Installing a Custom Fireplace Mantel

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.

In my opinion, the best way to build a fireplace mantel is to use a story pole, take good measurements and snap a few pictures. A story pole is a length of narrow board, often a 1×2. It was used long before tape measures were available and was usually cut to the height of a floor or a “story.” It is a layout tool for measuring, marking and transferring accurate measurements. It eliminates the tape measure and, once marked, can be used throughout the project to continually check your progress.

A mantel is basically a hollow box with hollow legs. The components were joined with wood biscuits and glue then secured with a variety of clamps.

I used the story pole to mark and locate all of the important measurements for the soon-to-be-built mantel. Make note of measurements such as depth, height, width, build-outs, recesses, etc. I use all four sides of the story pole to record measurements and label each side appropriately.


Mantel Dimensions

Opening height: The opening height of the mantel is determined by measuring the height of the fireplace facing material and subtracting 1/2 in. from that height to allow the mantel to overlap the facing material (granite).

Opening width:The mantel opening width is determined by measuring the width of the fireplace facing material and subtracting 1 inch (1/2 in. for each side).

Prior remodeling resulted in the walls sticking out 2 to 3” past the facing of the brick—an odd situation that required a custom-made mantel. The edges of the mantel legs were sized to accommodate this obstacle.

Depth of Return: This measurement ensures that your new mantel will cover your protruding fireplace facing materials. Fireplaces have facings of brick, tile, marble, granite or slate. You want your mantel to be installed flush to the wall and to cover this facing material. Measure the distance this facing material sticks out from the wall. Add 1/4-in. to the thickness of the protruding facing materials when figuring your sides on the fireplace opening side to allow the mantel to completely cover the fireplace facing material. On this installation the brick corbelling protruded past my fireplace facing materials. I used the corbelling as my measurement.

Shelf width and depth:For my project the moldings that we were using determined the width. I used a scrap piece of molding to determine the overall width of the molding returns, and then added for an overhang.

Once the main box is assembled, fill all seams with putty and sand the surface smooth.

Make sure you have the room for the shelf length. Sometimes the shelf length can be limited due to limiting factors such as situations with flanking book-cases, windows or doors.


Cut Components to Size

This mantel was going to be painted, so I used furniture-grade Birch plywood and solid poplar wood. I like poplar because it’s reasonably priced, relatively knot-free and easy to work with.

I start by figuring out what widths I need for stock and rip all of the parts at one time. Once ripped, I then run everything through the edge joiner and start working on the lengths.


Constructing the Mantel Frame

A mantel is basically a hollow box with hollow legs. There is a top shelf, top face and two legs with sides. In this situation I needed my top face to cover the brick corbelling and overlap the granite facing.

The first measurement you need is the inside width X height of the mantel, and then the overall width and height. (Follow your local code for fireplace combustible clearances.)

I needed to make sure the mantel would cover all of the brick and fit tight to the wall. In my situation the outer leg-side edges were going to be shorter than the inside leg-side edges. This was because the plaster walls were protruding farther than the granite face.

Using the story pole to get my measurements, I began to assemble the mantel box. The large top face and then the two face legs get assembled first. To do this I used a biscuit jointer with biscuits and glue, which make these butt joints strong and easy to align. Clamping the legs requires long pipe clamps. I have a limited amount of long pipe clamps, and in these situations I join two smaller pipes together with a pipe coupler.

This mantel was decorated with a variety of compound mouldings for a classic look.

TIP: When I need to clamp long parts together, I use a double-threaded union to join two clamps. I keep 3/4-in. diameter pipe clamps (2-, 3- and 5-ft.) in my shop and join them with the threaded union. This trick saves me from purchasing long clamps and then having to store them in my small shop.

Once on site I had to remove the old mantel shelf nailer. I cut it back with a recip saw and used a cold chisel and 3-lb. hammer to break off 3” of the top corbelled brick.

The clamping process takes a while, and the glue’s drying time slows the project. Once the face parts are clamped together I try to move onto something else. Other-wise, I have to walk away until the clamping process has dried.

On parts that were extremely wide, like the mantel’s top face, or on areas where the edges will be hidden, I use birch plywood. The plywood is more dimensionally stable and saves me from gluing narrow boards together to get a larger sized board.

Once dry, I rough-sand the glue joints and then add the outer and inner side panels. These are the sides that will fit tightly to the wall and the granite.

I purposely leave the inner panels narrower so they will not contact the granite surround. There is no need for this wood to be tight since I will later be adding a thin trim stock to this area. By leaving it shy of the granite, I ensure that it will not be in the way of getting the outer side trim boards to fit tight to the wall.

Add the outer and inner side panels using biscuits and glue. Very few nails are used. I sometimes use a brad nailer to hold boards in place until I can clamp them.

Assembling a mantel with tight seams and without nails requires a lot of clamps. The mantel side panels created a “U” shaped hollow leg. To keep these legs square and in place during clamping, I cut and inserted dry “clamp blocks” the exact width of the inside hollow. I removed these blocks when I removed the clamps.

To fit the mantel tightly to the uneven wall, I had to scribe and plane off some of the wood at the back.

After the mantel box was constructed, I then added a 6-in. vertical board on top of each leg to give the legs depth as well as to provide some outside and inside miters for the molding. The plinth blocks were added on top of these boards at the floor level.

TIP: Remember that it’s extremely important to continually refer to your story pole as you start assembling each part of the mantel.


Building the Mantel Shelf

I consider the next step to be the fun part.

The designer on this project gave me a photo of a fireplace mantel she liked. I placed the photo next to my favorite molding catalog and did my best to match the molding as close as possible.

I secured nailing boards to the wall, then drove finish nails through the mantel and into the nailers.

After buying the molding from a local mill shop I then began to make the mantel shelf.

For this shelf I used Birch plywood and then added a half-round Poplar nosing with mitered corners to the edges of the Birch plywood shelf. This gave me a nice radius nosing and a stable, sturdy, deep Birch plywood top shelf.

This mantel has a large molding and a smaller sub molding, so I needed to make a sub shelf of plywood under the top shelf.


Adding the Moldings

Once the rough shelf was attached I began attaching the moldings. Some of them needed to be built out to match other moldings, and this required adding strips of furring behind the molding. Anything that showed was made with Poplar and anything hidden was plywood.

I take time to sand the glue joints and fill any nail holes or imperfections. I attach all of the moldings with glue and brad nails, making sure to apply the glue to every contact edge of every joining piece of wood or molding. This will ensure a seamless, tight fit that will prevent movement as well as help hide the seams when painted.

The spacing of the lower smaller molding was determined by the designer, but we based it on one-third spacing of the larger space.

Filling Imperfections and Seams

I used wood filler to fill all nail holes and then sanded them later. I also used a putty knife to add filler to every glue joint seam. This helps make the seams flush so they disappear when painted.



Sanding is a constant process, and I find myself doing it every time I remove clamps or just prior to adding any moldings. It’s a lot easier to sand flat surfaces that it is to sand around moldings.


Adding Plinth Blocks and Base Molding

If I had my druthers I would have changed the baseboard trim and baseboard molding in the room so that it would match and wrap around the mantel plinth blocks. Nothing looks better than a room trim that dovetails with the centerpiece mantel. This particular job did not call for that, so I made my plinth blocks to return back into the mantel legs.


Final Sanding

I gave the mantel a final sanding with 150-grit sandpaper and checked for any imperfections, nicks, scratches, etc. The painter will be responsible for sanding further and also for sanding between the primer and top finish coats.


Installing the mantel:

Once on site and ready to go, one of the first things I had to do was to remove the old wood shelf nailer. The nailer was a 2×6 or 2×8 that was installed between two wall studs and buried behind the wallboard.

I installed thin trim pieces where the mantel meets the firebox.

I used a reciprocating saw to cut this slightly back from the wall’s surface. We used a HEPA vacuum as we cut this board to keep the sawdust from flying all over the room.

After this was complete I used a cold chisel and 3-pound hammer to break off 3 inches of the top corbelled brick. This was necessary to allow the mantel box to completely cover the bricks and fit tightly to the wall. (Place cardboard beneath your drop cloth to protect the hearth and floor from falling brick pieces.)


Cutting the Existing Trim Work

In this installation I needed to cut out portions of the wall’s chair-rail molding and baseboard molding so the mantel would fit flush to the wall.

I pushed the mantel up against these moldings, taking time to center the mantel over the fireplace granite. Note: Make sure that you have an equal amount of granite showing on both sides. Also make sure you are within your code for combustible setbacks.

Once centered, I marked the chair rail and baseboard moldings with a utility knife and then used a multi-tool to make the cuts. I purposely made a slight back cut on the moldings to create less friction when sliding the mantel past them. The back cuts also allow me to make micro adjustments with a utility knife if necessary.



When measuring for this mantel I checked the walls and hearth for plumb and level.

I decided to allow 1/4 in. for scribing to the walls as needed. I also made one leg slightly longer than the other to compensate for an out-of-level hearth. It turned out that I did not need to scribe to the wall but I did have to hand-plane some off the inner side trim edge that surrounded and covered the granite.

Note: when removing a large amount of wood to scribe to the wall, I use a jig saw to cut close to the scribe line and then a hand plane to get to the line. For small material removal when scribing I use a sharp block plane and create a slight back bevel when planing.


Install the Mounting Board

When installing mantels I always bring extra material for mounting-board nailers and also for those Oops! Situations. You never know when you might need to make an emergency repair or adjustment.

To secure the mantel to the wall, I installed two vertical layers of 3/4-in. plywood on both sides of the mantel legs. These nailers were attached to the wall studs with 2-1/2-in. screws.

On some situations I’ve had to install these mounting boards to the brick facing with Tapcon screws and construction adhesive.

TIP: If your mounting board falls on an area between studs, you can secure a block low at the floor level and fasten it to the wall’s shoe plate. Then use molly bolts or toggle bolts and construction adhesive in the upper section.

The nailers fit into the mantel leg hollows and allowed me to install finish nails through the sides of the mantel. The top shelf did not require a nailer since we had cut back the old nailer.

The top was fastened with finish nails toe-nailed into the old nailer.

Securing the Mantel to the Wall

I used finish nails to secure the mantel to the wall.

I shot finish nails through the legs on both sides and hand nailed the top shelf. On the top shelf I predrilled and installed

16-penny finish nails at an angle (toe-nailed) through the shelf and into the nailer that I had cut flush with the wall.

TIP: Make sure to keep your mantel tight to the wall when nailing. If hand nailing you may want to drill pilot holes before nailing to prevent damaging the mantel when nailing into position.

When finished there were eight finish nails in each mantel leg (inner and outer sides) and six 16-penny finish nails toe-nailed along the top shelf.

I filled and sanded all of the exposed finish nails.


Installing the Inside Scribe Molding

The inside scribe molding is used to cover the gap between the mantel legs and the fireplace facing material of brick, tile, marble, granite or slate.

In the shop I routed a 3/8-in. radius to the edge of a 3/4-by-6-in. wide board and then used my Delta thickness planer to bring the board’s thickness down to just about 3/8-in. of an inch thick.

This molding board was installed along the inner edge of the mantel opening and was scribed to fit tightly to the granite fireplace surround. The corners are mitered to allow a continuous 3/8-in. bead on all three sides of the mantel face.

I secured this scribed board with finish nails and then filled and sanded them.


Final Sanding and Caulking

I always fill my nail holes and sand them before walking away from a mantel project. Once done I dust and HEPA vacuum the project and then install caulking along the wall and shelf seams. This small step is important and helps make that mantel-to-wall connection look perfect. The goal is for the finished mantel to look like it was always there!


Editor’s Note: Robert Robillard is editor of A Concord Carpenter. Visit



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