Poured-concrete foundations--how difficult can they be?
One takes a bunch of boards and plywood and nails them together to make a form system and then pours in some concrete. How difficult can that be?
To be honest however, getting the foundation right is very important to getting the whole house done right. Perhaps this could be known as getting your home started on the right footing. There are so many things that have to be considered before you nail all those boards together.
1. Excavating to the necessary depth and down to undisturbed soils is criitical. Both of these can be difficult or easy depending on where you are digging. Of course getting the foundation hole all dug out and nice and level is a good part of the battle, but an ill-timed rain can make a mess of it that can lead to areas of “disturbed” soils. These mushy areas then have to be excavated to be filled with more costly concrete—hopefully. Many builders however just ignore the mushy spots and just pour away.
2. Depending on the soils you are building on, you will have to think about how wide and how tall the footing under the foundation wall is going to be. Geo-technical information will give you this information and is usually spelled out on the plans.
3. Will the finished height of the foundation be high enough to keep wood siding and trim components far enough above grade? This is a very commonly missed consideration.
4. How thick is the foundation going to be? This will also be spelled out on the plans.
5. Have you installed a concrete releasing agent on the forms so that the concrete does not stick to the forms? Whoops!
6. What strength of concrete and what size aggregate will you be using, as well as how much slump is allowed? (Makes you want to slump into your arm chair by now doesn’t it?) However slump will predict how many shrinkage cracks you end up with.
7. How much rebar will there be in the footing and foundation and what different sizes will be required? The house plans will tell this. You do have plans don’t you?
8. Will the foundation and footing be poured all together or will the footing be poured first and the foundation poured on top of the footing later.
9. You will also have to consider all the foundation anchor bolts and imbedded strapping etc.
10. And don’t forget the Concrete Encased Electrode.
11. There can also be penetrations that have to be considered like: crawl space vents, window openings, door openings, ductwork openings, plumbing pipe openings, gravity drains etc.
12. There can also be decorative items that have to be installed on the inside of the forms. You know---the lines that make the concrete look like something else?
13. Is the top of the foundation perfectly level? You better pay attention to this if you don’t want your house framing that is going to sit on your foundation to be a nightmare.
14. Have your protected the wall from evaporation or freezing after the concrete is poured?
15. Are all corners the correct angle? For example, in a rectangular foundation, the diagonal measurement from corner to corner must be “precisely” the same. Likewise, the lengths of opposite sides must be exactly the same. If they are not, your diagonal measurements will be useless. This can get very complicated when you are talking about angles greater than or less than 90 degrees and if there are many zigs and zags. If you have not managed all this properly---just wait until you get to the roof structure!
16. This list is just a few of the considerations to get you thinking about how simple the foundation is.
Today I want to talk about #5.
In my opinion it is best practice to install the foundation as a “monolithic pour.” That is when the footing and the wall are all poured as one unit. This method eliminates any cold joint that would be present at the footing/wall connection if they are poured separately. In this method of pouring the foundation, wood elements are used to span the boards used to make the footings, and the forms for the walls sit on top of these cross-boards.
Eventually, when the footing and foundation form boards are removed the spreader boards (cross-ties) remain in place.
Over time these boards will rot or become infested with wood destroying insects. They can also lead to points of water intrusion or become vermin entry points. Inspectors routinely call for these wood components to be dug out of the concrete and sealed with expansive concrete to prevent leaking and keep vermin out of the crawl space. They are considered conducive to wood destroying organisms--and are reported as such by any Licensed Structural Pest Inspector.
One solution to this problem is to make the spreaders (cross-ties) out of foundation grade pressure treated lumber. On a recent foundation I noticed that the builder had used pressure treated lumber. As I approached, the foundation, I could see a sticker on the end of one of one of the boards and I could read the words “Pressure Treated Wood.”
Well that was interesting, because my experience with foundation grade pressure treated materials told me that the color of the sticker and the appearance of the wood (color) were not quite right. Also, in the NW, most foundation grade pressure treated lumber has “incise” marks on the surface to aid in getting the preservative into the wood. (This is not true for folks on the East Coast that have access to foundation grade pressure treated Yellow Pine.)
When I got closer and could actually read the sticker, I could see that it was pressure treated lumber and that it had a “lifetime limited warranty.” The part that said for “Above Ground Use” meant that someone basically wasted their time and money on materials that will ultimately fare not much better than untreated wood.
These cross-ties now have a “limited lifetime warranty” as opposed to a “lifetime limited warranty.”
Poured concrete foundations--how difficult can they be?
Charles Buell, Real Estate Inspections in Seattle
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WA State, Home Inspector Advisory Licensing Board