It may seem like a simple part of the overall construction process, but getting the foundation right is incredibly important. The biggest reason for this is that any mistakes you make in the foundation will only get worse as you go up. It’s known as compounding defects and it means that mistakes grow. Here’s an example: You notice that your slab foundation is 3/4” out of square when you start framing. It’s also 1/2” out of level across the total building. You figure, “well, I can handle that. I’ll just adjust it in the framing.” As you complete the framing, you get up to the roof and notice that the building is now 1” out of square and 3/4” out of level. Bummer, but you figure you can capture it in the roof framing. By the time you metal roof shows up, the square panels don’t fit on your out of square roof and you have “to make it work.” In the end, everyone and your grandmother can see that the roof is out of square and the simple mistake in your foundation has ruined the look of the house. I’ve actually seen this happen to someone. It was a “simple” mistake and it just got worse and worse as he went up in the construction process.
Here’s another reason that your foundation deserves the highest focus and attention to detail: your entire house sits on top of it! If you skimp here and something fails, it’s not an easy fix. Is it worth the extra few dollars to add a bit more rebar to the slab? Yes. Ensuring that the foundation is built to the very best standards possible is very much worth it. Here are some simple things to look out for when building your foundation and/or slab.
- Make sure that the rebar is the right size and is laid out in the right spacing for your soil/geological conditions. A typical residential house in the United States uses #4 rebar in the foundation walls and #3 in the slab on an 18”x18” grid pattern. The layout for the foundation rebar depends on the size of foundation wall and the local codes.
- Do not allow any rebar to “daylight” or even come close. No rebar should end or be placed within 4” of the edge of the concrete. Rebar closer than that can draw moisture from the outside and rust. That rust will creep down the rebar and, over time, render the entire rebar system useless.
- Pay extra attention to the layout of your foundation and slab. Make sure that the corners are square and level. The closer to perfect you are, the better, but in no case should you be more than 1/4” out of square or level for a roughly 2000 SF house. Use a laser level if you have one or the best standby of all time: a water level. It’s the cheapest level you’ll ever buy. It’s basically some clear plastic tubing, water, and a little red food coloring to help you see the level lines better. You can even use this set by yourself.
- Wait until the water has evaporated off of the top of the slab before you start finish troweling. If you press that water back into the surface of the concrete, it will weaken it. Allow it to cast off the water it doesn’t “want” and then get on it for the finish work.
- If you plan to acid stain your concrete, don’t over finish the surface. If you polish the surface too much, you will seal it beyond what the acid stain can react with. You can definitely get the surface smooth, don’t get me wrong, the key is not to power trowel the heck out of the slab.
- Use a stepped foundation when applicable to minimize concrete use. If you have a sloped site, step the foundation up or down the hillside to work with the topography. Be sure to measure the steps and keep them in line with the bale courses so you can step the bales too down the road if that works with your design. This won’t apply in all cases, but if it does, it’s great to get it right when stepping the foundation to keep your bale work easy.
- Spend some extra time around your foundation bolts when finishing the slab. Many people don’t put a lot of attention here because they figure “it will be buried in the wall so who cares if it looks good.” This is one way that mistakes compound. When you add your 4×4 toe ups to a series of foundation bolts sticking out of poorly finished concrete, you will quickly find that the 4×4’s won’t sit flat. The thick 4×4’s won’t bend like a 2×4 to fit flat either, so you will be left with a toe up that’s up in the air in some spots and flat in others. This not only allows for air gaps through which bugs can also travel, but also messes up the framing before you even start it. Finish those areas well and you will be happy you did.
- Use Wedge Bolts or other “after cure” anchors for the interior toe ups. By adding the interior anchor bolts after the concrete has been finished you can get a better finish on the concrete (not only for the bolts as described in #7, but also for your floor which will come very close to the anchor bolt locations). Using the drill in bolts also makes the layout and installation of the interior toe up a lot easier and more accurate.
- Be sure to vibrate your form boards to eliminate “honeycombing” of the concrete. This not only improves the strength of the wall, but increases the beauty. This can be as simple as pounding a hammer against the form boards while the concrete is still wet. Do this BEFORE you finish the surface as the vibration can make the surface of the concrete drop a bit.
- Use adequate bracing for your pours. There is nothing worse than having a form board blow out during a pour. It means more concrete will be used and your nice straight line will be shot. Use lots of diagonal braces to support the forms during the pour.
About the Author
Andrew Morison is a specialist in straw bale and green construction. He has shown thousands of people how to build their own straw bale projects through his comprehensive series of instructional straw bale, concrete foundation, and plastering DVDs, as well as his hands on workshops. You can check these out at www.StrawBale.com/store.
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