Fireplace Makeover Progress: Building the MDF Frame and Mantel

Ready for more progress on the fireplace? Good—there’s been a lot of work since the last update when we tiled the hearth and part of the surround with marble subway tile.

Tiling was the straightforward part. The next phase—building out the rest of the surround—required many more materials and a little more planning.

We chose MDF for the frame because it’s lighter, more economical, and has a smoother finish than many solid woods. Modern MDF is also available without formaldehyde and can be FSC certified. A single 4ft x 8ft sheet gave us all the pieces we needed.

The first elements of the frame were the two vertical side pieces that sit on the floor and extend up to the mantel.

I cut the MDF to size and joined the pieces at right angles using a Kreg Jig and wood glue for extra strength.

I relied heavily on wood glue during this project. I often used a nail gun because it’s much faster than pocket screws, but nails don’t always hold as tightly long-term, so glue added peace of mind.

To join the two side panels, I cut a middle span on the table saw and secured the assembly with scrap 1 x 2″ whitewood pieces.

It may look a little confusing in close-ups, so here’s the frame positioned around the original brick firebox so you can see how it all fits together.

From above you can see the frame sits snugly around the brick. Removing the old mantel revealed the brick beneath, which was in rough shape, but it gave us a clean surface to attach the new surround.

Where the brick flared out under the old mantel, a small gap appeared between the MDF and the tile at the bottom. We added thin strips of MDF to fill that gap so the surround would sit flush from the side and look balanced.

Here’s the frame from the front so you can see the overall shape coming together.

Even though the structure was fairly steady, we secured it with masonry screws through the top and sides and ran a bead of construction adhesive where the MDF met the brick to make sure it won’t shift.

Next came the mantel. The old mantel was too small, and our initial plan to fasten a mantel directly to the brick failed because the brick wasn’t level. To solve this we built a flat top for the frame out of MDF, attaching it with glue and nails.

We then built a mantel top from a 1 x 10″, boxed on three sides with 1 x 2″ pine strips so no raw edges would show. We also used a few strips of lattice leftover from another project to raise the mantel slightly so it matched the previous height and covered the unpainted paneling behind it. The goal was to preserve the proportions we liked while fixing the hearth and side profile.

After the mantel was in place we planned to add crown molding underneath to give it more presence. We needed the crown to sit flush with future decorative boxes, so we attached a 1 x 3″ board to bump the crown into the correct position and provide a backing for the trim.

With the mantel set, the surround still looked a bit too flat compared to the rest of the room’s more traditional cabinetry, so we added baseboard and framed the face with 1 x 2″ trim to introduce simple, classic detailing.

Behind the baseboard we added scrap 1 x 2″ backing so the baseboard would sit proud of the MDF and align with the future trim layout.

From there we added vertical 1 x 2″ strips, then horizontal pieces to form rectangular trim boxes that add dimension without being overly ornate.

We also added a decorative trim piece where the vertical posts support the middle span to break up large vertical panels and balance the composition.

Trimming went quickly enough that we started caulking seams before a nap interrupting the workday. Caulk helps hide small gaps and makes the joint lines crisp once painted.

We still need to prime and paint, finish the trim at the bottom where the tile meets the cork floor, and add quarter round where the baseboard meets the floor. Once those details are complete the surround will look much more polished.

Even unpainted, we’re thrilled with how this fireplace makeover is coming together. For reference, here’s what the fireplace looked like before we started:

Give us another day or two to wrap up the finishing touches. We’ll return with the fully finished fireplace, a complete budget breakdown, and a collection of final photos to show how the details came together.