How to build Rock retaining walls
When it comes to rock retaining walls, this is something we really love doing. Why? Because it has two main functions, one is to create level usable area’s and the other is because there are so many natural stone options for materials and look’s when building those walls.
Our main focus in this post is natural stone for retaining walls, which will be Flagstone or Granite.
What Are Flagstone Walls?
Flagstone is a type of flat sedimentary stone that is usually delivered on pallets, and has been pre-graded by the supplier. It is usually more expensive than most stone, as it has passed through many hands to get onto the pallet, therefore being more money. Having said that, with it having been pre-graded, you can stack it and get more unique shaped and angles from it, that you might not get from Granite boulders.
What Are Granite Walls?
Granite is normally graded in either weight or by the foot i.e 1-2ft boulder, 2-3ft boulders. This natural stone material is more rugged in shape, and most quarries blast it out of the side of mountains and then send it to you or the supplier in trucks. Shapes are irregular and harder to work with. Most of the time to build any substantial sized rock retaining wall, you’ll need machinery like a bob-cat or excavator.
If your lucky sometimes you can find a quarry that doesn’t blast the rock, they actually chisel the rock using jackhammers on the front of big excavators. If this is the case, then you can get much more squarer shaped boulders, which will give you nice shaped walls and steps. They usually have more straighter edges, which lend themselves to a nicer ‘finished’ look.
Design and Location of your rock walls
When looking to build a rock retaining wall, first you have to know where your going to build the retaining wall, as well as how you will get access from one side to the other once it is built. This may require steps or a slope to be included in the building of the retaining wall. When deciding that, it is best to consider what is around you, and what will be your most direct route of travel. Do you need to get to a shed or a lower patio? if there is a shed in the landscape, then maybe consider where the lawn is in relation to it and do you need to get a lawnmower to the upper grade of the rock wall. Do you need access to the lower side of the rock wall with a wheel barrow for the planting bed? These are all thing to consider when looking at installing a wall.
Another thing to consider with retaining walls would be, does it have grass backing the wall. This can be a maintenance headache when it comes to mowing and using a weed wacker as you can’t get in between the boulders. In one landscape, the homeowner got us to fill the gaps with smaller rocks, and concrete them in place to prevent this problem. that takes a lot of time and can be avoided if you design your landscape using planting beds on both sides of the wall.
Garden with retaining wall
Building the Rock Retaining Walls
When building retaining walls, the first thing to do is to mark out where you want the rock walls to be. Then it is best to figure out what the finish grade of the lower area will be, as well as where the top of the wall will be. You should already know this, as you will have had to figure out how much rock you need to build the retaining walls.
When it comes to actually building the boulder retaining walls and you have your grades and location figured out, take a look to check the rock dimensions. With this you can figure out how wide your rock walls ‘footing’ needs to be. Typically, if i am using 1-2ft rock, then i would have a footing of about 2-2.5 ft wide. The footing should be dug out down to clay if possible and compacted in layers using a plate tamper or large hand tamper. Soaking the crush gravel before compaction is always a good idea also. If you have any irrigation or lighting wires to go under the wall, then adding some sleeves for them at this time will make your life easier later.
For a good footing I would have it start 4-6″ in front of the wall position and make sure its below finished grade. It is beneficial to have a good few inches of your wall buried, to help prevent pushing out. Having the wall sloping back slightly will also help for strength also. If you are worried about drainage then adding drainage behind the wall can be beneficial also, just make sure you have a point to quietly exit that drainage too.
As you build your wall, try to keep an eye out for extra nice pieces of rock that would make for nice corners or finishing caps. These should be put to one side, for when you get to the end or to the corners. Any wall can look good or functional once finished, but a wall that has crisp corners and a flat top, will always look more eye-catching once complete.
Whether you are building a retaining walls out of granite or flagstone, make sure that each rock is exactly where you want it before moving to the next one. Making sure each rock is not moving and backed in crushed gravel will only make things easier as you get higher. Taking time to completely fill the back of each row, soak and gently pack the crush will only make for a stronger base for the next row as you progress. Check to make sure the wall as a consistent back lean regularly, you don’t want to have to adjust rocks later.
Once you get to the last row of rocks in your wall, you may want to stop with the crushed gravel and fill any gaps in the back of your wall with a mix of gravel and small rocks/boulders. This is so you can have top soil right to the back of the wall and not have any of the crush showing once finished. the small rocks that fill the gaps of the last row will help to keep soil in place. This last row maybe more frustrating than the rest if you are going for the ‘level’ ‘finished’ look, but if you have saved yourself a number of flat rocks with nice edges, then its just a case of trial and error, till you get the top row looking how you want.
Once the wall is built, it is time for filling the small gaps. This may seem as a tedious and painstaking task, but it only makes the wall stronger. By filling in the smaller gaps with small rocks, there is less chance for movement, movement of the rocks and of the material behind. It is up to you how detailed you wish to be with this process, obviously the longer you take the better it will look. I have done some walls where all the small rocks had to be stacked neatly in the gaps. This process took forever and required alot of patience, but once finished, it really made the wall stand out.