|Home | Wiring | Plumbing | Kitchen/Bath
Making sketches is well and good, but sooner or later, imagining the real has to give way to realizing the real, and , unless it’s winter and the ground is frozen hard, that time may as well be now. So go to your site, measure off the dimensions of your outbuilding, bang in a stake at each of the four corners (straight pieces of tree branch are as good as milled wood, and a rock is as good as a hammer), and connect the stakes with twine. (The best twine for this purpose, readily available at most hard ware stores, is yellow mason’s twine, mainly because it’s yellow and stays yellow. Twine that’s already, or is likely to become, the color of your site will soon be invisible, at which point it will try to trip you or girdle your ankles and stanch the supply of blood to your toes.) You’ll discover that there are practical limits to how much precision the marking-off process will bear, so don’t tie yourself in knots over fractions of an inch. The main thing to strive for is a true rectangle, and the best way to ensure that its angles are all right angles is to keep both diagonals equal. Which brings up the fact that one tool you’ll have to buy or borrow for the project is a measuring tape that’s at least as long as a diagonal of the outbuilding. For geometry fans, that’s the square root of the sum of the squares of the length and the width, a dimension easily obtained with a pocket calculator.
But another thing not to tie yourself in knots over is calculations, at least not now, when you’re working with something as imprecise as earth. Regardless of what your calculated diagonals are, in the field just get them equal. True right angles are more important here than an inch or two of length or width.
Clearing Brush and Trees
Since you’re building on posts, you won’t have to do any grading, but tradition dictates that the next step after marking off the corners of the building is clearing the land. So cut down any trees inside your rectangle and keep the stumps shorter than the bottom of the building. If there’s a stump where any of your posts will go, remember that right now it’s much easier to move the building a few inches than to pull a stump. And give some attention to the trees just outside the rectangle. Trees in close proximity to a new structure tend to make it look less raw, less of an offense to the landscape. But follow them up with your eye. If any trunks or branches intrude on what will be the final volume of the outbuilding, get rid of them now. Once you’ve started with the carpentry, your momentum will be precious, and you won’t want to interrupt it to become an arborist.
If you want to make any minor siting changes, this is also your last good opportunity to make them. Putting up even a small outbuilding is a strangely momentous venture. It doesn’t always hit you at first, but eventually you realize that you’re altering a part of the face of the earth. A measure of care and humility is warranted. Marry the building to the landscape as best you can, because presently you’ll begin breaking ground.
Marking Out the Posts
Before getting repellently reverential, though, get inside your twine-bounded rectangle and bang in more stakes to mark off the centers of the nine posts. Start with the four nearest the corners of the building and use the dimensions from your sketch to locate their positions. (I’m assuming you’ve inset the posts at least a tad from the actual corners. If you haven’t, placing the first four post markers will be all the easier.) Then run some more mason’s twine from each of the four post markers to the next, and you’ll have a rectangle within a rectangle. If the sides of the inner rectangle aren’t parallel to the sides of the outer rectangle, adjust them accordingly while keeping the corners square. But again, don’t tie yourself in any knots. The bottoms of the postholes will be wide enough to let you do some fine- tuning before you carve your deck in pressure-treated wood.
Now find the centers of all four sides of the inner rectangle, mark them, and drive four more post stakes. You’ve now staked eight of the nine postholes. To do the ninth, just connect up the center stakes with more twine. The intersection of the two twine lines defines the location of the last, innermost post.
At this point you’ve driven thirteen stakes: four marking the corners of the building and nine marking the centers of the support posts. With an ordinary spade, cut a 2’ circle in the ground around each of the nine post markers and start digging. Be forewarned that digging nine deep holes yields rather less personal fulfillment than just about any of the tasks you’ll carry out in putting up the building. The work is boring, frustrating, and thankless, and it gets harder as the holes get deeper. Granted, if you’ve sinned against mankind recently and need some self mortification, digging nine deep holes may be just the ticket. But if you’ve behaved decently, reward yourself by calling local hardware stores or equipment rental services and seeing if you can rent a gas-powered posthole digger. Even if you’re working in the most compliant, rock-free soil, once you’re down a couple of feet you won’t be able to level your shovel inside the hole, which means you’ll be removing dirt a teaspoon at a time. Mechanical posthole diggers aren’t nearly as formidable as they look, and if you rent one you may be able to accomplish in a day what might otherwise take you weeks.
Oh, yes. The depth of the holes. Make them 3’ 8” deep—3’ for the post and another 8” for the thickness of the poured concrete. That way you’ll be protected against frost heave in all parts of the country, and you won’t have to brace the posts to keep them from leaning away from the vertical. When you backfill the holes after you put the posts in, the ground will provide all the bracing you need.
With the digging done, you’re ready now to pour your footings, those being the concrete pads that keep the bottoms of the posts from sinking into the ground beneath them.
If you’ve never fooled around with concrete or mortar before, there’s no reason to tense up. The stuff can be bulky and sloppy, but it won’t ask for any special skills. It’s true that when an application calls for concrete to be inordinately hard or free of porousness a certain degree of knowledge and experience is warranted. But all you require of your footings is that they remain more or less in one piece and not compress under the weight of the posts.
Just so you know what you’re working with, everyday concrete is a mixture of Portland cement, sand, and small stones and /or crushed rock. The last ingredient is called “aggregate.” While sand and aggregate are found in nature, Portland cement is manufactured. Limestone is pulverized with shale, clay, or marl and then heated to about 2,7000-deg F. After some time the mix fuses into a “clinker,” which in turn is ground into the fine powder that’s sold as Portland cement. Different grades of cement are obtained by varying the ratio of limestone to clay, but the two most commonly available grades are mortar-type cement, which you mix with clean sand (around three parts sand to one part cement for a good strong mix) to make brick mortar, and concrete-type cement, which, as we said, is mixed with sand and aggregate to make concrete. The proportions of cement, sand, and aggregate vary fairly widely from one concrete mix to the next, but a typical, middle-of-the-road mix might contain three parts sand and four parts aggregate to one part cement.
As for how cement hardens, it’s got nothing whatever to do with “drying.” In fact, Portland cement hardens just as well underwater as it does aboveground, and sometimes even better—a property that greatly facilitates the building of bridges and tunnels. While the various ingredients of Portland cement, most of them compounds of silicon, lie around in the ground for all those many millennia, wondering when they’ll be mined and dumped into cement kilns, they combine chemically with water. Then, in the high temperatures of the kilns, the molecules of water are driven off as steam. However, the ability of those compounds to combine with water once again is unimpaired. So when you add water to Portland cement, they do combine with it—chemically—and the resulting crystalline mass is what gives concrete its stone-like strength. In fact, concrete permitted to dry out while setting isn’t as strong as concrete that’s been kept damp until it hardens. But unless you’re putting up your outbuilding in the desert, the ground at the bottom of your postholes will almost certainly not be dry, so your footings will enjoy the benefit of damp-curing without your having to take any particular pains one way or the other. And if you are building in the desert, you might wet the stuff down a little once or twice a day for a couple of days—just enough so that it never completely dries out.
We’ll do some very rough calculations now. Figure the bottom of each of your postholes to be a circle about 1’ in diameter, and figure their thickness to be around 8”. That would make the volume of each of your footings a shade over half a cubic foot. (In feet, pi x h = 3.14 x (1/2)^2 x (2/3)= 0.5233 ft^3). Which means you’ll need around five cubic feet of concrete to pour all nine footings.
Premixed Concrete (Add Water and Stir)
If you don’t have any Portland cement around, the easiest way to do the footings is simply to go to your hardware store or local brickyard and buy nine 80-pound sacks of premixed concrete (Portland cement, sand, and aggregate are already sifted together; Sakrete is the best-known brand but hardly the only one) and allot one bag per footing. Each 80 pounds makes a shade over half a cubic foot, and all you need to do is follow the directions and add water, mix until fairly uniform (if you don’t have an old wheelbarrow for this purpose, you can use a garbage can or even one of those plastic washtubs), shape the dirt at the bottom of each hole so it forms a circle that’s roughly a foot in diameter, tamp down the bottom of the hole with the end of a board so the dirt is fairly compressed, then pour in the wet concrete. Make sure to add sufficient water so that the top surface of the concrete levels more or less by itself. You can encourage additional leveling by slopping the end of a stick in the wet mix. And while you’re doing that, look for any chunks of aggregate that prominently protrude above the surface and push them back down. When the mix hardens you’ll want a reasonably smooth surface on which to move the bottoms of the posts around as you make the final adjustments to their position. But also keep in mind that all you’re making here is a buried footing, not a skating rink. A modicum of fussing is appropriate, but don’t tip into compulsivity.
Mixing Your Own Concrete
If you do happen to have some Portland cement around (or if you prefer to mix your own concrete just as an exercise), a mix of one part concrete-type Portland cement to three parts clean sand and four parts small stones should do fine. Proportioned at that rate, a single 94-pound sack of Portland cement should make about 8 or 9 cubic feet of concrete, which is more than enough for the whole job. Portland cement is available by the sack at brickyards and masonry supply houses and even at a fair number of hardware stores, but unless you already have some sand and aggregate near your building site I wouldn’t recommend going to the trouble. Even if you do have the sand and aggregate you’ll save only a few dollars by making concrete from scratch. Most masonry suppliers won’t deliver sand in quantities of less than a cubic yard (that’s 27 cubic feet). As for aggregate, I assume you’ll be using your own stones, which you really ought to pile up and hose clean before you mix them into concrete. Still, if you’re adamant about making concrete, mix the dry ingredients until they’re fairly uniform, then add water a little at a time and use a hoe (or something hoe-like) to draw them into the puddles of water. Keep doing that until everything is wet, then—still only a little at a time—add water until you can move your hoe fairly easily through the whole mix. When it has the consistency of fresh oatmeal you’re ready to pour it into your postholes, their bottoms shaped as described above. Also as described above, do what you can to encourage the concrete to make a level surface without any prominent bumps. Home-grown aggregate tends to be more chunky than the store- bought kind, so try to push the bobbing heads of any stones down into the wet concrete, however cruel it makes you feel.
Your footings will need at least a day—maybe two or three days; it depends on the outside temperature—to get hard enough to work with, so what’s wanted now is some measure to prevent your postholes from caving in onto the footings. The chief danger here is a severe rainstorm, not because the concrete will be harmed by the water but because the rain may cave in the walls of the holes and bury your footings prematurely. So look around (the dump is a good place) for scraps of plywood or even heavy cardboard. Lacking those, make some mini-tarpaulins out of clear plastic—10x25 rolls of 4-mil plastic sheeting are often sold in hardware stores for only a few dollars—and secure them over the holes. You can use stones to hold them down, or you can peg them right into the earth with sharp sticks. However you handle the problem, try not to be the problem. Which is to say, don’t accidentally cave in the dirt around the sides of the holes while you’re trying to prevent the dirt around the sides of the holes from caving in.
Your footings are setting now, you’re thinking ahead to the next step, and somehow you find yourself harboring the wish to have the building rest on posts made of some more durable material than wood, albeit pressure-treated wood. Well, just so you can’t say you weren’t alerted to the possibility, you can always go to your local masonry supplier, buy cardboard tubes manufactured to serve as concrete forms, and pour concrete posts to your taste. You’ll have to mix a lot of concrete or have some ready-mix delivered, and you’ll also have to be much more careful with placement of the forms. Once they’re full of concrete they’re not going to move around as easily as pressure- treated 6x6s. In fact they’ll never move, and any mistakes you make will literally be etched in stone. And I strongly suspect that if you opt for concrete posts you’ll have barely begun making them when your wish for archival permanence will evaporate abruptly. Pouring nine of them by yourself is a messy, expensive, time-consuming, and generally unattractive job. The great thing about pressure-treated posts is that you cut them, drop them into the holes, line them up and get them more or less plumb, and there they are, ready to hold up a building. Besides, all over America insects and microorganisms are sampling chunks of pressure-treated wood and going “phooey” if they’re lucky, dying if they’re not. And if the posts do rot out over the course of time (probably a good couple of generations), so what? Imagine how delighted your heirs will be at the prospect of a juicy restoration project. Meanwhile the epic struggle between modern chemistry and the ancient agents of decay will be playing itself out right under your feet, and the outbuilding will seem to crackle with its energy. Go with it.
Also see: Do-it-Yourself Log Home / Cabin
|Top of Page||HOME||Prev: Making a Sketch||Next:
Sunday, November 14, 2010 19:16 PST