The side of our house if continually overrun with weeds. I'm thinking of fencing it in and laying some concrete to prevent the weeds and provide some storage area. Currently there is red lava rock over there (under the weeds). Pretty much this stuff (not my photo):

Lava Rock

I have never laid concrete for any reason, though I've dome some tiling and other stuff that makes me think I can handle it (famous last words, I'm sure). I haven't measured it yet, but let's say it will end up being a 4' X 30' slab, if that matters at this point.

The question I have is whether I can use the lava rock as a base for the concrete rather than using something I'd have to buy. I suppose there might be concerns given the size of the rocks (1" or so versus the smaller stuff I've seen used as a base) and the porous nature of the rocks.

I understand that I'd still have to remove the rocks and prepare the area by removing weeds, removing some soil, etc. before putting the rocks back as a base.

Thoughts? Suggestions?

3 Answers 3


The key to a good base is compacting and stability. If you could compact the rock adequately there's no reason it couldn't work but appropriate concrete base is so cheap, and cracked concrete such a pain to repair that it makes no sense not to use exactly the base intended for the purpose.

  • Thanks for the response, Matthew. The main reason for trying to use this landscaping rock as a base is to avoid disposing of it, which is a pain itself. Secondarily, it would save a little money.
    – Joey
    Commented Sep 1, 2012 at 16:28
  • As The Evil Greebo says, isn't lava rock very friable? Won't that work against compacting and stability? I like your other advice.
    – bib
    Commented Sep 2, 2012 at 0:35

By itself I would not rely on it, as this is lava rock which isn't the strongest - it's prone to breakage.. However, if you cover it with stone dust to help fill in the voids which will help prevent shifting (which encourages breaking) and then solidly compact it with a power compactor, then follow up by covering it with a layer of sand on top, compacted again, then I think you'll have a good solid base for your slab.

What really matters, though, is how deep it goes. You need the base to go down below the normal frost line for your area. If you don't dig down deeply enough, then come winter, the freezing ground under the slab will heave and destroy your concrete, which will be a huge waste of money.

So if you need to dig down more, which you'll find out quickly (if you're in N CA go 16-24", if you're in the southern part, 1' will be fine) then dig down, and fill with this first, then gravel, then stone dust and finally sand - compacting each layer as you go.


If the goal is to prevent weeds and obtain storage, why not consider putting down paving blocks. Assuming the storage is not overly heavy, you could fill in with sand. or decomposed granite or crushed limestone (fine base material) topped with sand, followed by paving blocks that are much easier to install than poured concrete, and much more forgiving of freezes and thaws (and revisions).

You may need to do a litte weed pulling or a bit of spraying (such as Roundup), but the main goal could be met with a reasoanbly quick fix. You could also use a weed blocker (such as landscape cloth) in combination with the above to suppress weeds for several years.

  • I appreciate the suggestion. We have a little paver'ed area in our backyard. I have landscaping cloth and sand underneath them. In the last two years, the weeds have prevailed and we're reluctant to spray there due to proximity to vegetable garden. I'd like to conquer these weeds once and for all (at least on the side of the house).
    – Joey
    Commented Sep 2, 2012 at 3:26

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