Here is some fodder to help you decide if pouring a concrete slab foundation is right for you and some in-depth information on slab construction.
Definition of Concrete Slabs
Concrete slabs are common foundation/floor systems in many parts of the world, particularly in warmer climates with soils that are stable (i.e. not expansive or otherwise active).
Typically, building a concrete slab consists of (from the ground up) 4" of sand, gravel, or crushed stone; a layer of 6 mil polyethylene used as a vapor barrier (seams to be overlapped a minimum of 6"); a layer of wire mesh - usually #10 wire welded in 6" squares (6x6-10/10) which is embedded in the concrete; and 4" of 3000 psi concrete. The edge of the slab is insulated, typically with a rigid foam product.
Should You Build Your Home With A Floor Slab?
Some people prefer slab construction, because it can be less expensive than other systems. Also, with a slab, you never have to worry about a squeaky floor!
Slabs are pretty ridgid structures. They don't have any "give" like a wood floor has. For that reason, people with back problems have often attributed increased aggrivation to standing on concrete floors.
Most of the literature suggests that this problem occurs when people are forced to stand on concrete floors for extended periods - as in an eight hour shift at a factory.
These problems are usually relieved with the addition of a cushioned rubber or "ergonomic" pad. If your home is carpeted, and you don't plan to stand directly on the exposed concrete for hours on end, you shouldn't have any problem with a concrete floor slab.
Some think that a floor slab is cold. Studies have shown that most of the heat loss in a slab is through the edge. The edge of the slab can be insulated to reduce the heat loss through the slab into the ground.
Anyway, a cool floor may be just what you want if you're building in Florida!
In addition, you may choose to heat your new home with a radient heating system, embedding the heating coils in the slab. This scheme insures a warn floor in winter.
Types of Slabs
Slabs are either designed to be monolithic (turned down) or
poured within a typical (concrete or masonry) foundation.
In a simple monolithic slab, the slab and the footing are one and the same, with the footing (at the perimeter of the slab and under any load -bearing walls) being "turned down" a lilttle deeper than the floor area.
The footing portion of the slab may or may not be reinforced with steel (rebars), depending on soil conditions and local code requirements.
The footing is set up and poured at the same time as the slab. This is done by digging the footing and forming the outside edge - typically with 2x lumber held in place with stakes and bracing until the concrete cures.
Post-Tensioned or Engineered Slabs
Another form of monolithic slab is the post-tensioned or "engineered" slab. This slab has steel cables running through it in both directions. Once the slab has cured, the cables are stretched and secured at each end so that a force is placed on the slab.This makes the slab tougher and more crack resistant.
We recommend that this type of construction be designed, inspected, and certified by a qualified engineer. That means your engineer, or his representative, should inspect the slab setup just prior to actually pouring the concrete. In the case of a post-tensioned slab, he should also supervise that process after the slab is poured.
Engineered slabs are more common in areas where active soils or soils with very poor bearing capacity are present.
Slabs With foundation walls
In this type of slab, the edge of the slab is supported by the foundation wall. In the middle it will rest directly on the ground.
With this type of slab, chances are there will some backfilling of dirt within the foundation walls to provide a supporting surface for the slab. This fill material should be good clean soil (no debris or vegetation) and compacted with a mechanical tamper to minimize settling away from the bottom of the slab. In areas where termites are a threat, get your pest control sub to treat the sand and soil under the slab for termites before laying the poly.
In cases where the filled area is especially deep (over three feet), additional support for the slab can be easily provided by digging down to undisturbed soil with a post hole digger every eight to ten feet on center.
That will allow the concrete to flow down into the holes and provide additional support for the slab. Another method of providing additional support for the slab is to build some piers for it to rest on.
Which Type Should You Use?
Don't know which type you should you consider? First look at some local construction in the area where you plan to build. See what others are doing. Chances are there are good reasons for what is typical in your area. These reasons may include climate and soils prevalent in the area. Also talk to your local building official. He'll tell you what the local requirements are.
There may also be site conditions which dictate which type of slab is best for your lot. A flat lot could lend itself to a monolithic slab, while a lot with significant slope would favor one with a foundation wall. A very high water table on your lot may dictate a post tensioned slab that "floats" on the lot.
Speaking of soil conditions, if your area tests high for radon, take a look here for design techniques to reduce the radon threat with slab construction.
Other Parts Of A Slab
Your slab will also have to be set up to accommodate the structure that will rise above it. You'll have to tie the wood framing to the slab to keep it from blowing away, and you'll have to take care of plumbing supplies (water) and waste.
When laying a slab, 1/2" x 8" anchor bolts are pushed down into the concrete along the perimeter of the slab so that the sill plate of the exterior walls can be bolted down to the slab. These bolts are to be spaced within three feet of each conner and then every six feet on center.
The supply and waste lines which pierce the slab are put in place before the slab is poured - in fact, before it is even set up to be poured. The most important thing to check here is the location of all pipes which the plumber has installed. You can do this by measuring from the foundation walls (if these are used) or the forming for the edge of the slab.
Precise positioning is important so that pipes to be located in walls actually end up in the walls, and toilets are properly located.
If a pipe ends up in the wrong place, after the slab has been poured, it may have to be jack-hammered out and relocated - a time-consuming and expensive process.
Copper water supply pipes will be under pressure once the water is turned on. To prevent the chance of leaks in inaccessible places, there should be no soldered joints in them either in or under the slab.
All pipes passing through the slab should be protected with a rubber sleeve to minimize abrasion and possible damage from natural vibrations and movement.
Floor drains may also required for the washer and the water heater. Check to make sure these are in place where they should be.
The plumbing inspector will check the installation before the slab is poured to make sure it meets the plumbing code.
The wire mesh may be held up off the poly by some artificial means (rocks, pieces of wood, etc.) or, more typically, it will simply be lifted slightly by the concrete finishers as the concrete is being poured. Ideally, the wire mesh will be located about one third of the way up from the bottom of the slab.
The poured slab will be finished with a power trowel machine until it is smooth and flat. If the garage is poured at the same time, it should slope towards the door so that water will drain out.
Planning on pouring your own slab? Here's a great article from Readers' Digest's Family Handyman magazine.
For additional information on house foundations,
see Lesson Five and Lesson Fourteen of our online course
Successful Home Contracting.
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