FOOTINGS
I was told many years ago, a footing that is uniform and 6 inches deep is better than one 12 inches that is not. This is true because of weight distribution and settlement. Keep in mind that the concrete in that hole may not be seen, but it is the foundation for everything on top. If you mess up on floorcovering and need to have it replaced, no sweat. Try pulling the footing out from under the building!
Unless you are using a pick and shovel to do the footings, it may be best to have a professional do the work. Take it from someone who has dug footings by hand, the cost is worth it to rent equipment or pay someone else.
Width and depth of footings are controlled by varying climates and soil conditions. The bottom of the footing should be below the frost line. Here in the south, that is about eight inches and requirements usually call for the shallowest to be twelve inches below finished grade. Another factor is the soil material and compaction. It is great to have your footing sitting on virgin clay, but that may not be possible. We won’t go into those requirements for this down and dirty (catch the pun) calculation. If you are working from approved plans, the requirements should be there.
A rather standard footing is 24” wide x 8” deep. If we are building a 40’ square building, the math is 160 x 2 x .67 = 214.40 cu. Ft. Now, divide by 27 ( the number of cubic feet in one yard ), and we get approximately 8 cubic yards of concrete. Keep in mind, there is a large waste factor with concrete. Many people calculate 20 percent more for that. If you were ordering material for this job, and of course, if the footings were dug uniformly, 9 yards of concrete would probably be what to order. Never skimp on concrete by ordering less and hoping it will be enough. When the plant has to send another truck out with a half yard, they generally charge you extra.
Steel required in footings will vary for the same reasons as depth and width. You should estimate it based on getting 18’ from a 20’ bar times the number of bars required. Each rod has to be lapped by the next bar and tied with wire, thus you don’t get 20’ of running footage from a bar. Rod chairs, those little wire things that hold the bars of the ground, are figured about every four feet of footing. It is very important to keep the steel in the concrete.
Although footings are not a finished product, it is important to keep them smooth and level for the person working behind. Laying brick and block on rough concrete that is high in some areas and low in others presents obvious problems for the mason.
It is not possible to cover every factor about footings in this small area. Please, direct other questions to our forum.
FOUNDATIONS
There are many variations of foundations. I will only cover two.
The first foundation has been the standard for years and is about the same whether you are going with a concrete slab or wood frame. In olden days buildings were placed on piers that were spaced 8 feet or more apart. The floor system was wood with doubled joists at the perimeter. Where this saves material, it doesn’t render a good situation for weight distribution, nor keeping the joists from sagging under the outside walls. Unless you are building a very light temporary structure, I don’t recommend doing this.
If you are concerned about the strength of the concrete in your footings, you may have taken test cylinders to be broken by a materials testing engineer. If that is the case, you should wait at least a week after the concrete is poured and the test is performed to see if the footings are sufficient. Although concrete doesn’t reach its maximum strength until a month later, 85-90% is reached in the first week. Most areas call for a minimum of 2800 pounds per sq. inch for footing concrete. If you do not have concerns about strength, it is fine to start laying block the day after the pour.
FOR WOOD FRAME FLOORS:
The normal cinder block used for residential construction is 8” x 8” x 16”. Calculating block is rather simple. Coverage in a wall is 1.33 feet long by .67 feet high. A 40’ x 40’ building would be 160’ around. Divide that by 1.33 to get the number of blocks for one complete course around. If you have steps in your footings due to grade variations, it will be necessary to measure the distances for each partial course and divide each of those by 1.33 for the block count. Your last course of block should measure at least 24” from the ground beneath the house to the top at the highest point of elevation. This is necessary to allow the proper clearance for crawl space.
Masonry cement is calculated at a rate of three bags per one hundred blocks. Sand is calculated at one cubic yard per 20-24 bags of mortar. We will not discuss anchor bolts or straps, as there are different products that can be used for this. Your local inspector should be consulted about this, if you do not have plans that give the information. It is important to have some way to strap the treated lumber to the foundation, and is required in most areas.
FOR CONCRETE SLABS
Slabs are practical when the pad for the building is relatively flat. The block are lain the same as for wood floors except for the last course which is a header or form block. This is a block that has been notched out 4” on the inside to allow for the slab to pour inside. It is only necessary for the top of the last course of block to be 8” above the highest point of ground on the outside of the building.
We will discuss the slab itself under concrete slabs, as well as the monolithic slab.
For other articles on footings try here.
