It may seem like a simple part of the overall construction process, but your foundation is the most crucial part of your entire project. Any mistakes you make in your straw bale house foundation will only get worse as you build up. It’s known as compounding defects, and it means that mistakes begin to pile upon one another, making the problem worse.

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.”

After completing framing, you’re on 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 your 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.

That simple mistake in your foundation has ruined the look of the house. This happens and becomes a permanent feature of the home. A “simple” mistake that gets worse and worse as a builder goes up in the construction process.

Your foundation deserves the highest focus and attention to detail because 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.

Is it worth having an engineer, at the very least, review the foundation design? Yes.

Ensuring that your foundation is built to the very best standards possible is very much worth the extra time and attention to detail. When done properly it saves you the headache and expenses of dealing with those compounding defects, or worse yet, a failure down the road. Luckily, there are LOTS of examples of foundations done properly and plenty of resources to get this crucial step in your straw bale house build to produce the result you want.

Here are some simple things to look out for when building your foundation and/or slab.

Tips on Building Your Strawbale House Foundation

1 - Make sure that the rebar is the correct size, lapping sufficiently, and is laid out in the proper spacing for your soil and geological conditions. A typical residential house in the United States uses #4 or #5 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.

2 - 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. As it is the rebar that provides the tensile strength to the structure, not the concrete itself, compromised rebar is less than optimal!

3 - 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. 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 it by yourself. (Here’s a reference from wikiHow on How to Use a Water Level.)

4 - Wait until the water has evaporated off of the top of the slab BEFORE you start finish troweling. Pressing that water back into the surface of the concrete will weaken it. Allow the concrete to first cast off the water it doesn’t “want” and then get on it for the finish work.

5 - Planning to acid stain your concrete? Be sure not to 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.

6 - 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 straw bale courses so you can more easily step the bales down without a lot of custom work. This can be accounted for in the design phase. This won’t apply in all straw bale house foundations, but if it does, it’s great to get it right when stepping the foundation to keep your bale work easy.

7 - Spend some extra time on the areas near 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 of those sneaky places for a compounding defect to impact you later. For example, when you try to add your 4x4 toe-ups to a series of foundation bolts sticking out of poorly finished concrete, you will quickly find that the 4x4s won’t sit flat. The thick 4x4’s won’t bend like a 2x4 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.

8 - 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. (Here’s more on Anchor Bolts.)

9 - 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.

10 - Use adequate bracing for your concrete pours. There is nothing worse than having a form board blow out during a pour. If a board blows out, 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.

You can learn even more about pouring foundations in our comprehensive How-To Instruction Video Series.

It contains over 10.5 hours of top-notch, professional quality instruction on everything from foundations to framing, to baling (post and beam and load bearing), to plaster. All delivered instantly to your inbox.