How to Add a Mantel to a Fireplace

Between the twelfth and nineteenth centuries fireplaces were practical devices used for heat and and cooking. Fireplaces were still used even after other sources of heat and cooking became available. During the mid twentieth century home design went through a modernistic stage which was slightly cubist and stark with wide open spaces. Furniture was metal cubes or one piece plastic sculptures. Fireplaces dropped all pretense of practicality and were sometimes simple openings in brick, rock, or wood walls. Mantels were largely abandoned. Sometimes a few shelves of metal and glass replaced the mantel. As a building and custom trim contractor I added many mantels to these rather plain fireplaces. Here’s how I built and added a simple box mantle to my own 40 year old home.

The box mantel is exactly what it sounds like; a simple box of appropriate size and shape attached to a wall or fireplace. This is an easy method for installing shelves as well. The enclosed box provides size and presence to the mantel and lets us use a hidden installation method. Length doesn’t matter, but the depth, or width of the shelf probably shouldn’t be more than 12 or 16 inches unless you install legs or pillars for support. For this explanation I will use the 8 inch by 6 feet dimensions of my mantel for convenience. Adapt the dimensions and type of wood to your situation. The National Building Code requires all combustible materials to be at least 12 inches above the fireplace opening. Keep this in mind.


  • Four pieces of 2 by 4 cut 72, 6 1/2, 6 1/2 and 69 inches.
  • Two pieces of 1/2″ plywood 8 by 72 inches.
  • Two pieces of 1/4 inch mahogany plywood 8 1/2 by 72 inches.
  • Eight feet of 5 1/4 by 3/4 inch solid mahogany lumber.
  • Eight feet of decorative mahogany trim for the top edge of the mantel.
  • Appropriate finish nails or screws and glue.
  • Filler, stain, and varnish.

Building steps

  • Stand the 6 1/2 inch pieces of 2 by 4 on end. Nail or screw the 72 inch piece to these. Lay this assembly on edge and make sure that the 69 inch piece slides between the short pieces. Don’t attach the 69 inch 2 by 4 yet. You will have a simple square frame 72 by 8 inches.
  • Glue and nail the 1/2 plywood top and bottom but not to the 69 inch 2 by 4. Remove the 69 inch 2 by 4. You will have a box with one open side.
  • Glue and nail the 1/4 inch mahogany plywood on top and bottom of the box. You will see that the plywood overhangs on the open backside by 1/2 inch. The overhang is used for scribing to rock, brick, or a badly warped wall. You can eliminate the scribe overhang if it’s not necessary.
  • Attach the 69 inch 2 by 4 to the fireplace or wall at the appropriate height. Make sure you meet all building codes. Use screws or nails securely attached to studs or masonry surfaces.
  • Slide the “box” over the attached 2 by 4. Scribe to the wall, remove and cut. Continue these steps until you are satisfied with the fit. Temporarily attach the “box” to the wall 2 by 4 with a couple of nails or screws. I prefer screws.
  • Trim the exposed face and ends of the box with the solid mahogany lumber. Use 45 degree angle cuts at the corners. Glue and fasten securely. The solid mahogany will hang down 1/4 inch on the bottom but be flush on top.
  • Add the decorative trim to the top face edge as appropriate. Use glue and appropriate fasteners. I use 4d finish nails.
  • Remove the mantel to sand. Stain, fill holes, and varnish.
  • Permanently install the mantel with screws at the top and bottom.

You’re done! Stand back and admire.

More from Gerald:
For the Home Toolbox: Wood Chisels
Spring is a Great Time for Exterior House Painting
Fix the Falling Towel Bar

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