Intro: 40 Inch (Inner) Diameter Retaining Wall Fire Pit ~ $110
As stated in the title this fire pit cost me about $110 for the raw materials. The inner diameter is 40 in (102 cm) while the outer is 54 in (137 cm). I purchased materials from both Home Depot and Lowes and I recommend visiting both because they had different block colors and sizing if you want to create a variation of this pit.
Raw Materials: 6 bags of .5 cu ft of drainage gravel (Lowes)
42 Retaining Wall Blocks (Home Depot)
Angle Grinder with concrete-ready blade (only needed if you get blocks that do not have two level faces)
Step 1: Place 14 Blocks in Circle
You could use a string and two pencils to mark out a more perfect circle, but I found it easiest to just lay the blocks in the best circle I could. Then, use a shovel or a stick to mark the outer and inner edges of the blocks.
I would also like to note at this point that you could use any number of blocks for different size fire pits and 12 would actually be the easiest to set up because the angles work the best. I chose 14 simply because I liked that size.
Step 2: Remove Blocks, Dig, and Lay Gravel
Dig out in between the two circles you marked to a depth of 2- 3 inches. Fill this with gravel. It will help when you need to level the blocks and it also helps with drainage.
Dig a hole in the center of the pit about 20 in deep and 15 inches in diameter. Fill this with gravel for drainage.
Note: My pit was located in an area with wood chips so the area already drained well, hence less drainage rocks were needed. If you have clay soil I would advise following the instructions from this old house on the trench/ hole depths. Or you can use your own discretion.
Step 3: Remove "Tags" With Angle Grinder
This step is only necessary if you decide to use retaining wall blocks that have little maybe 3/4 inch ledges on one of faces which I call tags for some reason.
Yes it makes life easier if you use blocks that do not have these "tags", but in order to have this color at from my local Home Depot it was my only option.
So to get rid of the tags I cut across with the angle grinder then "scraped" any remaining part that prohibited the face from being perfectly level.
Step 4: Lay Blocks on Gravel Ring
Move the blocks back onto the gravel ring and tap them down with the rubber mallet. Use the level and the rubber mallet to make sure all blocks are level. Then you simply add the next two layers of blocks with the centers in line with the gaps from the previous levels.
Step 5: Add Remaining Gravel
I dumped whatever gravel I had leftover into the pit to get a full coverage of about 2 inch depth. Again use your judgement on your particular soil type to whether you need a deeper layer or gravel or if you would like to dig the pit down a couple inches before adding gravel.
The one thing I was worried about was cracking of the blocks. I have had 3 fires in the pit so far without any cracking. I will post an update if the blocks do ever crack. Overall for a little more work I saved at least $300 because a similar sized kit would have cost me at least $400. The kits do have a steel ring which helps form a perfect circle, but I was told by a local stone retailer that it was not necessary in order to protect the blocks from the fire.