Things You'll Need
Masonry drill bit
Clay flue liner
Building your own outdoor fireplace can save you a great deal of money on contracting costs while adding an aesthetic and practical element to your outdoor space. A competent DIY homeowner can build a simple outdoor fireplace from cement blocks and a few extra materials, including fire bricks and a lintel. Covering a cement block fireplace in stucco creates a decorative exterior for an otherwise very plain surface.
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Create an approximate dimension for the fireplace. You can build a fireplace to any size you want. Measure an indoor fireplace in your home to get an idea of standard fireplace measurements.
Measure the outside perimeter of your fireplace on a flat concrete space, using a tape measure. Mark the perimeter of the outline on the surface using chalk.
Place a concrete block on the surface so the inside rim of the block lines up against the outside edge of the chalk outline on the floor. Draw a line on the outside of the block, using chalk.
Create a second perimeter around the first perimeter, using the chalk line drawn on the outside edge of the cement block as a guide.
Sweep the concrete surface clean with a broom.
Laying Concrete Blocks
Mix mortar in a bucket, following the manufacturer's instructions. Purchase a mortar designed specifically for use with concrete blocks for optimal performance.
Pour some mortar onto the concrete surface so it fits between the two chalk lines drawn on the floor. Spread the mortar in a thin, even layer across one side of the perimeter, using a trowel.
Place cement blocks on top of the mortar until they cover all the mortar on the surface, with the holes in the blocks oriented vertically. Cut blocks to size using a circular saw and diamond-tipped saw blade if you need partial blocks to fill out the row.
Repeat the process until you cover the entire outside perimeter.
Fill in the area within the perimeter with concrete blocks, using the same process as before, cutting blocks as necessary to fit.
Check the level between each pair of blocks to ensure flatness. If the juncture of two blocks does not create a flat surface, adjust the blocks within the mortar before it dries.
Mix concrete in a bucket, following the manufacturer's instructions.
Fill all the holes in the concrete blocks with the concrete mixture. Flatten the concrete in the holes with a trowel until the concrete is even with the top of the block.
Mix more mortar and apply it in a flat, even row across the top of the concrete blocks. Check the flatness of the mortar with a level and install a second layer of concrete blocks on top of the first.
Continue installing layers of concrete blocks until you reach a height ideal for the bottom of the fireplace, based on your personal preferences.
Install only the outermost layer of concrete blocks to the full height of the actual fireplace, and only at the back and sides of the structure – leave the front open. Stop when you finish the layer for the top of the fireplace.
Installing the Fireplace
Install a steel lintel across the front of the top layer of concrete blocks framing the fireplace. The lintel should clear the entire fireplace opening plus a few extra inches on either side to attach to the row of concrete blocks surrounding the fireplace.
Affix the lintels to the top-layer cinder blocks using a drill, masonry drill bit and concrete screws, drilling through the lintels and into the blocks.
Place a layer of mortar across the concrete blocks forming the bottom surface of the fireplace's interior, in a flat, even layer. Check the flatness with a level.
Install a layer of fire bricks across the top of the mortar. Start by lining the outside perimeter of the fireplace with bricks, then work inward from the perimeter, as you did with the cement blocks. Cut bricks to size with a circular saw if necessary. Fire bricks resist the high temperatures found in fireplaces, which can damage concrete blocks.
Line the walls of the fireplace with fire bricks by stacking rows of bricks in a single vertical layer around all sides. Apply mortar between bricks as you did between the concrete blocks. Check the level of each row and adjust accordingly.
Place mortar across the flat surface of the lintel. Install a layer of concrete blocks across the top of the lintel.
Place a clay flue liner across the top of the fire brick rows installed at the outside perimeter of the fireplace. Purchase flue liners that match the dimensions of the inside of the fireplace, so they'll sit across the layer of bricks.
Stack two or three flue liners on top of one another to create a chimney.
Continue installing layers of concrete blocks, as you did in previous steps, until they reach the top of the chimney.
Allow all mortar and concrete poured in spaces in blocks time to dry before beginning stucco installation. Refer to the manufacturer's information for drying times.
Mix stucco in a bucket or wheel barrow. Stucco is a cement-like substance you can apply directly to concrete blocks.
Place a large clump of stucco on a hawk, or a wood block with a small handle.
Transfer small amounts of stucco from the hawk to the fireplace structure using a trowel. Apply stucco in a flat, smooth layer.
Continue transferring stucco from the bucket to the hawk and from the hawk to the fireplace structure until you cover the entire surface of the fireplace in a layer of stucco. Apply the stucco in a ½- to ¼-inch-thick layer.
Scrape a scarifier across the surface of the first coat of stucco to create ridges in the material. Make a scarifier by driving nails at ½ inch intervals through a 6-inch-long piece of wood. The ridges in the stucco help the top coat bind with the bottom coat.
Apply a second coat of stucco on top of the first coat, in the same manner. Make the second layer approximately ¼ inch thick.
Allow the stucco time to dry before using the fireplace.
Concrete block structures require a concrete base. If no concrete base exists, create one by pouring a simple concrete slab. Always place about 4 inches of gravel beneath a concrete slab before pouring to aid drainage.
Base the dimensions of the fireplace on the dimensions of your concrete blocks to avoid cutting blocks to fit.
Building a concrete block fireplace requires lots of masonry work. Practice masonry work with some scrap blocks before starting to avoid botching the job, or contact a local mason to assist you.