Question
QUESTION: Hi Marc,    I live in Southern California and want to have a raised garden beds for my backyard.  I have priced wood and WOW its expensive so I started to research River Rock (I have tons of it in my yard and enough to do several 4x8x8 beds)  My question is to verify I used larger rocks on the bottom, and use small and large ones (mine range from football size to small beach ball size)  Will I need a foundation or can I dig a little trench and start putting them in.  Also would you suggest I use mortar? Or do you think stacking will be just fine?  If you suggest mortar can you tell me a little more about it.  I have done much research but cant find much on the mortar.  Thank you very much for your help.  Darlene

ANSWER: The best rock material for a wall is that which has angular or fractured surfaces.  The "potato" type rocks obviously don't want to stack up as neatly,or knit together like broken pieces do...But you can, with careful placement, get a pretty nice looking gravity wall out of them.

You are right - start with the big ones at the bottom, and add the smaller ones on top.  

If you want to use mortar, I'd use it only on the back of the stone that doesn't show, so it still looks like a "loose-stacked" wall. Use it if you are planning to go heigher than about two feet, or in your case, you've got "slippery" rocks.  

In no case should you go heigher than 4 feet without a permit, due to overturn issues, and code restrictions - those pesky earthquakes!!

All that mortar is a limestone plaster/cement mixture (plus some other stuff like alumina & silica) with some sand mixed in for good measure.  The lime makes it more "workable".  Its like frosting that hold the cake together...no substitute for structure, but it sure helps keep things in place.  It can be cound in bag form at just about any hardware/supply store.

Here is a pretty good link to help with the design portion of the task:

http://www.cornerhardware.com/articles/art59.html

Shows you the starting trench, and off you go!  Best results are if you lay down a row, then go back to the beginning, and lay down another row, set back slightly - called "batter" - until you get to your desired height.  

Its best to "dry-fit" each piece before committing to mortar, just to make sure it looks the way you want it to.  You can use a mason's chisel or hammer to "trim" bits off, if needed.  Another great look is to fit small leftover pieces into the cracks after the big rocks are in (called "chinking").   Give is t areally finished look.

Let it cure, then backfill with you thopsoil of your choice.

Hope it helps ~M



---------- FOLLOW-UP ----------

QUESTION: Thanks so much, you helped me a lot.  How long would you suggest to "Let it Cure" ?
Thanks
Darlene


Answer
Mortar/concrete holds its shape in a few minutes, and usually  "sets" within 24 hours.  It reaches full strength in 28 days...you can probably back-fill your wall, depending height, in as little as a day for a two-footer, and maybe two days for a four-footer.  

~Marc