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Getting started



Foundations:


There's an abundance of old adobe ruins scattered throughout the lonely places throughout the Southwest. Reconstructing how they came to such an end can sometimes be instructive. One cause that that's seen is a failure on the builder's part to have given the structure a proper foundation. (The same can be said of relationships, but we won't go into that here.)


Here's some food for thought. If you're planning an modest earthbag structure - let's say 10' x 10' with eight-foot high walls - and if the bags average a mere 25 lbs each - you'll be using approx. 1,000 bags with a combined weight of about 24,000 lbs (12 tons, or nearly 11,000 kilos), with a compressive load of around 700 pounds per square feet. Now, increase the bag weight to 30 or 40 lbs, raise the wall height to 10 feet, and recalculate.


The point is this: the potential soil compression of such a weighty structure can cause differential compaction (uneven settling) of the ground, depending on the ground's makeup (rocks here, sand there). This may over time cause your structure to start sagging. You might not notice this until your doors & windows start sticking and then your plaster starts cracking. We'll leave it to you to ponder what the worst might be.


Your ideal goal, then, should be to strive for perfectly vertical walls, with the stress load being as close as possible to being perpendicular to the ground. If you're building on a slope, you'll want to dig your foundation trench deeper on the downhill side so you can maintain that perfect verticality. Additionally, if you might be dealing with rainwater or snowmelt (on a slope or in a hollow), incorporate a french drain into your plan to channel water away from your foundation.

Dig your trench where your walls will be (a little wider than your bags, to allow exterior drainage), and then fill it with something that won't shift or compact over time. Good available choices include gravel, rocks, or small, broken-up chunks of concrete rubble. How deep? It depends on the size of your structure & how solid (or mushy) your ground is. It also depends on if you have hard freezes in your region; if so, you'll want to go down below the frost line so that your structure's foundation won't be prone to frost heaving or the freeze/thaw cycle. Generally, 18"-24" (45-60 cm) is a good depth.


If you want to dig it deeper (which might incidentally allow you to lay your first few courses below ground level, sitting atop your foundation material & adding significant structural strength), then so much the better. If your fill is irregularly shaped (such as rubble), it'd be a good idea to pound it with your tamper or a sledge, so the chunks will "lock" together & have less likelihood of shifting after bearing weight. Additionally, you can mix & pour concrete over your fill, adding enough water to the mix that it will seep down & fill the void spaces before it sets up.


Stem Walls:


Stem walls typically consist of the first 1-3 feet of your walls above grade and may be considered an extension of the foundation. Their purpose is to prevent water on the ground or in your foundation trench from wicking up into your dirt-filled bags, and to add a measure of protection against rain-splash or piled snow from saturating (and weakening) your earthen bags. How high you build them depends upon your local conditions.


At their simplest, these can be bags filled with gravel (instead of dirt fill). Rounded gravel or river rock may be preferable; the sharp-edged stuff can tear your bags as you're tamping them. On the other hand, sharp-edged gravel, as mentioned above, does have the virtue of locking together when tamped, preventing movement or the contents. You decide.
They don't have to be gravel bags; you can fill your earthbags with wet cement, or you can just use bags of cement straight from the hardware store (average dimensions 20"W x 30"L x 4"H). Poke a few holes, saturate the bags with water & they'll harden right up. Use barbed wire in between courses before the bags harden to help lock them into place.


Fill Material:


Some people want their fill to be as natural as possible. Others just want a solid structure & don't really don't care. Dirt needs a component to help it to "set up" or harden, so your bags won't slump over time & jeopardize your structure's integrity.


Natural fill: You'll ideally want relatively clay-rich sandy soil. Soil typesClay is what helps your mud to set up hard. About 20-30% clay is good. A higher percentage is not good, since some clays swell & shrink as they absorb moisture and undergo drying. If you have too much clay, you can "dilute" it by adding sand. We find the adjacent diagram useful. A "sandy clay loam" is a good thing to shoot for. What you're really doing is making an adobe mud (minus the straw) to pack into your bags that will harden.


To help determine how suitable your native dirt is for building & plastering, we offer two tools: a link to the USDA's NRCS Web Soil Survey, and a basic guide to testing your own soil here.


If you need to add clay, you might be able to find clay-rich dirt in different areas of your property, or at different depths. You can also check out your neighborhood (river bottoms & flood plains are good places to look). Worst case, you can buy powdered clay in bags from potter & ceramic suppliers, though this might be expensive. Currently, we can find it for as low as $0.64 per pound.


Lime and cement amendments: If adding clay isn't an option, and if you're not partial, you can amend your soil with hydrated lime, cement, or a combination of the two, You can get both of these very reasonably in large bags at your local hardware store. Using cement alone (road construction crews call this "soil cement") has a tendency to crack, so if you opt for this you might try adding a little lime (which adds flexibility) to your cement. Start with 10% & work your way up to about 25%. Experiment. Even if all you have is 100% silt, beach sand or gravel, amending it will work to a large degree if you add sufficient stabilizer to it.


Because all dirt is not the same, we can't offer you a universal formula. Make samples. Start out small by making a few different little batches (or fill a few bags), maybe using samples from different corners of your property or from different depths, adding clay or sand or stabilizer or whatnot. Be sure to label them.  Set them out in the sun & let them cure for a few hot days. If it's still winter or early spring - still cold outside - then bake them in your oven at 150 or 200 degrees (keep the heat low if your samples are filled bags so they don't melt in your oven or catch fire!).

After they're hardened,  test them & write down your results, referring to their labels. Press them between your fingers, press down on them with your palm against a hard surface, try twisting them to shear them apart. Let it drop from waist or shoulder height onto the ground (dirt or grass, not cement) & see what happens. Shattering is bad, cracking into two halves is better). At the upper end is to try gently tapping a small nail into your sample. If you can do this & not have your hardened shatter or splinter, you're on to something good. If your best sample shatters, add less sand. If it cracks into several pieces, maybe add a bit more clay or lime, then try again until you find a mixture that's optimum for your dirt.


The trade-offs of using cement-stabilized dirt include factors like poor plasticity (cracking due to settling,  earthquakes, vibration from road traffic, etc.) and lousy thermal insulation (anyone who's ever lived in a cinder-block house knows how cold they are in the winter & how hot they are in the summer). Adding lime lends plasticity, but it's caustic & it can be a pain to work with. Your choice.


If you prefer to have fill brought in to your building site, one common alternative to dirt is road base or "engineered dirt", available by the cubic yard from many aggregate yards. They're engineered to have a good sand/clay ratio, though road base often has gravel or rock that you might want to sift out (this sifted rock or gravel would be great for your foundation trenches or for your stem walls).


Scoria: Also known as lava rock, this is another good alternative (but not available everywhere). It's much lighter than dirt, and the air voids help provide insulation.


Planning:


A frequently-asked question we get is, "how many bags will it take to build a (for example) 2,000 square foot building?"


Hard to answer, because a 2,000 square-foot building can have different configurations (wall widths & lengths) and - more importantly - different wall heights, as well as various numbers & sizes of doors & windows.


If you're trying to calculate the number of bags you'll need for a given project, it'd be easier for you to calculate the dimensions (height x width) of each wall. Look also at how many doors & windows you plan to have; whether the walls will go below grade or sit atop your foundation; whether you plan on having interior walls to create separate rooms; and whether you'll be adding stabilizing buttresses to long walls.

The bags we use for our estimating are 14" x 26" (about 35.5 x 66 cm) when empty and average about 30 lbs. when filled. When laid out & solidly tamped down, these bags measure approximately 12" long, 12" wide, and 3" high. We maintain this standard because it leaves enough space in the bag for the fill to be evenly distributed when laying horizontally & tamping (a full bag obviously won't work). It also allows us to provide calculations for estimating. Additionally, 30 lbs is a pretty manageable weight for most everyone, particularly when lifting or swinging them over your head as your walls rise.


If you fill your bags to a different standard (say, 2/3 or 3/4 full), you'll find that your tamped dimensions will be different from what we're providing here. Obviously, bags with a tamped height of 4 inches high will constitute 3 courses to a foot, instead of 4 courses to a foot. Likewise, a filled & tamped bag length of, say, 16 or 20 inches long will be... well, you get the idea. Even so, you may find the below calculations useful.


Rules of thumb:


For basic construction (a simple wall), based on 30 lb., 12"x12"x3" filled & tamped bags. You can add more fill to each bag - up to 60 lbs., which will give you more length/height. Bag widths remain the same at  approx. 12 inches. Be aware, though, that heavy bags can be harder to manage & require more effort to hoist as your walls go up.




                                                      Height                                             Length (course)

   1 bag  3 in. high
1 bag length 12 inches long
   4 bags  1 foot high
2 bags (end to end) 24 inches long
    8 bags  2 feet high
3 bags (end to end) 3 feet long
    12 bags  3 feet high
6 bags (end to end) 6 feet long
    16 bags  4 feet high
9 bags (end to end) 9 feet long
    20 bags  5 feet high
12 bags (end to end) 12 feet long
    24 bags  6 feet high
15 bags (end to end) 15 feet long
    40 bags 10 feet high
18 bags (end to end) 18 feet long


*** Note: see comments on "buttresses" under Wall Length (below)


   Example #1:

Imagine a wall that's 5 feet high and 12 feet long.  With bags at 12" long, you'll need about 12 end-to-end bags to span the wall's length of 12 feet. This is one "course". You'll need 20 courses of 12 bags each, one on top of another, to reach the wall's height of 5 feet. 20 courses times 12 bags each = 240 bags (not counting bags for buttressing).


Example #2:


Consider a wall that's 8 feet high and 36 feet long. Each course (row) will have 36 end-to-end bags, and you'll have 32 courses. 36 x 32 = 1152 bags (again, not counting bags for buttresses).

These figures are guidelines. If your wall is 35 feet long, you'd simply dump out one or more bags and use partials. And, again, an alternate way to estimate how many bags you'll need is to look at our calculation page, which again is based on 30 lb bags. If you're filling your bags more (say 4-5 inches high, or 18" long, or whatever) then you'll of course have to make adjustments.



Stability:


center of gravity

It can't be emphasized enough how important it is to frequently check your work with a level to ensure that your walls are vertical as you add courses.


Consider that a 8 foot by 8 foot wall is going to weigh over 4,500 lbs (over two tons). If the weight is directed straight down, towards the foundation, your wall (and everything that's tied to it) will be stable. If your wall is "off vertical" even just a few degrees, the error will magnify as your wall increases in height & you run the risk of the wall eventually failing - either by shifting (causing cracks throughout your structure) or collapsing.


With wood, you can make adjustments with shims as you go along. With earthbags, you can do the same with how you lay your bags (a little thicker on one end than the other) & how you use your tamper. Using barbed wire to lock the bags together is recommended up to 2 feet high; over this, you should consider it mandatory.



Wall Length &Buttressing:


Despite whatever care you give to leveling your foundation, and despite the stable 12" width of our dirtbags, you'd be well advised to consider the addition of a buttress every10-12 feet or so for straight, free-standing walls. Emphasis upon "straight"; sinuous walls are more stable.


The rule of thumb is 2:1; one-foot thickness of buttress for every two feet of wall height. A 6-foot high wall should have a 3-foot thick buttress every 10-12 feet. (The hypothetical 36-foot long, 8-foot high wall mentioned above should have 2 to 3 buttresses, each 4 feet thick, adding an extra 96 bags per buttress to the 864 bags estimated, for a grand total of 960.) Of course, the buttresses should have their own gravel or rock-filled trench foundation to sit upon so they don't shift.


Optionally, there are those who have built double-thickness walls (24" thick), and one person even spaced the two walls 6" apart & filled in the gap with packed earth for 30" thick walls. With that wide a footprint, buttressing might be far less critical. But they do look cool and can lend themselves to secondary uses such as benches or planters.


Doors & windows:


When figuring out how many bags you'll be needing, you'll want to factor in the number & sizes of your doors & windows and then subtract these spaces from your total square footage.


Let's say one wall is 10 feet high and 15 feet wide; that wall will be 150 square feet, and - based on our calculations page - will require about 450 bags. If that wall will have a 3x6' doorway and a 2x3' window (18 square feet + 6 square feet = 24 square feet), then that wall will result in being 126 square feet and will require approximately 375 bags... a savings of about 75 bags. (Note that the bags you save here can then be used for  benches, planter boxes, covered porches or buttressing - see below.)


You'll be wanting to frame out your doors & windows with wood, and build your bags around the frames. The frames will serve several purposes. The most obvious is providing a guide for stacking your bags & provide something for the bags above the lintels to rest on. They'll also increase the strength of the wall (the sheer weight of the bags pressing against the sides and pressing down will firm up the openings). And finally, the wood will give you something to nail or screw hinges into for doors, shutters, etc.


Speaking of which, we should mention gringo blocks, which are blocks of wood that are the same approximate dimensions as your bags (height & width, if not length). By substituting these in in your wall in place of bags, you'll have a wooden surface to which you can nail or screw shelves or whatever.



Roofs:


Best practice for roofs is to start with installing a bond beam atop your walls. These come in essentially two flavors.


Wood bond beams are just that; heavy wooden beams, tied together at the corners, and well secured to the tops of the walls. Drilling holes in the beams & pounding 18-24" lengths of rebar down into the top courses of bags is a good approach. If you have access to an electric drill & if your bags - once cured - can handle having holes bored into them without splitting, then this will give you a secure bond beam that you can then attach your roof to & have a high level of confidence that it won't blow off in a big storm. You'll want this to be bomb-proof.


Some building codes will call for a cement bond beam. This consists of installing vertical lengths of rebar in your upper courses (especially at the corners), then clamping plywood forms to both sides of your walls, creating a trough all the way around. Once your trough is ready, you'll then pour mixed concrete into the trough to a typical depth of 8-12". The cement will flow around the rebar When it cures, you can remove the plywood forms & you'll have a solid concrete beam that's securely attached to the tops of your walls and into which you can drive bolts to secure your chosen roofing.


Another approach is to crenellate the tops of your walls - like castle battlements - and then lay thick timbers in the openings, spanning the roof. Here in the Southwest, these timbers are commonly 6-12" diameter tree trunks called vigas. You'd want to space your crennelations accordingly so the vigas are a nice fit. After they're in place, you can wedge them so they don't move, and then plaster around them. Once they're in place, you can nail or tie latillas (smaller limbs) at a right angle over the vigas, then finish the roof however you like - wood, thatch, dirt, mud - whatever.



Floors:


The simplest option is a tamped earthen floor. Plastic sheeting covered with tamped dirt will provide a vapor barrier to keep cold, dampness, and critters at bay. Next might be a mud floor a couple of inches thick. The Spaniards in this part of the country mixed mud with ox blood, but you could get away with using mud or adding wheat flour or eggs to make it hard. Alternatively, you could polish it smooth and then add a couple of coats of polyurethane to protect it.


Pavers used for patios or bricks are great (set these in sand). Cement floors are easy to pour, and you can then add tile, flagstone or slate, pebbles, beach glass - pretty much anything that'll be flat and easy to keep clean.


Raised wooden floors on studs are also a possibility, and give you the option of utilizing the space underneath for plumbing and/or wiring. You may want to excavate your foundation before you start building if you're considering this option.



Plumbing, wiring, attachments, etc:


Since the face of your stacked & tamped bags are scalloped (rounded with crevices in between), those crevices can be ideal for running pipes, conduit or wires. Think ahead & give yourself some access panels for when/if you need to make repairs.


Attaching things to your earthbag walls can be done in several ways:


1.) As you're building your walls, you can lay baling wire, threaded rod, or strong twine between the bags so they dangle on both the inside and the outside. You can then tie in your attachments.


2.) You should be able to gently tap stakes through your walls if you did a good job with your fill. This might be easier when your bags are semi-dry, but you usually can do it between bags (remember the barbed wire) if your stakes are flat.


3.) We mentioned "gringo blocks" above. You can selectively use gringo blocks (wooden blocks the same size as tamped earthbags) to secure heavy things like door frames, counters, cabinets, and the like. If you do, drive nails top & bottom to wrap the barbed wire around as you're working on that course so you can secure the blocks in place and help prevent them from pulling out if your counter or cabinet or whatever ends up bearing too much weight. Worst case, you want it to fail, not the wall that it's attached to.



Framed Construction:


Some jurisdictions (state, county, city) have proscriptive regulations against non-conventional construction materials & methods. If you want to use earthbags, one possible way around this is to build a traditional framed structure and use the bags to fill in the walls between the studs.


Benefits include savings on cost over a traditional frame building, superb insulation value, and maintaining control over your materials (bearing in mind that some "approved" building materials on the market may later be found to outgas or aggravate allergies). Plumbing, wiring, and attachments will be straightforward and won't require creative measures as is common with earthbags alone.


Since earthbags are 12" wide, your frame components (top & bottom plates, studs, etc.) should be wider than traditional 2x4s or 2x6s, meaning more wood. Because of earthbags' superior load-bearing quality, you could probably use 24" spacing between your studs instead of the traditional 16". Obviously, you'll have trouble tamping your bags as they rise towards your top plate (no room for the tamper handle).


Overall, a hybrid framed/earthbag structure will be more costly than one constructed with earthbags alone. It can stand to be less costly than a traditional framed structure. It will be easier to get permits (and homeowners' insurance) for a hybrid as opposed to a structure made solely of earthbags. And if you're hiring a contractor to do the work, they'll understand the concepts. This might be about the extent of how much you want to involve traditional contractors with earthbag construction, though. A good contractor will be understandably wary about possible code violations & potential liability concerns/ You, for your part, should be wary about someone building your house using alternative materials & materials that they might not understand or fail to exercise proper care.




Again, this is just an introduction. None of this is not meant to be a comprehensive guide to earthbag building. If you're serious, do your research. There are great resources in the library and on the Web. By all means, do experiment, seek out advice from people who're knowledgable about both earthbag and conventional construction, and please be safe.




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New Mexico Dirtbags                   Albuquerque,  NM 87106          (505) 750-3478 (DIRT)