Question: I want to dig a fire pit somewhere in our yard and line it with retaining wall blocks. Three questions:
1. How deep does it need to be?
2. I’ve heard that if you dig a fire pit in the wrong area, you can actually start an underground root fire; is that true?
3. Are retaining wall blocks safe for this purpose?
– William David, via e-mail
depth really depends on what you want and how into your project you’re going to get. For instance, if you just want a basic fire pit, dig about 6 to 8 inches down and call it good. You can go deeper if you want, but keep in mind that you don’t want the hole so deep you can’t enjoy watching the fire. Some people like to add about 4 to 6 inches of pea gravel or sand to their fire pit, as this helps with overall drainage. Of course, this would then require a slightly deeper hole.
A good reason to choose sand over gravel? The root fires you mentioned.
Oftentimes, we might think a fire is out, when really, down in the ground, roots may be still smoldering at scorching temperatures. These roots can, and do, re-ignite and turn into fires.
So to avoid this problem, it’s best to try to build your fire pit far away from any trees. (Usually the root system of a tree extends to about the same width as that of its branches.)
Addtionally, a deep layer of sand on the bottom of your fire pit will cut off the oxygen needed to start and sustain a root fire.
As to those retaining wall blocks you’re thinking of: It would be better to go with firebrick, something that is made to withstand extremely high heat. If you were just placing the retaining wall blocks around the rim of the pit, they’d serve the purpose well enough, but lining the pit with those types of blocks might one day result in you being clobbered by flying pieces of hot concrete.
See also Making a Fire Ring