DIY Guide: Build a Custom Fireplace Mantel Step-by-Step

Ready for the next stage of the fireplace makeover? There’s been a lot of progress. Previously we tiled the hearth and part of the surround with marble subway tile.

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That tiling was the straightforward part. The next step — building out the rest of the frame — required more materials and a bit more planning.

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We chose MDF for the frame because it’s lighter, smoother, and less expensive than solid wood. Modern MDF is also sold without formaldehyde and is often FSC-certified, which was a plus for us. A 4′ x 8′ sheet cost about $35, and we only needed one sheet to cut all the pieces for this build.

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

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

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Wood glue became indispensable. I used a nail gun for speed, since it’s faster than pocket screws, but relied on wood glue as insurance because nails can loosen over time.

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To join the two side pieces, I cut a middle span on the table saw and attached everything together with a few scraps of 1×2 whitewood.

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Here’s a clearer view of the frame in place so you can see how it’s coming together.

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From above you can see the MDF frame snugly fits around the original brick fireplace. Removing the old mantel revealed the brick underneath, which looked rough in spots.

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There was a small gap where the MDF met the tile because the brick flared out under the mantel and tapered toward the floor. To fix that, we added thin strips of MDF where needed so the frame would sit balanced and flush from the side.

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Viewed from the front, the MDF frame already looked solid and true.

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Even though it sat well, we secured it with masonry screws through the top and sides and added construction adhesive where the MDF met the brick to make sure it wouldn’t shift.

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Next came the mantel. The old mantel was too small to reuse, and an attempt to attach a new mantel directly to the brick failed because the brick wasn’t level. We resolved that by adding a flat MDF top to the frame, securing it with glue and nails.

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We built the new mantel top from a 1×10 board and framed the exposed edges with 1×2 pine strips so no raw wood showed. We used the same technique from other projects and added a few strips of leftover lattice to raise the mantel slightly so it matched the old height and covered the unpainted paneling behind it. The final mantel height matches the original, which helped maintain proportions and conceal imperfections.

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After installing the mantel, we wanted some visual weight beneath it, so we planned to add crown molding. We couldn’t mount the crown directly to the MDF because it needed to sit flush with the decorative boxes we’d add later, so we attached a spacer board to position the crown correctly.

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We used 1×3 boards as the spacer so one inch would act as the crown’s backing and the remainder would serve as the top of the 1×2 boxes. The nail gun made quick work of the installation.

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With the mantel in place, the surround looked more complete. The flat-fronted look felt a bit too modern compared with the more traditional kitchen cabinetry, so we planned to add baseboard and framed trim to soften the design.

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Behind the baseboard we installed scrap 1×2 boards so the baseboard would sit proud of the MDF and line up with the future trim details. This ensures consistent reveal and depth across the surround.

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Next we added 1×2 vertical strips to create the framework for simple boxed details.

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Then we installed horizontal pieces between the verticals to form the boxes. The added trim started to give the surround a balanced, tailored look.

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We also placed a decorative trim piece where the vertical posts supported the middle span to break up long vertical boxes and bring more visual interest to the design.

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The trim stage moved quickly, and we even had time to start caulking seams before the baby woke from her nap and wanted to know what we were building.

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Still to do: prime and paint the surround, finish trimming the bottom where tile meets cork flooring, and add quarter round where the baseboard meets the floor. Those final touches will make the project look polished and complete.

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Even before paint, we’re thrilled with how the makeover is progressing. For comparison, here’s the fireplace when we started.

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Give us another day or two to wrap up the finishing touches and we’ll share the fully finished result, a detailed budget breakdown, and plenty of final photos.