Build a Backyard Fireplace

With a preformed system like this one from FireRock, an outdoor fireplace is a great “D-I-Why-not?” project. No previous masonry experience is required.

For a backyard escape or a front-yard social spark, a wood-burning fireplace can’t be beat. Sure, a fire pit may cost less and be easier to build, but an open-burning campfire can’t hold a candle to a fireplace when it comes to comfort, safety and stature. The chimney directs smoke up and away from onlookers; the hearth corrals flames, sparks and ashes; and the stone-covered structure provides an attractive focal point for your landscape.

If you’ve lived with a smoky, inefficient fireplace, you know that good design is essential to the structure’s performance. Therefore, building a brick-and-mortar fireplace is best left to professionals. But precast kits like the FireRock model we used (which is precisely shaped and sized to promote proper burning and ventilation) can make constructing an outdoor fireplace almost as easy as stacking building blocks and frosting a layer cake. It’ll require a little more muscle and sweat, but the results will be smokin’.

Lay the groundwork
To make this fire feature, plan on working for portions of three weekends: one to form the base, another to assemble the components and a third to apply the stone or stucco veneer. Order the materials and gather tools and equipment ahead of time (see materials and tools lists in PDF below). And be sure to round up some help: Pouring the slab and assembling the fireplace requires two people (preferably brawny ones). Here are the initial steps:

Choose a site - Whether you live on a city lot or a country estate, check ordinances related to recreational fires and placement of a fireplace structure. Local code requirements may drive location decisions and will define any rules of operation for an outdoor fireplace. Of course, consider convenience and aesthetics as well as orientation with the landscape and the direction of prevailing winds, which could affect how smoke draws.

Build the base - Here’s an opportunity to practice forming a concrete slab. We outlined the footprint of the fireplace (with hearth and side extensions) on the site (see illustration in PDF below). Next, we excavated 9 in. of soil before installing the gravel, sand and concrete. (For how-to steps, see “Pour a Pad,” in PDF below.) The key is to have a smooth, stable and perfectly level surface.

Build the core
Once the concrete slab has cured, take some time to first dry-assemble the components (as a “dress rehearsal”) to prevent delays once you’re applying mortar. When it comes to actual construction, FireRock says a generous application of its adhesive mortar (specified for its durability under intense heat) to all joints is essential for bonding the blocks and preventing air leaks.

To create the raised hearth, we extended the foundation and installed two courses of concrete blocks (CMUs) before setting the base plate (see illustration in PDF below). We also opted to beef up the width of the fireplace by adding CMUs to the sides of the firebox. The assembly sequence is shown in photos 1 through 8.

Apply adhesive mortar to the joints between and under the CMUs. Check often to ensure that the tops are level and sides are plumb; pound high spots with a mallet and add mortar to areas that are low. To block air movement, fill the voids between the ends of the CMUs with mortar.

Use a cutting tool with a masonry blade to trim the concrete blocks as needed. (When measuring, remember to leave space for mortar between blocks.)

Apply a heavy bead of mortar to the block foundation just inside the perimeter of the space for the base plate. (A mortar bag — similar to a pastry bag — works best.) Carefully set the base on top of the mortar and foundation.

It’s vital that this unit be level side-to-side and front-to-back. Use the mallet and/or additional mortar, as needed, to make adjustments. Scrape off excess mortar that oozes out of joints.

To build the firebox, set the first tier of the sides and back, following the above procedure of applying a generous amount of mortar and adjusting for level in both directions. Continue with the next three tiers.

Once the four tiers of the firebox are complete, you’re ready to add the eight smoke-chamber components. Set the front, the back and then the sides, mudding all joints and checking for level with each addition. Repeat for the second course.

After adding the chimney base (Q), we installed six courses of CMUs on each side of the firebox to add width to the fireplace (see illustration in PDF below). This step is optional, depending on the design and scale of the look you want to achieve.

Four chimney blocks complete the construction of the fireplace core. Because this is an outdoor fireplace, a damper is not needed for controlling the flow of air. (Refer to the manufacturer’s instructions for more construction details and for safe operation.)

Add the charm
Although modular fireplace systems are “cookie-cutter,” no two will look alike once the decorative layer has been added. The veneer options (stucco, manufactured stone or brick) offer endless potential for a custom design. The finish also unifies the concrete components into a single attractive assembly. We used a stone reproduction veneer from Barkman Concrete (see SOURCES ONLINE). Follow the manufacturer’s instructions for any finish you choose. The basic steps to install stone veneer are shown in the photos starting below.

Install the corner stones, leaving 1/2-in. spaces above and below for mortar joints. Using a trowel, spread a 1/2-in.-thick layer of Type N mortar on the rear surface of a stone and jiggle the stone as you press it onto the surface. Scrape away the mortar that oozes out along the edges.

To achieve a finished firebox interior, cover the floor, sides and back with veneer firebrick (which is heat-tempered to withstand direct contact with fire). Be sure to use mortar and grout that can withstand high temperatures.

Add pieces to fill in the front and side faces, trimming stones as needed with large nippers or a mason’s hammer. Again, generously “butter” the backs with mortar and leave 1/2-in. grout joints at the ends. (Keep the concrete substrate slightly damp by misting it occasionally with a garden sprayer or hose.)

Using a grout bag, squeeze at least a 1/2-in.-thick bead of mortar between the stones; then use a brick jointer or wooden dowel to press and smooth the grout lines.

When the mortar is slightly dry, clean the stones’ surfaces with a whisk broom. (Wiping with your hand or a cloth can smear the mortar into the stone.) Then brush between the stones to create a natural and consistent texture in the grout lines.

Apply a generous amount of adhesive mortar to set the hearth and ledge stones. Check and adjust for level in both directions, pounding high areas with a mallet and adding mortar under the low spots. For a custom fit, cut the stone using a masonry saw equipped with a diamond blade.

Enjoy the results
Whenever you’re operating a fireplace, safety is paramount. Always take these precautions:

  • Keep water or a fire extinguisher nearby.
  • Make sure any combustibles are a safe distance from the fireplace.
  • Do not leave the fire unattended.
  • Carefully monitor pets and kids.
  • Burn only clean fuels (no trash, leaves, etc.).
  • Be sure to extinguish the fire completely before you leave.
  • Dispose of ashes only after you are certain they are cold.

One more rule: Fires are more fun when they’re surrounded by friends and family. So invite your loved ones to celebrate your latest addition with a fireplace-warming party.