. Assembled, the six panels should form two three-dimensional right-angled triangles.

Assemble the two triangles using proprietary construction adhesive, applied and allowed to set and cure according to the manufacturer’s instructions. Put the triangles in place on top of the flat-topped wings, and step back to confirm they are at the angle that you desire and that suits the aesthetics of the room, particularly the height of the ceiling. This is the time to revise your plan if the shape is not ideal.

Cut four more panels of cement board, sized and shaped to form backs and fronts to the two triangles. Glue these panels in place, effectively making enclosed five-sided boxes, and allow the adhesive to set and cure. Fix the boxes in place on top of the wings using the same construction adhesive or a mortar mix, and allow to set.

Veneer the entire fireplace, the chimney breast, both wings and both concrete board structures in a suitable, fire-resistant outer layer. As noted, fieldstone is a popular and attractive choice, as are ceramic tiles and faux bricks. Home improvement warehouses often have free project sheets that elaborate on the details of such projects. They're generally available in the aisles that sell the materials and the mortar and grout necessary to make a first-class job.