How We Built Our Outdoor Fireplace

Our backyard underwent a major transformation this summer. We added a sprawling deck to our backyard and furnishings and finally, built an outdoor fireplace as the finishing touch.

As a reminder, just a few months ago, the space looked like this:

We went between the idea of fire pit vs. fireplace, but in the end, a fireplace had the architectural elements that a fire pit didn't offer: the height and the fact that even when it isn't being used, it still looks great and intentional especially sold us on the idea. I loved the look of a few board-formed concrete fireplaces I had seen, so with inspiration in hand, we built our own in a week for around $1,300. Here's how we did it:

First, Building code may vary from area to area, so always make sure your plans meet the requirements of where you live.  Our town didn't require a permit, but they did give us recommendations about nearby branches to trim back, etc. In the end, our fireplace ended up being about 7ft wide x 2ft deep x 7.5ft tall, so all material quantities listed may vary, based on how large you choose to make your fireplace.

Step 1: The Concrete Foundation

Cost of Materials Needed: $175
- three 2x56x8 studs
- wooden stakes
- six bags all purpose gravel
- one wire mesh sheet
- eight 2ft rebar
- galvanized wire
- sixteen bags concrete
- magnesium float
- concrete mixing tub
- flat shovel

• Measure out the area of your foundation and place stakes in the four corners, using a square to ensure the corners are an exact 90 degrees. The foundation will be topped with cinder blocks, so ensure the size of your foundation will work with the cinderblocks you plan to use, so you don't have to cut any cinderblocks.
• Dig down a few inches and level the area of the foundation with a shovel until level, smooth and compact. If the soil is loose, you'll want to compact the soil down until hard and stable.
• Measure and cut the outside forms using smooth 2x6 boards. Square each form to ensure all angles are 90 degrees, and stake the boards in place once they are level with each other. If you screw the stakes to the forms, be sure the screws are attached from the outside if possible.
• Add 1.5-2 inches of rock in the bottom of the foundation area and spread out evenly.
• Measure and cut concrete wire mesh sections to fit the foundation area. Set them in place using wire mesh stands, wiring them together securely.
• If the foundation will extend above the ground around it, add 2-3 pieces of rebar in the corners in a crow's feet pattern and wire them in place. This will reinforce the concrete and prevent cracking in the corners.

• Following the instructions on the bag, calculate how many bags of concrete mix you will need for your foundation. For larger jobs, renting a cement mixer may be a good idea, but using a cement tub and shovel for small slabs is usually sufficient. If using a tub and shovel, it's a good idea to mix one bag at a time.
• Following the instructions on the bag, mix and pour the cement until the foundation area is full. Run a long, flat board (or concrete screed) across the top of the concrete, laying it on the board edges to keep the board level. Move the board back and forth as you slide it across the concrete, adding more concrete to fill in any gaps around the edges. Tap the sides with a hammer to release any bubbles that may be trapped in the concrete.
• Using a magnesium float, smooth the top of the concrete every 45 minutes or so for 3-4 hours.
• Allow concrete to cure for the time specified on the bag (different kinds of concrete will have different curing periods).

Step 2: The Cinderblock Base

Cost of Materials Needed: $68
- two bags mortar
- fifteen 16x8x8 concrete blocks
- fifteen 16x8x4 cap concrete blocks
- mason trowel
- concrete tub & flat shovel from laying the concrete

• Calculate how wide your cinderblock base will be once installed (figuring for roughly 3/8-1/2 inch in between each block for mortar), and make sure it is not wider than your concrete foundation.
• Mix your masonry mortar using the instructions on the bag.
• Work quickly to lay the cinderblocks, ensuring there is mortar along the entire base and in between each block. Use cored cinderblocks for the first layer, then lay a second layer of cap cinderblocks, which are solid. Choose blocks that correspond in size so they cover the same area, but lay the cap blocks cross-wise over the core blocks, to prevent cracking along the seams. Level each block as it is laid, because there's no doing it later.
• Allow the blocks to dry in place for 48 hours.

Step 3. The Cinder Block Heat Shield

Cost of Materials Needed: $43
- eight 16x8x6 concrete blocks
- eight 6x8x8 concrete blocks
- one plated steel slotted angle bar
- leftover mortar from cinderblock base, as well as trowel and concrete tub/shovel.

Because of the openness of the finished fireplace, the cinderblocks are there only to block direct exposure to the wooden frame, which will be added later.

• Set the fireplace insert in place, ensuring it is exactly centered on the cinderblock base.
• Dry lay the surrounding heat shield cinderblocks in place, offsetting the top block the depth of the slotted angle bar. Set the bar in place to ensure the fireplace sits flush against it. Keep in mind the angle bar will sit higher once the blocks are laid, as the mortar will raise each block slightly. Remove the fireplace insert, and mark where the cinderblocks sit.
• Lay the cinderblocks with mortar, leveling as you go. Drop excess mortar evenly into the center of the blocks to provide additional stability. Once the blocks are in place, measure where the angle bar sits to ensure there is room for the fireplace insert to fit. If the bar needs to raise, add mortar to get it to the correct height. Allow it to dry for 24 hours.

Step 4: The Fireplace Insert

Cost of Materials Needed: $645
- fireplace insert kit
    (includes insert, Quickrete glue and chimney stack)

You can definitely build your own insert with brick, but because we aren't professional masons (or even close to it), this insert made the whole project infinitely easier.

• Run a line of Quickrete glue around the edges where the fireplace will sit against the cinder blocks and angle bar. Run a line of Quickrete glue along the feet of the fireplace insert as well.
• Set the fireplace insert in place. If the angle bar is knocked loose, get the insert in place then put the angle bar back and secure it down with some of the Quickrete glue.
• Run a small bead of glue along the sides and top of the fireplace insert to create a seal.
• Using a miter saw equipped with a metal-cutting blade, trim the pieces of the chimney to desired height. You want the chimney to be about 1 inch shorter than the total overall height of the front of the finished structure (the roof will slant down toward the back, so the chimney will still stick out of the roof without being visible). Be sure to cut the proper ends of each chimney section so it will still assemble together after all pieces have been cut.

Step 5: The Structure

Cost of Materials Needed: $130
- twenty-four 2x4x8 pressure treated boards
- concrete wedge anchors

The inner structure of the fireplace will likely be exposed to the elements year round, even after the siding is in place. For this reason, it's recommended to use pressure-treated lumber to build the structure.

• Measure and build the front wall of the structure. Leave about 6 inches of clearance above the top of the fireplace to prevent combustion. Again, traditional fireplaces, indoors, that would be a no-no, but this fireplace is very open and will trap and hold less heat. Studs should be no more than 16 inches apart.
• To secure the wall to the cinderblock base, drill 1 hole (3/4 inch) into the bottom plate of each side of the fireplace. Set the wall in place and use a pencil to mark the cinderblocks, where the hole is. Use a hammer drill and 1/2 inch masonry bit to drill a hole about 2 inches deep where you marked the cinderblock. Hammer a 1/2 inch concrete wedge anchor bolt into the hole, being careful not to hit too hard, or the cinderblock could crack. Lift the wall up and set it in place with the bolts going through the holes in the bottom plate. Secure with a washer and nut. If the wall is unstable by itself, make a kickstand for the wall by screwing a longer board into it and setting that board on the ground to provide stability while you build the other walls.
• Build the side walls the same height as the front wall and secure them in place similarly. Level and square each wall as they're added and tack them together with a single screw to hold them in place until all walls are up.
• Build the back wall 6 inches shorter than the front (this is so when you add your roof, it will be angled to promote water run off) taking care to not have any studs exposed to the backside of the fireplace insert - they should be behind the cinderblocks you put in place. For this specific part of the wall, it's ok if the studs are more than 16 inches apart (they likely will be).
• Once all the walls are secured in place, level and square, screw them together with 4 screws from top to bottom where the walls meet.

• Make roof supports by cutting boards so they match the angle of the roof when screwed in from the front wall to the back.

Step 6: The Roof

Cost of Materials Needed: $87
- three galvanized drip edges
- corrugated steel roof panel
- spark arrestor/chimney cap (the one used is not found online, but this is another option)
- roofing shingles
- foam

Nobody is living in this fireplace, so don't feel like the roof needs to be like a home or even a shed. That said, you want the structure fairly protected from rain, so take care to keep the edges tight and materials overlapped properly.

• Nail down galvanized steel drip edge along the edges of the roof.
• Cut your corrugated steel roof panel so the ridges guide water down the back of the structure. Allow for a few inches overhang on the back.
• Secure the roof panels in place, cutting around the chimney and so the front section tucks under the drip edge.
• Install a spark arrestor over the chimney opening.
• Wrap the side edges with roofing shingles and cut to fit.
• Seal around the chimney and spark arrestor with heat-proof, weather-proof expanding foam.

Step 7: The "Board-Formed Concrete" Planks

Cost of Materials Needed: $147
-  sixteen pieces of James Hardy Siding

James Hardy siding is a composite and concrete-based product that holds up well in extreme weather and is fire resistant, making it the perfect siding for an outdoor fireplace. Plus, it looks a lot like the board-formed concrete planks in our inspiration photos. You can lay it traditionally, but for this tutorial we will be laying it in a plank fashion, with the backside facing outward (the front side has text indented along the edge of it).

• Measure and cut the first piece of siding for the front. This first piece will be placed just above the fireplace insert and will serve as the guide for the rest of the pieces.
• To secure the first siding piece in place, apply Quickrete glue to the metal angle bar where the bottom edge of the siding will sit. The siding is 1/4 inch thick, so ensure you cut your siding piece 1/2 inch longer and allow it to stick out 1/4 inch on each side (so the side pieces will tuck behind the front). Put the first piece in place and nail it with 15 gauge galvanized finish nails (use an angled finish nailer at around 100 psi).
• Cut and secure the other siding pieces in place.
• To secure the bottom piece in place on the cinderblocks, use a small, 3/16 masonry bit to drill a hole and secure in place with liquid nails and masonry screws.

Step 8: (Optional) Paint the Fireplace

While we loved the variance that came with the hardie board planks, we didn't love the barcode inked onto a few of them. We tried a few methods to remove the ink with no luck, and finally settled on painting the whole thing--which I ended up loving even more. With a small foam roller, I painted on a light coat of Benjamin Moore's Winterwood in an exterior flat finish. It dries the color of warm concrete and looks very natural.

TOTAL COST: $1,295

The fireplace is functional, of course, but even when it isn't lit, it adds so much to our outdoor space. Day or night, you can find us out here eating, lounging or chatting.

We've had a few questions about the durability of the furnishings out here (sources below). Almost everything is weather-rated except the leather pouf that we bring in, and the dining chairs that are holding up great. We have outdoor furniture covers that I mostly try to get on if I see rain coming, but Idaho is a desert climate so we don't get much rain. It's also a great idea to spray cushions with this outdoor ScotchGuard and if it comes down to it, all the covers are removable and can be thrown in the wash.

RELATED POSTS

Would You Rather: Real Wood vs. Composite Decking, Deck on Deck!, Our Finished Deck!, Our Outdoor Sofas are Here + What’s Next?, First Look: Building Our Outdoor Fireplace

SOURCES

Decking--Trex in Clam Shell color; Area Rug--Rejuvenation; Wood and white outdoor sofas--World Market; Black and white outdoor sofa--Crate&Barrel; Dining Table--Ikea; Striped Umbrella--Home Decorators; Black Tabouret dining chairs--Overstock; White wire dining chairs--Rejuvenation; Concrete planters--Rejuvenation; Leather Pouf--Lulu & Georgia; Black planter--Crate & Barrel; Teak lanterns--Crate & Barrel; String lights--Rejuvenation