How to Build an Outdoor Fireplace
Building an outdoor fireplace is a substantial project that sits at the upper end of what a skilled DIYer can accomplish independently. It requires accurate masonry work, an understanding of refractory materials and flue engineering, a properly poured reinforced foundation, and compliance with local building codes throughout. The reward for getting it right is a permanent, high-value outdoor feature that transforms the patio and serves the property for decades.
This guide covers the full construction sequence for a standard masonry outdoor fireplace, from permit and planning through foundation, firebox construction, flue and chimney assembly, and exterior finishing. Homeowners who are not comfortable with masonry work or who have not completed a comparable structural project before should seriously consider the prefabricated kit approach detailed in the best outdoor fireplace kits guide, which significantly reduces both the technical complexity and the scope of site work required.
Before You Build: Planning and Permits
Check Local Requirements First
A permanent outdoor fireplace requires a building permit in virtually every US jurisdiction. The permit process involves submitting construction drawings showing the fireplace location on the property, setback distances from structures and property lines, chimney height calculations, and materials specifications. Permit requirements vary significantly by municipality, some require stamped engineering drawings for structural masonry; others have standard prescriptive requirements that can be self-certified.
Contact your local building department before any design work or material purchasing. The information you receive shapes critical design decisions including chimney height (typically required to extend at least 2 feet above any structure within 10 feet horizontally), setback from the house (minimum 10 feet from combustible wall surfaces in most codes), and foundation depth (typically required to extend below the local frost depth).
Homeowners association (HOA) rules may impose additional restrictions on outdoor fireplace design, including height limits, facing material requirements, and placement limitations. Checking HOA covenants alongside municipal requirements avoids expensive redesigns after work has begun.
Choosing a Location
The most important location constraint for an outdoor fireplace is the relationship between the chimney top and nearby structures. The standard rule, chimney top must be at least 2 feet above any structure within 10 feet horizontally, means that a fireplace positioned close to a pergola, covered patio roof, or the house eave requires a taller chimney stack to meet code. Positioning the fireplace away from overhead obstructions simplifies chimney design and reduces overall structure height.
Prevailing wind direction affects both smoke management and draft performance. A fireplace positioned so that prevailing wind blows from behind the structure toward the firebox opening benefits from positive wind pressure that enhances draft. Wind blowing into the firebox opening from the front can cause downdraft and smoke spillage into the patio area.
Sizing the Firebox
The firebox opening dimensions determine how much heat the fireplace projects and how easily it can be fueled. A firebox opening of 36 inches wide by 24 inches high is a practical standard for a residential outdoor fireplace that serves a seating area of four to six people. Wider openings allow larger fires and more heat output but require proportionally larger flue dimensions to maintain adequate draft.
Firebox depth (front to back) should be approximately 60 to 65 percent of the opening height, for a 24-inch-tall opening, a firebox depth of 14 to 16 inches is correct. Too shallow a firebox causes smoke to spill forward through the opening; too deep reduces heat output toward the seating area.
Materials You Will Need
Structural Materials (CMU Surround)
Concrete masonry units (CMU blocks, typically 8x8x16 inches) form the structural shell of the fireplace body and chimney column. Standard CMU is used for all areas outside the firebox interior. CMU block work is laid in running bond with Type S mortar, which is suitable for below-grade and above-grade outdoor masonry.
Refractory Materials (Firebox Interior)
The interior of the firebox, the back wall, side walls, and floor, must be lined with refractory firebrick set in refractory mortar. Standard concrete block cannot withstand the thermal cycling of sustained wood fires and will crack and spall within a season if used without a refractory lining. Refractory firebrick is denser and more heat-resistant than standard brick and is rated for continuous exposure to firebox temperatures. It is typically 2.5 inches thick and laid in a running bond pattern using refractory mortar only, standard mortar fails at firebox temperatures.
Flue Liner
The flue liner carries combustion gases from the firebox throat to the top of the chimney stack. Clay tile flue liners are the most commonly used material in residential outdoor fireplaces, they are code-compliant, durable, and available in standard square and rectangular section sizes. Stainless steel flexible or rigid liner systems are a practical alternative, particularly for prefabricated firebox kits where the liner is sized to the firebox by the manufacturer.
The flue cross-sectional area must be correctly sized relative to the firebox opening area. A general rule of thumb is that the flue area should be approximately one tenth of the firebox opening area, for a 36×24-inch opening (864 square inches), a flue area of approximately 86 square inches is needed, corresponding to a 10×10-inch clay tile liner or an 8-inch round stainless liner at minimum.
Facing Material
The exterior of the fireplace structure is finished with a facing material that covers the CMU blockwork and gives the fireplace its visual character. Natural stone veneer (granite, limestone, fieldstone, or travertine), brick veneer, and stucco are the most common choices. The facing material does not carry structural load and is attached to the CMU surround with mortar and metal ties. The hearth extension in front of the firebox opening is typically faced with the same material as the surround, or with natural stone or tile that can withstand foot traffic and incidental heat.
Step-by-Step Build Guide
Step 1: Excavate and Pour the Foundation
Excavate to below the local frost depth, this varies by climate region but is typically 12 inches in the South, 24 to 36 inches in the Midwest and Northeast, and up to 48 inches in northern states. The excavation footprint should extend 6 inches beyond the planned outer dimensions of the fireplace structure on all sides.
Install 1/2-inch rebar in a grid pattern with approximately 12-inch spacing, positioned on chairs that keep the rebar in the middle third of the slab depth. Pour 3,000 PSI concrete to a minimum depth of 8 inches (12 inches is preferable for larger structures) and finish the surface flat and level. Allow the concrete to cure for a minimum of 7 days before loading it with masonry.
The hearth surface, the area in front of the firebox opening, needs its own slab extension at finished patio grade if the patio surface is not already a continuous concrete slab. A 16 to 20-inch hearth extension in front of the firebox opening is standard.
Step 2: Build the Base and Side Walls to Hearth Height
Lay the first course of CMU block on the cured foundation using a full mortar bed. Check for level constantly, errors in the first two courses propagate upward and are increasingly difficult to correct. Build the two side walls and the back wall of the fireplace body to hearth height (typically 16 to 18 inches above finished grade, corresponding to two or three courses of standard 8-inch CMU).
Leave the front of the structure open at this stage, the hearth face and firebox opening are framed in the next phase.
Step 3: Frame the Firebox Opening
The firebox opening is spanned by a steel lintel, typically a 3×3-inch steel angle iron of adequate span length, which carries the masonry above the opening. The lintel seats on full courses of CMU on both sides of the opening and provides a flat bearing surface for the masonry courses that form the arch above the firebox mouth.
Some fireplace designs use a true masonry arch over the firebox opening rather than a lintel. A masonry arch is more traditional in appearance and carries load more efficiently, but it requires form work and careful cutting of tapered voussoir bricks. For most DIY builders, a steel lintel is the more practical choice.
Step 4: Construct the Firebox Interior
Once the side walls reach the top of the planned firebox opening and the lintel is in place, line the firebox interior with refractory firebrick. Start with the firebox floor, laying firebrick flat in a running bond pattern set in refractory mortar. Then build the two side walls and the back wall of the firebox interior, again using refractory firebrick and refractory mortar only.
Refractory mortar joints should be kept thin, 1/4 inch or less, as tight joints perform better thermally than wide ones. Allow refractory mortar to cure for at least 72 hours before lighting any fire.
Step 5: Build the Smoke Chamber and Install the Damper
The smoke chamber is the transitional zone between the firebox opening and the base of the flue. It is built by corbeling, stepping each successive course of masonry inward over the firebox opening, until the chamber narrows to the flue liner dimensions. The corbeling angle should not exceed 45 degrees from vertical on any face.
A cast iron or fabricated steel damper plate is installed at the throat, the narrowest point between the firebox and the smoke chamber, before the smoke chamber courses close over it. The damper allows the flue to be closed completely when the fireplace is not in use, preventing rain, cold air, and animals from entering. It is one of the most important functional components of the fireplace and must be installed at the correct throat dimensions to work properly.
The smoke shelf, a ledge at the back of the smoke chamber just above the damper, helps prevent rain and cold downdrafts from entering the firebox. It is formed by allowing the back of the smoke chamber to step back slightly before corbeling inward.
Step 6: Install the Flue Liner and Build the Chimney Column
Beginning at the smoke chamber, set the first flue liner section in a full mortar bed. Build the CMU chimney column around the liner, maintaining a minimum 1-inch air gap between the outer face of the liner and the inner face of the CMU. This air gap allows for differential thermal expansion between the liner and the surrounding masonry and prevents the expansion pressure of a hot liner from cracking the chimney column.
Continue building the chimney column upward, setting a new flue liner section in mortar at each course where the liner joint falls. Check for plumb constantly, a leaning chimney is both structurally and aesthetically problematic and becomes progressively harder to correct the higher the work progresses.
The chimney must extend at least 2 feet above the highest point of any structure within 10 feet horizontally. In most residential settings this means a chimney top height of 10 to 14 feet above the patio surface. The flue liner should project 2 to 4 inches above the last course of chimney masonry to allow a properly fitting chimney cap to seat correctly.
Step 7: Apply the Facing Material
Once the structural masonry is complete and cured, apply the chosen facing material to all visible exterior surfaces. Natural stone veneer is set in mortar against the CMU substrate with metal wall ties anchored into the mortar joints of the CMU block for additional mechanical attachment. Brick veneer is applied in the same way. Stucco is applied in two or three coats over a metal lath substrate attached to the CMU.
Work from the base upward, ensuring each course is level and that corner joints are handled with appropriate corner pieces or careful cutting. The hearth facing is typically applied last, after the main surround is complete, using a flat material, stone tile, brick, or natural flagstone, that can accept foot traffic.
Step 8: Chimney Cap, Curing, and First Use
Fit a purpose-made chimney cap over the flue liner projection at the top of the chimney. A properly sized cap prevents rain from entering the flue, deters birds and animals from nesting in the chimney, and reduces downdraft. Stainless steel chimney caps are the most durable and weather-resistant option.
Allow all mortar to cure for a minimum of 28 days before lighting any fire. Then cure the fireplace using the same progressive fire-building sequence described for chimineas, small fires on the first three sessions, building up to full capacity only after the refractory mortar has been fully heat-cured through the gradual moisture-driving process. For the detailed curing sequence, the how to use and season a chiminea guide covers the same progressive fire approach that applies to any new refractory masonry installation.
Choosing the Right Firewood
A well-built outdoor fireplace performs best with dry, seasoned hardwood. The firebox geometry and flue draft of a properly constructed outdoor fireplace are optimized for a clean, hot burn, burning wet or softwood produces the excessive smoke and creosote that the design is engineered to minimize. Oak, hickory, maple, and ash are the best species for outdoor fireplace use. For a full guide to firewood species selection, moisture content, and buying options, the best firewood for fire pits guide covers all the relevant information for outdoor fireplace use equally well.
When to Use a Kit Instead
Homeowners who want a permanent outdoor fireplace but are not comfortable with the full masonry construction process described above should look seriously at prefabricated fireplace kits. A kit provides factory-engineered refractory firebox components that are assembled on site in a fraction of the time required for a full masonry build, then finished with a surround of the homeowner’s choosing. The engineering work has been done, the firebox dimensions, flue sizing, and damper placement are all specified by the manufacturer. The best outdoor fireplace kits guide covers the leading options with full evaluation of installation complexity and finish quality.
Part of the Outdoor Fireplaces hub. See also: Best Outdoor Fireplace Kits | Outdoor Fireplace vs Fire Pit | Fire Pit Safety Tips