How to build a Retaining Wall.
The purpose of your retaining wall will determine how you will build it as there is no set rule, the best we can do is give you the principles and guidelines we work with to make our decision on the type of wall and it’s procedure. There are a few aspects to a wall that you must consider for everything to flow together in your landscape plan and so I will be outlining these here.
- The first thing we consider is what exactly are we retaining, for example is it loose soil, clay, or building a wall to just face a rough rock.
- Next, is the ground cut out already or are we bringing in fill to build up to the level of the wall, this will really only effect the final stage of the wall as drains have to be put in directly at the back of the wall and so this becomes a little more difficult.
- What material we are going to build the retaining wall from. This will really be governed by the look and the cost of wall, remember that if you are going to be looking at the face of the wall or walking past it you might want to consider a material and style of wall that is appealing, whereas if you don’t see the face of the wall the look is not that important.
- The wall must be able to more than handle what you are requiring it to do in the worst condition you encounter, The design has to be purpose specific, there’s no one fits all in this case.
Sometimes these varying circumstances will need a specific design of wall along with the building specifications and the Government Authorities involvement. The height of the retaining wall may be subject to building codes etc. therefore you will need to get Council approval and they will require a construct plan drawn out and designed by a structural engineer, this of cause just ads to the cost, but is well worth it if it doesn’t mean having to dismantle it under a council court order.
Timber walls are still the most common to build especially for the DIY as it’s probably less complicated but still here are a few pointers.
- If you are setting posts for rails to either be fixed behind or in front (personally I prefer behind with the post in front, for the look and structural soundness) then posts should be fully set in concrete and as an average be 600mm (2′) in ground, this will of course vary to the height of the wall and the varying solidness of the ground you are setting the post in, solid rock for example needn’t be as deep as loose soil. they are usually 1200mm (4′) centre to centre but this can also vary according to how high the wall is, generally the higher the closer the posts are.
- The size of the posts will also vary to the height and the purpose of the wall, what it is retaining etc. I generally use a gauge bigger than the rail that I’m using, so if I’m using sleepers and the rail is 150 x 75mm (6″ x 3″) then I will use a 200 x 100mm (8″ x 4″) post
Posts are always set to a string line and vertical when the wall is no higher than 900mm (3′). If the wall is higher then we can put a batter on the face which just means they are leaning back towards the fill. For higher wall the engineer will advise what is needed.
You can make sure that the posts are always set taller than the wall has to be and then cut them off after the wall is finished, I usually cut them with a chain saw.
- You can see that I cut off the top of the posts on a slight angle, this is just to let the water run off easier, the chain saw can be used but keep a close eye on the cut so as not to mark the top rail with the chain.
- If the ground permits I like to fan the end of the wall with small posts set to hold the ends of the rails. you can vary this to suite.
Of course we cannot forget the look of a rock wall, these provide a very natural look and with rocks being very durable, they provide a good alternative to the timber. The work however is not only more time consuming but is harder to do.
I will always set the base run of rocks in a wet concrete footing, anchoring the wall to the ground. This footing will also vary in size accordingly to the height of the wall, the rocks used for the base run can vary in size just like the rest of the wall, don’t think they have to be all the large ones as the wall won’t look very nice, you really have to mix it up when positioning the rocks.
All the cementing is done from the back, personally I don’t do much cementing in the front, I like to keep the rock looking at natural as possible, but behind the wall it looks like a disaster, there’s cement, broken bricks and other stones I cement in place to brace and connect it all together. don’t be afraid to even cement a one brick pillar up the back as well, this adds great strength to the wall, especially if you have some thin rocks.