When Oklahoma homeowners ask about outdoor fireplaces, the conversation eventually turns to the structural materials: brick versus concrete block. Both appear commonly in finished fireplaces, and both can produce excellent results — but they are used differently, for different structural purposes, and understanding the distinction separates homeowners who know what they are getting from those who are guessing.
The Role of Concrete Block in Outdoor Fireplace Construction
Concrete masonry units — CMU block, often called cinder block — are the structural backbone of most properly built outdoor fireplaces in Oklahoma. The firebox, smoke chamber, and the structural core of the chimney are built with CMU because block provides exceptional compressive strength, thermal mass, and dimensional stability. Block cores can be filled with concrete and reinforced with rebar, creating monolithic structural sections that resist the thermal expansion and contraction cycles that Oklahoma’s temperature swings impose on masonry.
CMU is not the finished aesthetic face of the fireplace — it is what sits behind the finished surface. The average homeowner never sees the block because it is covered with brick veneer, natural stone, stucco, or another finish material. When a contractor describes building the fireplace “out of block,” they are describing the structural system, not the finished appearance.
The Role of Brick in Outdoor Fireplace Construction
Traditional full-brick outdoor fireplaces use solid brick — not block — as both the structural and finished material. This is the old-school approach and it works well when executed with the right mortar, correct brick selection, and adequate footing. Solid brick construction is more labor intensive than block-core construction and typically costs more for equivalent size and quality.
Brick veneer over a block core is a common hybrid approach: the block structure provides the strength and thermal mass, and the brick veneer provides the traditional aesthetic finish homeowners expect. The veneer is attached to the block with mortar and metal ties; it is not structural, but it looks and weathers identically to a solid brick fireplace from the outside.
Refractory Materials for the Firebox
Neither standard brick nor standard CMU belongs directly in the firebox — the area that takes direct flame and intense heat. The firebox lining should be firebrick (refractory brick) set in refractory mortar. Standard brick and block crack under repeated thermal cycling at firebox temperatures. Firebrick is designed specifically for these conditions and can last the life of the structure when properly maintained.
The smoke chamber — the tapered area above the firebox where combustion gases collect before entering the flue — should also be built from or lined with refractory material. In Oklahoma where wood-burning outdoor fireplaces see significant use across the outdoor season, refractory liner quality in the smoke chamber and flue directly affects draft performance and long-term structural integrity.
What Oklahoma Conditions Require
Northeast Oklahoma’s freeze-thaw cycling, heavy spring rains, and clay soil expansion all stress masonry. The correct footing — typically a reinforced concrete slab extending below frost depth — is as important as the materials above grade. A beautiful brick fireplace on an inadequate footing in Broken Arrow or Tulsa clay soil will settle, crack, and eventually fail regardless of the quality of the masonry above grade. Ask your contractor specifically what the footing design includes — depth, reinforcement, and concrete mix — before they pour.
