Outdoor Fire Pit Project - Part 1

My in laws have been doing some upgrades at their trailer by the lake which have lead to the need to move the fire pit.  I decided that a fun project would be to make a masonry fire pit.  I did quite a bit of web searching and found a picture of something similar to what I wanted to achieve at this link.  I did some more searching a found a plan for building a stone or masonry fire pit at fire-pit-and-grilling-guru.com.  This site has some really cool projects, advise, and links.  I started to plan to build a masonry fireplace according to the plan on this web site.  I also found a book titled the Complete Guide to Home Masonry by Black & Decker at Half Price Books.  This book has a lot of cool masonry projects in it along with descriptions on how to do things.  It actually even has a project for an outdoor fireplace / grill that is similar in concept to a fire pit.

I started the project this past weekend.  The first step was to create two concentric circles with spray paint in the desired location.  The smaller circle is the diameter of the inside of the pit, and the larger circle has a 12 in. bigger radius.  The next step was to dig the ground inside the circles about 10 in. deep all the way around.  I then put 4 bags of gravel at the bottom of the hole and tamped it down real good.  This step was actually not in the plan, but I decided that it would be a good base at the bottom, plus I needed some extra dirt to fill in the old fire pit.  Finally, I filled the hole with about 6 in. of concrete as the plan specifies.  I made the inner diameter of the pit approximately 40 in., although it ended up being about 42 in. after the digging.  It took about 13 60 pound bags of concrete to fill this in.  the concrete is about 2 in. below the ground level now.  I calculated the concrete needed by computing the volume:

Area = 33^2 * pi - 21^2 * pi
Volume = Area * 6 (cu. in.)
Volume = 7.07 cu. ft.

According to the bag, a 60 pound bag of concrete yields about 0.5 cu. ft.  Based on this calculation I needed 14-15 bags of concrete.  I used ready mix concrete (not quick setting) and mixed them one at a time in a wheel barrow.  It ended up taking 13 bags to fill the trench up to 2 in. below the ground.  After the concrete was within a couple of inches of the top I placed pieces of rebar that were cut to approximately 2 ft. lengths tangentially in the concrete and then covered those up with more concrete.  The rebar will add extra stability and prevent cracking.  I leveled the concrete it with a custom made contraption out of some spare lumber that I attached a level on top of and could rotate it in the trench to screen off the excess.  I figured that it didn't need to be too perfectly level since the bricks would be set in mortar and the mortar thickness could be used to level of bricks which is really what is important.  The purpose of the concrete is to provide a reasonably flat base that won't move too much for the bricks to sit on.

I'm estimating that the total man hours so far was broken down as follows:

2 - getting materials and tools
6 - digging, mixing concrete

Total man hours so far = 8

The next step of the project is to find a source for the fire bricks, the refractory mortar, the regular bricks, and the regular mortar.  I have already determined that the home stores do not sell these materials (possibly by special order, but I didn't check that).  The concrete now needs to set for about 5 days, which is fine because I won't be working on it until next weekend at the soonest.

My one open question about this whole plan is that I believe that it should be necessary to prevent the bricks and mortar from cracking to have a stable base that will not move with the frost in the winter.  The outdoor fire place in the Black & Decker book clearly states to make the concrete base deep enough so that it is below the frost line in your local area.  In this case, 8 in. will definitely not be below the frost line in my area.  So one thing of interest to keep an eye on in the future is how much cracking I will end up seeing in this fire pit.

One last thing is that I made the fire pit quite large at 42 in. diameter.  A couple of times during the process I had second thoughts that maybe this was too big.  One thing I know for sure is that I like big fires, and you can always build a small fire in a bigger fire pit, but not so much the other way (big fire in a small pit).

A wise proverb from my Boy Scouts days: "Indian build small fire, huddle in close to stay warm.  White man build big fire... carry wood to stay warm!"