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Installing a Custom Fireplace Mantel

Construction How-To, FirePlace, Heating & Fireplace Inserts, Mantels. Trim January 16, 2013 Sonia

By Rob Robillard

Sage advice from a seasoned carpenter.




In the finish carpentry world, fireplace mantels are often the aesthetic showpiece of a room. People use mantels to display family photos, treasured artworks, knickknacks and, more recently, flat screen televisions.


Stock Mantel or Custom Made?

Many fireplaces built in today’s homes are a standard size and easily accept a catalog-purchased mantel. These mantels usually come shipped in a box in three pieces. The three pieces are easily assembled with twist cam locks and a screwdriver. This mantel is then nailed to mounting boards that you fasten to the wall.

In some situations a catalog mantel will not fit. In these non-conforming situations special attention is needed. Custom mantels are generally more expensive. Many companies that make stock mantels will also modify their product and make you a custom-sized mantel based on the styles they offer. These mantels typically cost twice as much and take approximately 4 to 6 weeks to deliver.

Mantels come in many styles and sizes. Determine your design and cut the components to size for assembly.

When pursuing this option, your measurements must be absolutely accurate and communicated to the company just as accurately.

As a DIY’er, it’s sometimes easier to custom-build your mantel so you can continually check yourself against the existing fireplace.


Building a Custom Mantel

I was recently asked to build a custom fireplace mantel to cover up a painted brick fireplace with a wood top shelf. The existing fireplace brick was corbelled out to create a 6-in. shelf, and the wood shelf projected additional depth to that.

Many times my solution is to order a standard fireplace mantel from a manufacturer and simply install it.

In this case, unfortunately, a standard fireplace mantel would not fit. A few years ago the homeowners finished off the basement where this fireplace is located. At that time the contractor installed 2×4 walls and 1/2-in. wallboard along the perimeter basement walls. This resulted in the walls that flank the fireplace sticking out 2 to 3 inches past the facing of the brick—an odd situation that required a custom-made mantel.


Installing Fireplace Facing Material

Material immediately surrounding a fireplace must be non-combustible. Materials such as brick, tile, marble, granite and slate are all acceptable. Typically mortar or thin-set is used to install these materials to the face of the fireplace.

Prior to constructing the new mantel, we had a local granite shop install a slab of black granite for the hearth and a brick veneer wall. The result was a dramatic facelift.


IRC Code

In the 2009 Edition of the IRC (International Residential Code) Section R1001.11 Exception 4: “Exposed combustible mantels or trim may be placed directly on the masonry fireplace front surrounding the fireplace opening providing such combustible materials are not placed within 6 in. of a fireplace opening. Combustible materials within 12 in. of a fireplace opening shall not project more than 1/8 in. for each 1-in. distance from such opening.”

Note that state and local building codes and ordinances can vary from the IRC or may use standards from older versions of the same code. Check your local building authority first when installing fireplace facing material. Ask about the mantel opening and hearth extensions in order to be compliant with local building codes on fireplace combustible materials.


Measuring for the Mantel

In my situation, with the walls built out past the brick facing, there were many things to consider. The fireplace top also meant I had to deal with the brick corbelling along the top.

Once ripped, I ran the stock through the edge joiner and started working on the lengths.