So you'd like to build a garden wall...

March 23, 2009 6:00 AM

Is there an 'ask metafilter' on the internet for home/yard projects? Not that I'm disputing the collective mind here, but I'm looking to build something in my backyard and it seems to me there must be some group of contractors who all get together somewhere on the internet to tell amateurs that they're doing it wrong. Of course if you KNOW how to install a yard retaining wall, then, by all means, speak up.

So here's the thing: I'm renting a house with a backyard. A very sloping, bumpy, uneven back yard. And I'd like to put some gravel and picnic table down. But to keep said picnic table from sliding down the hill to the neighbor's chain link fence, I thought I'd try to even it all out.

So here's my plan.

Put in some deep posts - like five feet tall - along the bottom part of the hill slope. Sink them halfway down into the the hill. Then, put some boards from one post to the next. nail em in. Fill area with dirt.

Then, after, I figured I'd get some fruit trees from that orchard that's always giving them away and put them BELOW the posts and aim them so they grow towards the fence and eventually support it.

All in all? I'd say it's a fool proof plan. Except when it comes to this sort of thing, I'm kind of a new fool to the arena. Some have told me I need a specific kind of lumber. Others said I have to put the posts in concrete. Which I'm guessing I have to make. Others said the opposite, pointing to the non concrete much MUCH smaller version of said wall that stood for a long time previous.

So do you know the answers here? And if not, do you know where I can GET the answers?

posted by rileyray3000 to home & garden (24 comments total)

My surveyor/civil engineer grandfather's mantra was "land is liquid." You need to think of the slope as a very slow moving, very heavy wave of liquid oozing down into your yard. How would you hold that back?

Due to the sheer weight of the material, boards and nails are not going to hold it back. You are going to need something solid for a five foot tall wall, something like reinforced concrete, masonry, or railroad ties (or treated 6x6 or larger lumber). You are going to need to allow water that seeps down through it to escape, so you will need drainage at the bottom of the wall (otherwise the liquid becomes even more viscous). Last you are going to need to calculate the volume of the fill, which is going to get pretty large pretty quick, particularly in cost.

I'm not telling you to discourage you here, I just want you to think of scope of the task. You may want to speak to your landlord before initiating such a large, expensive project also.
posted by Pollomacho at 6:32 AM on March 23 [1 favorite has favorites]

Unlike residential construction where you have strict codes that tell you how you're allowed to build, a retaining wall is an engineered structure that can have serious consequences if it fails, including, but not limited to, landslide, collapse of adjacent structures, etc etc.

Are you sure your landlord will allow your proposed earthworks project? Are you sure you want to spend the amount of money and labor it will take to accomplish this?

So do you know the answers here? And if not, do you know where I can GET the answers?

Yes, I'm a civil engineer. Get an engineer.
posted by electroboy at 6:35 AM on March 23 [1 favorite has favorites]

SomethingAwful has DIY forums that are very active and have very knowledgeable people posting. Check there.
posted by unixrat at 6:38 AM on March 23

Tenants attempting to improve the landlords property usually never ends well, even with the landlords initial approval. My advice, is to request (nicely) that the landlord do a little something with the backyard so you can enjoy it. If he is hesitant (since you did rent it the way it was) and you are desperate to have the backyard you dream of, then offer to put in a little bit of money for the work. Do not, under any circumstances, do the work on your own. Its a good way for the landlord to turn around and cause you grief if he/she does not like what you have done or if you have a falling out. His property=he is the one to make the changes, not you.
posted by scarello at 6:40 AM on March 23

You might try the forums over at Gardenweb, which isn't all about flowers. There are forums on landscape design and a whole other sub-site on home related topics.

And yes, what they all said. A retaining wall is not something to do half-assed. If you do your homework and do it correctly, you can DIY though. My brother built one with RR ties and a metric shit-ton of fill. It wasn't trivial, but it's held up. Seems like a lot of work for a place you don't even own.
posted by bondcliff at 6:48 AM on March 23

Before you hire an engineer to get drawings done - how tall is the wall you are putting up?

No matter what tho, your plan is not a good one.
posted by JPD at 6:48 AM on March 23

I have done some landscaping and my parents are avid gardeners so I have some experience with retaining walls. The first consideration is how high it will be. Higher walls are obviously more difficult, so you may be better off building a series of terraces. According to this PDF from Fairfax County, VA any retaining wall over 2 feet requires a building permit and inspection (in other places 3 feet is often the cutoff). The link also has examples of different types of construction and materials that are acceptable. As you can see it is not a simple process, which is why keeping it below 2 feet or whatever your locality specifies would be best. As for materials treated lumber is fine, as are old railroad ties. Concrete, bricks, and stone are other options. Precast concrete bricks like this are widely available for this sort of thing. Your local nursery or building supply store will likely have someone who can help you with planning and supplies. Depending on your site you may need to consider drainage issues and need to put a bed of gravel under your wall. Finally, using trees to support the wall is not a good idea because they could die of disease and leave you with no support; if built properly they will not be needed (and would be healthier planted on flat ground anyway). Hope this helps.
posted by TedW at 6:51 AM on March 23

To help your research, what you are proposing is called a retaining wall; here is a pretty typical article about DIY-ing a timber retaining wall. Your city's planning office will be able to tell you how high, and of what materials, you are allowed to go before you need an engineered wall -- it varies tremendously by location. Because you are on a slope, remember that the wall will be supporting a tremendous weight of soil, so you will need to build a stronger wall than if you were on a flat site.
posted by Forktine at 6:51 AM on March 23

Better to leave the hill where it is and build a small deck. Less engineering, less material, less likelihood of collapse.
posted by jon1270 at 6:52 AM on March 23 [3 favorites has favorites]

jon1270 has got the right answer. There's plenty of resources for deck building on the web, or at your local big box hardware store.
posted by electroboy at 7:10 AM on March 23

Speaking of big box stores, Lowe's actually maintains a decent DIY site. (But I agree, terracing your landlord's yard seems amibitious if not actually insane. A deck sounds like the solution. Be prepared for him to raise your rent once you have a usable back yard.)
posted by nax at 7:44 AM on March 23

A retaining wall is no small feat, and if you were to do such a thing, you would be best getting an engineer involved, and you would probably need a permit even if you DIY. This is not something to take lightly - if the wall collapses - that's a lot of land that is going to slide down into the neighbours chain link fence, yard and possibly house. More trouble than its worth!

I second the alternate suggestions to do some terraces or build a deck. You can do some cool cheap terracing with railroad ties - my neighbour just did this with the slope in her garden - I keep looking enviously at it through the chain link fence :)
posted by Joh at 9:44 AM on March 23

There are three major structural problems with your plan, if I'm understanding it correctly:

1. You are only planning the vertical portion of the wall. Effective retaining walls will always have some kind of horizontal component to tie the wall into the hill they're holding. Otherwise no matter how you work your posts they will be pushed out and away from the hill quicker than you think, especially if your design alters the way water currently flows and holds it back.
2. On the point of drainage, you don't detail any plans on this. This is a major concern when building your wall, and if left to chance will destroy what you built.
3. You cannot "aim" trees.
posted by odinsdream at 10:00 AM on March 23

If you actually go through with building a wall, Forktine and Joh have nailed a couple of the issues you're going to deal with. Implied in their answers are some dealings with your local building department, which can have both beneficial and detrimental factors for your project. First, the nice stuff: at my local building departments, they'll frequently have standardized retaining wall details for walls of certain heights that you can use and pretty much get an instant permit, instead of having to go and hire an engineer. All you have to do is give them a drawings showing the locations of the retaining walls and how tall they'll be at certain points.

The nasty stuff: If I remember correctly, my building department (and, essentially, the building code) requires permits for walls over a certain height, which I think is in the 1-2 foot range, but I'm not sure. If that's correct, your wall require a permit, and unless you use one of the aforementioned standard wall details, you'll need to hire an engineer. Also, every building department I've ever dealt with has requirements that only the owner of the property can meet and sign off on, so this isn't something you can just do without getting the owner's approval (which would be a collossally bad idea in the first place). In some cases I've been required to produce a deed and a title report proving that the land belonged to whom we said it did.

You are only planning the vertical portion of the wall. Effective retaining walls will always have some kind of horizontal component to tie the wall into the hill they're holding. Otherwise no matter how you work your posts they will be pushed out and away from the hill quicker than you think, especially if your design alters the way water currently flows and holds it back.

Not necessarily. If the posts are driven deep enough relative to how much they're retaining or if it were a particlularly massive concrete/masonry wall, there wouldn't be a need for any kind of extended footing (the proposed wall meets neither of these conditions). People build retaining walls just from I-beams driven into the ground and wood planks all the time.
posted by LionIndex at 12:00 PM on March 23

Trees grow straight up, no matter which way you point them. I'm not even going to go into the silliness of using sapling trees to support a structure of any type, let alone a retaining wall.

Where I live, the maximum height of any retaining wall can only be 36"- after that you must hire an engineer. What I'm writing below only applies to walls 36" or lower:

I'm guessing you're talking about a post and lagging wall. Yes, your posts have to be anchored in concrete- the weight of the concrete helps keep the wall from overturning. You need to know the basic ratio of slope to determine how deep to sink the posts and the minimum spacing of the posts. At minimum, your posts should be sunk 24" inches into the ground, with a general rule of thumb that is equal to height (36" wall, sink posts 36". This is multiplied for anything over a 4-1 slope, which if you say is "very sloping" absolutely needs to be determined.) You also need to have proper drainage behind the wall. Make sure your lagging is behind the post, otherwise it is only the surface area of the washers around the bolts holding back the soil.
posted by oneirodynia at 3:22 PM on March 23

Then, after, I figured I'd get some fruit trees from that orchard that's always giving them away and put them BELOW the posts and aim them so they grow towards the fence and eventually support it.

I missed this on my first read-through. Sorry, but as mentioned above, this part of your idea just won't work.

But (and this is the constant problem with construction questions on AskMe), there's a lot of strong assertions in the well-meant advice in these responses that aren't all that true, either. Retaining walls don't necessarily need horizontal components, concrete foundations, or an engineer to do the design, for example. (Of course, as the wall gets higher, the slope steeper, and the soil more prone to slide, those things -- most especially the engineer -- become increasingly critical.)

The really, really important thing that is being said here, I think, is that you can't do major earth works without the involvement and permission of the property owner and the local planning authority. That, and the fact that even a low retaining wall involves surprisingly large forces that can create real problems if you build it wrong. (Also, have you calculated the cost of delivering and moving the tons of topsoil you will need? Filling, leveling, and landscaping the area behind your 2.5 foot retaining wall will probably cost a lot more than you would want to spend on a rental property.)
posted by Forktine at 5:26 PM on March 23

I figured I'd get some fruit trees from that orchard that's always giving them away and put them BELOW the posts and aim them so they grow towards the fence and eventually support it.

Think for a minute on this. Have you ever seen a retaining wall secured in this way?

In general, when one is attempting a complex project, it is a good idea to start with things other people have tried and found to work.

Even out a little spot just large enough for the picnic table, and skip the expensive terracing on property you don't own.
posted by yohko at 7:53 PM on March 23

Okay quite a few things.

1) It's a small wall I'm shooting for. Like two feet. MAX.
2) It's a very small yard.
3) I didn't know that about trees. I saw something about indians bending trees to make benches as they grew. I figured I was shooting for something like that.
4) I'm all for an engineer. But where does one get such a person who specializes in gardens?
5) My landlord said I can do what I want. He seems pretty indifferent.
6) There's free fill down the road. And I've got a truck.


I think that covers most of it. As to why I'd put money into something I didn't own...well I like working on stuff like this. And it's cheaper than that classic car I used to throw money at.
posted by rileyray3000 at 11:54 PM on March 23

1) It's a small wall I'm shooting for. Like two feet. MAX.

Probably it will still require planning oversight. Don't skip this; if a neighbor turns you in and it turns out you are in violation (perhaps the city considers your backyard part of a vital wetland or something like that) it can get expensive and ugly.

4) I'm all for an engineer. But where does one get such a person who specializes in gardens?

Phone book. You just need a qualified civil engineer; it's a really little project so you'll probably have the best luck with a small operation rather than a huge company that specializes in huge government and commercial projects. But don't talk to the engineer until you have talked to the planning department -- they may be able to ok non-engineered solutions for a short wall, which saves you money.

6) There's free fill down the road. And I've got a truck.

Calculate what you'll need in cubic yards; a regular pickup can carry about one cubic yard (more if you have a one-ton flatbed, less if you have one of those little baby trucks). That'll give you an idea of how much shoveling is involved. Don't forget that you will want to compact the soil as you add it, and you will want the top layer to be high quality topsoil, rather than just "fill," if you want to be able to grow a lawn or garden.
posted by Forktine at 6:18 AM on March 24

I worked landscape construction for all of high school and college summers - you don't need an engineer for something of that size unless code demands it. You should be able to find directions on what you need to do online (maybe on the LOW DIY sight posted up thread). Probably use railroad ties.
posted by JPD at 7:32 AM on March 24

Well, you'll probably need two walls to create a level area, one on the uphill side, one on the downhill. The DIY that forktine posted looks like a decent guide and if you're only talking two feet, it's unlikely that you can go too far wrong. Probably no need for the deadmen, just pressure treated posts embedded in concrete should do you just fine.

Also, oneirodynia and odinsdream are mostly incorrect. If you look at the graphic from the wiki entry, the wall you're proposing would be a piling type wall. Concrete is generally used to replace the compacted soil you removed when you dig the post holes. It's possible to replace the soil, but effectively compacting it would be pretty difficult. The horizontal component of the soil load is resisted by the buried portion of the post, but not by the weight of the concrete.

The only time the mass of the wall really comes into play are in the gravity and cantilever types, where the downward force from the wall's mass resists the overturning forces applied by the soil behind the wall.

It is possible to shape trees using guywires and pruning, but not for any structural purpose. The most benefit you would get is having the roots prevent soil erosion.

If the permit office does require sealed engineered drawings, it's likely you'll want to skip the retaining wall. Even a pretty simple sketch is going to set you back about a thousand dollars due to the time necessary to do a proper site assessment, drawing prep and assumed liability. You need a general civil, site civil or geotechnical engineer to design the wall. Two feet though, that's pretty much a glorified curb. I can't imagine they'll hassle you too much.
posted by electroboy at 7:33 AM on March 24

At less than two feet you are unlikely to run into any code issues, but check to be sure; you local planning and building department (or whatever it is called in your neck of the woods) might have that info online as in my link above. Also think about how it will affect drainage; if it adversely affects your neighbors yard they might be able to make you take it down; also you might need to allow for drainage at the bottom of the wall so that your nice level terrace doesn't turn into a mud pit. Otherwise, what you are proposing is in many ways similar to a raised planting bed and you can look at plans for those for ideas. I built one in my yard very similar to this one with railroad ties.
posted by TedW at 7:58 AM on March 24

electroboy: Thanks for clarifying the types of walls. I stand corrected. I just hope the OP has the resources to bury a sufficient amount of these pilings into the soil.
posted by odinsdream at 8:01 AM on March 24

You had the basic concept right, just the explanation a little off. The driven I-beam and lagging is pretty typical for heavy construction, but it's best accomplished in this case with a post hole digger and concrete.
posted by electroboy at 8:06 AM on March 24