Concrete Slab Foundation: Pros and Cons & Home Guide

It’s easy to overlook the foundation when you’re buying a new home, the kind of foundation and its condition can have a big impact on how your new home ages and what repair costs you might have down the road. Concrete slab foundations are very common but not necessarily the best kind of foundation for all homes.

You will get efficient and thoughtful service from Zolo.

This guide explores the pros and cons of concrete slab foundations, paying particular attention to common problems they develop and how they compare with other kinds of foundations.

What is a Slab Foundation?

Slab foundations are the most common type of foundation because they’re cheap and simple to build. A slab foundation is just a 6 to 8-inch concrete pad — or slab, if you will — that your house sits on top of. The concrete is poured on top of soil that’s been prepared with sand or gravel to assist with drainage.

How are Slab Foundations Built?

Slab foundations are the easiest type of foundation to build. The simplest type of slab foundation is constructed by pouring directly onto prepared soil, but some slab foundations have extra components like supporting concrete feet or insulating foam. All types of slab foundations are fundamentally similar, featuring a large block of concrete without any open spaces underneath.

What Are the Different Types of Slab Foundations?

There are three common types of slab foundations that all feature a slab of concrete at their core: slab-on-grade, T-shaped, and frost-protected.

• Slab On Grade
• T-shaped
• Frost Protected

Slab On Grade

A slab-on-grade foundation is probably what you think of when you hear the phrase “slab foundation.” It consists of a single monolithic chunk of concrete that’s poured directly on top of prepared soil. It doesn’t feature any bells and whistles and is as simple as a foundation gets.

T-shaped

If you live somewhere where the ground freezes in the winter, a common practice is to build a t-shaped support structure out of concrete below the frost line. Construction involves setting inverted t-shaped feet in the ground and building walls on top. The slab is then poured inside the frame. T-shaped slabs are more expensive than slab-on-grade foundations and take longer to finish, but they also offer more structural integrity and better support for load-bearing walls.

Frost Protected

Frost protected foundations are designed to protect homes in colder climates from frost heaves. The primary advantage they have over t-shaped foundations is a shallower depth, which makes construction easier and requires fewer materials. Frost-protected foundations use polystyrene sheets to insulate the edges of the slab and the ground around the foundation walls, effectively raising the frost level and eliminating the risk of frost heave.

What Are the Pros of a Slab Foundation House?

Slab foundations offer several major benefits over homes with other types of foundations.

• Price: Slab foundations are extremely cost-effective per square foot, offering a sturdy base for your home at much lower prices than basements or crawl spaces. The typical cost for a slab foundation is significantly lower than the alternatives.
• Dries Quickly: Slab foundations are also easier to build than other kinds of foundations since many can be completed with a single pour, which means less waiting time and faster home construction.
• Lower Risk of Infestation: Since there aren’t any open spaces under your home with a slab foundation, the risk of infestation from termites and other pests is much lower. Of course, insects and critters might find another way into your home, but at least you don’t have to worry about finding any nests in your basement.
• Low Maintenance: As long as you keep an eye out for cracks, there’s not much you have to do to maintain a slab foundation.

What Are the Cons of a Slab Foundation House?

Despite the relatively attractive advantages slab foundations have, they also come with some pretty sizable drawbacks.

• Gas and Water Lines: One of the biggest downsides to slab foundations is that there isn’t any space to run gas and water lines, which means they have to be embedded in the foundation. When your pipes wear out, you’ll have to open your foundation to replace them, increasing the cost and lengthening the project.
• Prone to Ice Damage: Even if you have a frost-protected or t-shaped foundation, slab foundations are prone to cracking in the winter. Even minuscule amounts of moisture can cause serious cracks in a slab foundation, which makes them a slightly risky option in colder climates.
• Hard to Access: Slab foundations are low-maintenance compared to basements and crawl spaces, but it’s much more of a hassle to work on when things go wrong.

What Are Common Issues of Slab Foundation Homes?

Homes with slab foundations have some unique problems that don’t affect homes with other types of foundations.

• Cracks: The number one problem affecting slab foundations is cracking. Cracks can be caused by freezing temperatures, shifting soil, tree roots, and natural disasters like earthquakes.
• Settling: If the soil under your foundation shifts and settles, it can cause your foundation — and, therefore, the rest of your home — to shift and become unstable. This can lead to a host of serious problems, like sagging ceilings, bowed walls, and cracks in both the foundation and exterior walls of your home.
• Upheaval: Freezing moisture in the ground under a slab foundation can cause it to rise, creating similar problems to settling. To combat this, slab foundations are built on a layer of sand or gravel to encourage water to drain out from underneath your home.

How Do Concrete Slabs Compare to Other House Foundation Options?

Here’s how slab foundations compare to other common kinds of foundations.

Slab Foundation vs Crawl Space

Compared to a slab foundation, crawl spaces take a bit more time and effort to construct, but offer a few advantages. Homes with crawl spaces are less prone to flooding since the living space sits above the ground, although proper drainage is still important. Check out our article to discover ways to boost yard drainage.

For more Concrete Slab Formworkinformation, please contact us. We will provide professional answers.

Crawl spaces are also perfect for housing utility lines and air conditioning ductwork and easy access to perform maintenance or replace pipes when something goes wrong.

Building a home with a crawl space takes a bit longer and is slightly more expensive than building one with a slab foundation. A crawl space also needs to be regularly inspected to make sure it isn’t developing mold or mildew and that no animals have decided to take up residence.

Slab Foundation vs Basement

Basement foundations have similar advantages over slab foundations to crawl spaces; the differences are just more dramatic. Full basements offer storage space and are perfect for housing utility equipment like water heaters and furnaces. They’re also great places to keep your clothes washer and dryer, freeing up space in your home’s main living area.

Unfortunately, basements are trickier to construct, take longer to build, and are more expensive. They’re also susceptible to flooding, so you need to make sure your basement is properly sealed and that your landscaping encourages water to flow away from your home. Vapor barriers can also help, although a basement isn’t a great choice in an area with a high water table or a region with a damp climate.

So, Are Slab Foundations Worth It?

Slab foundations are a good affordable option, although they need to be constructed properly to avoid catastrophic problems down the road. Colder climates pose a challenge to slab foundations, but t-shaped slab foundations and frost-protected slabs offer some protection against cracks caused by freezing moisture and frost heave.

Basements and crawl spaces are more involved to build and cost more than slab foundations, but they also provide entryways for pests and can struggle with moisture-related problems. Overall, a slab foundation is a good choice for the right home in a warmer climate.

One of our Building Designers asked me the other day if a 10% “shrink factor” should be used when advising how much concrete it takes to pour a concrete slab on grade. This particular Building Designer “in a previous life” had been a building contractor.  It had been his practice to always order 10% more concrete for a pour, than calculated! In my neck of the woods, concrete is pretty darn expensive. I only want to order what is actually needed to do the job.

First, let’s talk about concrete slab thickness. Concrete floors and slabs on grade are called out by their “nominal” thickness. Just like a 2×4, a four inch thick slab is only 3-1/2” thick. This is so the edges can be formed by using a 2×4 and the area to be prepared can be graded off the same way, by using a 2×4. The same goes for a six inch thick slab, being actually 5-1/2” thick.

Second, let’s do the math. Concrete is purchased by the cubic yard. A cubic yard would be three feet in all directions, or 27 cubic feet. If pouring a four inch thick slab, we need to spread this one yard cube, across four inches of thickness. As four goes into 12 three times, we can multiply 27 by three and get 81 square feet four inches thick.

For a nominal four inch thick pour, I would divide the square footage of the total pour, by 81 to get the number of yards required. Let’s consider a 24’ x 36’ pole building. 864 square feet of area divided by 81 equals 10.67. Round up to the nearest whole yard and order 11 yards.

Hold it…..but a four inch thick concrete slab, is only going to really be 3-1/2” deep in real life!

In an “ideal perfect world” a 3-1/2” nominal thickness floor would allow 92.57 square feet of area to be covered by a yard of concrete. However, there is just no prepared site which is perfectly level and compacted so tightly as to not have some of the concrete going into making it level or filling voids in the fill area. The concrete mixture also includes water, which will be absorbed into the surface below or evaporate as the concrete cures.

The summary is – for a nominal four inch thick pour, divide area by 81. For a nominal five inch pour divide by 65, six divide by 54. As long as a good grading and compaction processes have been followed, these numbers work every time to give you “just enough” for your concrete slab.

Want more information on Concrete Steel Formwork? Feel free to contact us.