Conventional Concrete Slabs

Slabs are the concrete on which we place buildings. In the old days, concrete was not as readily available as it is now, and most floors were wood. When making a decision for either wood or concrete, keep in mind a few things.

If the grade is extreme, the slab is not cost feasible. You should consider the wood floor over a crawl space. With the wood floor system, you have the added area under the house to put things. (Usually, things you will forget and never see again.) Also, from a termite control standpoint, it is easier to retreat later with a crawl space.

Plumbing is also easier to maintain with a crawl space. With slabs, plumbing pipes are sometimes bumped and moved during the pour. Those mistakes have to be dealt with after the concrete has set. With a wood floor, the majority of the rough plumbing goes in after framing assuring less chance of error.

Disadvantages to wood floors are the inevitable creaks and pops caused by curing, changing temperatures, moisture and gremlins. If you have a water leak that you are not aware of, as in a slow leak, the wood may rot. Even with plastic on the ground under the house, temperature variations can allow condensation and decay, especially in areas with high ground water.

If you have a level grade, slabs are the usual choice because of cost. Floors don’t squeak. One thing that I like is the close proximity to the outside elevation. If you have to jump out a window, it’s not that far to the ground. When we get to the point in life that we can’t get around as easily as we would like, climbing a lot of steps just to get in the house becomes a challenge.

In a traditional slab, the blocks are lain on a footing the same way as you would with a wood floor system. The only difference would be the final block course which would be a header (or form) block. Inside the foundation, the ground should be approximately 8” below the top of the header blocks and compacted to about 95%.

Depending on how the building will be framed, cross ditches may need to be dug for grade beams beneath interior walls. If you are working from plans, these will be indicated. Also, they need rebar and rod chairs the same as the footings. This will need to be done after termite treatment and 6 mil poly have been installed.

After plumbing is roughed in, along with any electrical or HVAC that may be necessary, 4” of gravel or sand should be placed over the whole interior. All plumbing lines should be at least 12” below the top of the block work to allow for cross ditches. If gravel is used for the fill below the concrete, it just has to be placed and graded to 4” below grade. If sand is used, it should be tamped with a plate tamper.

Always have your slab treated for termites, unless it is going to be a very small temporary structure. Everyone tells me that the current chemicals used are not very good. However, if you don’t treat it’s the same as yelling “supper time” for the little varmints. Prior to digging footings, you should remove any tree stumps and other wood materials that attract the termites. After the pre-treat, you should not disturb the material below the slab again.

Place 6 mil polyethylene over the entire area, lapping the edges by at least 12″.Holes that are cut for plumbing pipes should not be excessive.
Now the decision comes for reinforcing wire or the use of fiber in the concrete. I have done both, but probably prefer the wire. If you are planning on the fiber at least use wire where you will have exposed concrete, such as porches and garages. The hairy fiberglass that sticks out of the concrete is difficult to get rid of, and is obtrusive. I have actually gotten the stuff in my foot by walking over it barefoot.

Place any other steel for cross ditches, and you are ready to pour.

Watch the weather, and plan the pour. Be sure to have enough finishers to do the work.

During the pour, you will need to place anchor bolts around the perimeter to attach the rat sill (or bottom plate) of your outside wall. How often they are placed is determined by your area. Other than actual bolts, there are some other alternatives that work well. Simpson tie makes straps that are poured into the concrete and wrapped over the bottom plates.

Talk to you later.