I'll try to break this down into clear steps of what we did from design planning to lighting the fire upon completion. Again these are the steps WE took... and we consider ourselves novices. So I welcome any comments/feedback. Have fun and be safe if you decide to tackle a similar project!
Step 1: 1. Design
We knew wanted a circular pit with some sort of walls that could seat about 10 + people. After a few nights of adult beverages and tossing ideas out, and napkin sketching, I used the CAD software that i use at work (Solidworks) to draw up the pit with it's different components. We then did some basic geometry calculations for materials we'd need, made a list and calculated costs for everything. This original design incorporated a built-in concrete center pit sculpted to look like a tree stump. After a few weeks of trying to figure out how to make it we decided to do some Value Engineering and just have it be a flat patio. That way we could utilize any off-the-shelf fire pit... and could always remove the paver stones in the center and create the tree stump look later.
Step 2: 2. Materials & Tools
With our list of materials we called up a few local hardware stores to find the best prices. i don't remember exactly the total quantities of each material but we tried to over estimate to make sure we didn't run out. Our main challenge was getting the literal tons of material all the way up to the cabin, 2 hours from civilization. Luckily we had the use of a flatbed truck from my work, and had the gravel delivered. The basic tools we had, and borrowed the more specialized tools from friends or rented them. The rights tools makes the job so much easier and faster... or at least more accurate.
*Base: Gravel, 5/8" minus
*Foundation: Concrete pad 4" thick by 20" wide (to accommodate the 16" wide masonry blocks with 2" extra on each side), with wire mesh for reinforcement
*Wall: Masonry blocks 8x8x16, filled with concrete and the occasional rebar rod hammered into the still soft concrete pad
*Center: Sand base, with paver stones on top in some sort of pattern
*Wall Cover: Slate stones adhered with mortar
TOOLS / MATERIALS
*Metal Claw Rake
*Wood Bender Board for Concrete Forms
*Laser Tripod Level
*Electric Concrete Mixer
*Wet Saw for cutting Paver Stones
*Standard Bubble Level
*Mini Sledge Hammer
*Tarp for the unpredictable Northwest weather
Step 3: 3. Base & Foundation
First we located where we wanted the pit to be and about where the opening would face. We then determined the maximum diameter our base could be given the location and trees/roots in the way. I hammered a rebar rod at the center of the circle to use as our centroid for measuring and laying out the foundation and wall. We came up with roughly 13ft diameter at the concrete, with a 20" wide concrete foundation.
We tried to find the highest point in the yard, then hauled in and spread a bunch of gravel. We tied a string line to the rebar center rod and made knots in the string at 13ft and 11ft-4in and used that as our gauge to mark the edges of the foundation. We dug down about 4 inch into the gravel and tamped and leveled the area using our tripod laser level. We then hammered stakes around the outside of the circle every 4 ft or so, and around the inside of the circle. We then tied our bender board to the stakes as a basic concrete formwork.
After cutting and placing some wire mesh around the foundation (bending to make sure it didn't lay totally flat), we mixed and poured the concrete foundation, making sure to write our initials for recognition by future generations.
Step 4: 4. Masonry Block Wall
After a few hours of letting the concrete foundation cure, we started the Masonry blocks. Using the center rebar rod with string still attached, we made new knots for the inside and outside radius of our masonry blocks, and began placing the blocks on the fairly level concrete. We placed the first block perpendicular to the rest and aligned it roughly with where we wanted the end of the wall to be. We aligned the corners of the blocks first, then checked the radius for distance, then made the center of each block parallel with the string. The blocks ended up laying out just about right at the end so we didn't have to re-adjust the whole thing.
When the first inner row was done, we moved to the back and followed the same procedure. There were large gaps in the backside of the wall but we figured we could later fill those in with rocks/mortar/concrete. Then we moved to the second level, overlapping the blocks like legos as best we could. We made a third level at the rear side of the firepit just because we felt like it...
After the wall was assembled and checked for level (we allowed for a little slop towards the back of the pit for drainage through the pvc pipe we placed in the concrete foundation), we started placing rebar rods in the center of every other block hole. We hammered the 2ft rods through the still soft concrete, and into the gravel below.
We then backfilled the blocks with concrete to finish the wall structure. This thing isn't going anywhere! At least for the summer. Once the rains and snow start, we'll see how well it holds up.
Step 5: 5. Paver Stones
The following weekend we removed the bender board formwork, removed some gravel from the center of the pit, tamped it, added a bunch of sand, and leveled it as much as possible. This was our first time installing pavers so the process took quite a while. We changed layout designs about 3 times and finally settled on a diamond shape in the center with the red pavers and then laid the gray pavers in a linear pattern from the center, and lined the edge of the wall with red pavers as well.
We started with one red paver in the middle, and ran short of gray so tore it all up and put 4 in the center, were still short, tore it up one more time and finally put 9 in the center. We were still short gray ones in the end but 9 looked the best so we called it good, and made fire in an old steel burn barrel to pre-christen the pit.
Lesson learned: order more than you think you need, you can always return them... and look up how to level sand for paver stones on you tube and follow those examples.
The next weekend we borrowed a wet saw (no pictures) and cut all the odd shaped gray ones around the perimeter, along with all the wedge shaped red ones on the very perimeter. The saw worked fantastically but still it took all day for it to look just right... with breaks for adult beverages of course.
We then spread sand over the top of the whole patio to work into all the cracks, then sprayed the whole thing to help the sand settle. We ended up doing this 2-3 times before all the cracks were full and stopped settling.
Step 6: 6. Mortar the Slate Stones to the Masonry Block Wall
The next weekend we tackled the pain staking task of mortaring the masonry block wall and adhering the big slate stones. We started by taping plastic down on the whole patio to prevent staining the pavers. Then started mixing 2 bags at a time of mortar, slapping the mortar on the side walls first and trying to hold the slate stones in place for a few seconds, jiggling the stone the disperse the mortar behind it. Then we moved on to the top of the wall and tried to use as many of the big stones as possible to provide big flat seating areas. Inevitably the mortar mix was never perfect and would either be runny and sloppy or too dry. Took a few batches before we perfected the mix and got into a rhythm. We also filled as many big gaps in the masonry wall with gravel/rocks/grout so there weren't too many voids.
I think the slate stones were a great choice and so varied in shape and size that we able to cover just about every square inch of wall with whole stone pieces, except for a few filler pieces. So, basically it was like a giant jigsaw puzzle with heavy, sharp, fragile pieces that didn't want to stay where you put them and would never break the way you wanted them to. But nevertheless we finished the puzzle and it looked pretty fantastic at the end. We wiped the mortar off as we installed them but will eventually go back later and use an acid solution to clean the mortar residue off the stones.
I forgot to mention we placed pvc pipes in the masonry blocks before pouring concrete. Then were able to place tiki torches in the holes at the end.
The resulting fire patio/pit looks awesome and well worth the effort. Have fun if you try the same, but be ready for a lot of work!