How is a Slab on Grade Foundation Made
A look at the steps in making a typical slab on grade foundation for your home.
I either forget to bring or I forget to use my camera on home inspection job sites. I am trying to be better about taking photos of homes, particularly since I want to post more to this site. I still have to sort through some to add. Early yesterday morning I was examining a new foundation for a client, and I did have the camera. I have discussed foundation problems; however, I thought it might be of interest to some what goes into the creation of a foundation of a new home.
Site Preparation
The first step in preparation for building any subdivision is leveling the ground in the area. Specifically to your lot, the builder will have the house sited on the lot, laying out its position with string attached to stakes. The crew comes in with a backhoe to dig out a grid pattern of channels with one channel going around the perimeter of the building. The depth of these channels are determined by the frost line (how deep the ground freezes) in your area and by an engineer who is calculating what is needed to handle the weight of the structure. Then a medium, like sand, which will allow water to flow away is placed over the area of the foundation. Then a barrier to help prevent moisture from effecting the foundation is installed. Around the foundation, a wooden frame which allows the foundation to go above the grade of the soil is installed.
At this point plumbing pipes (supply and waste) that go through the foundation are laid out. Rebar is a metal bar which helps strengthen the cement foundation, and they are laid out through the grid. Current slab foundations also contain cables which run through them called post tension cables. These cables are sheathed in a plastic coat, and they are placed under tension as the cement cures. Like a rebar, this is a technique to provide extra strength to a cement foundation. The other item that you will see looks like a black plastic barrel. These are spacers; they create a void in the foundation, which will be used by part of the plumbing system when the fixtures are being installed. All of these parts have to be carefully laid out per the plan and engineering specifications. Once the cement is set, mistakes can be costly.
Pouring the Foundation
Depending on the size of the home, you could have ten to fifteen cement trucks come for the pour. A crew will set guide lines to show them how high the foundation needs to come in the form. This crew directs one truck at a time to ensure the cement is going into the proper location. They use a rake like tool to begin spreading the cement evenly over an area. It takes an hour before all the trucks have unloaded. During this process a vibration tool to clear out air pockets is used along the edge of the foundation. When a section of the foundation has been poured, the crew can begin to level off the cement with a screed. In this case, it is a rectangular metal tube that is used as a straight edge. Two crewmen move it along the guidelines for the foundation height. Once this task is complete, the final smoothing of the cement surface is undertaken. A long pole with a flat metal trowel which can be tilted to a 45 degree angle is used. At this point, no one walks in that area, but the crew ensures that the cement is coming to the edges of the forms , and that there is a clean line between form and cement. The final step is to insert the tie downs for the sill plate. This has a shaft which goes into the foundation, so it is firmly in place. Tabs are left out of the foundation, which bend over the sill plate to help attach it to the foundation.
Last Steps
Now the cement has to cure, before anything can be built upon it. Cure times can very depending upon the weather, the cement mix, and the amount of water used. The usual amount of time is about a week. I should mention that driveways, walkways, and slabs for porches were not poured at this stage, since they are handled differently. Tightening the post tension cables and removing the forms are the next tasks. At this point, the removed boards do not leave a smooth finish, so a buttercoat is added. This is a smooth coat to make the foundation look good; it does not serve any structural purpose. Cement caps will be placed over the post tension cable ends to prevent them from rusting. The sill plate is then attached to the foundation. The framing of the house is connected to the foundation through this wood beam.
Pictures for the foundation procedure have been included under the Foundation photo section. At this point, you cannot tell if the foundation was designed correctly or installed well. There could have been problems with the curing, or some element may not be functioning as it should, or damage may have been done to the curing foundation. Once the weight of the framing comes to bare upon this base, we can see if any cracks occur. As the home is completed, we may find mistakes made or damage done, so a final determination through the home inspection process cannot be made on the foundation till the house is complete.
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