Here’s How to Decide if Your New House Should Be on a Slab
Thinking about building your dream home, retirement or empty nester home, or starter home – if you’re just beginning life on your own, perhaps with a young family? One of the first decisions you’ll have to make is what kind of foundation you should build the house on.
A home’s foundation is the main load-bearing portion of the structure, and its footings need to extend below ground to a point deeper than the regional frost line. For residential construction, there are three main types of foundations:
No matter which type you choose, the foundation of the home has three main responsibilities:
Support the structure above
Keep out groundwater
Act as a barrier to vapor (from soil and water).
Here, we’ll discuss the most common type of foundation: the slab.
Something important to keep in mind is that foundations are forever. Because of this, we will cover both the pros and cons of a slab foundation so that you can decide whether this kind of foundation is right for you.
This 3-bedroom, 2.5-bath rustic Ranch style home is designed with a concrete slab as the standard foundation. Only homes built on a poured-concrete slab can appear so low to the ground and have a no-barrier (i.e., one or no step) entry (Plan #196-1206).
What Is a Slab Foundation?
A slab foundation is a large, thick slab of concrete that's used as the substructural base of a home, which is the lowest load-bearing part of a building and extends below ground level.
It is usually at least four inches thick in its center, although it can sometimes be up to six inches thick. However, the edges – or footings – of a slab foundation are thicker than the center and extend below the frost line to allow for extra strength and stability around the perimeter, as this is typically where the load-bearing walls of a home will rest.
The term monolithic slab foundation might sound like it’s something out of the stone age, but “monolithic” simply means single-poured. Pouring the concrete all in one go makes a slab foundation an attractive option for DIY projects like a tool shed or pool house.
When building a home, however, there is a bit of prep involved that requires a professional.
To begin, contractors will pour concrete footers about 24 to 48 inches below the projected finish grade (or deeper if the frost line goes deeper), which in layman’s terms is the elevation of the ground surface after all the work is finished.
After the footers, contractors may add two layers minimum of concrete block.
Once that is set, builders are able to add all internal piping. Usually, builders must install the drainpipes and vents that connect to the sewer or septic-tank lines, much of the water supply lines, and sometimes some of the electrical conduit before the final layer of concrete is poured. This means that the plumbing pipes and conduit lines are actually embedded in the slab.
The Floating Slab
You might hear of a “floating slab” when looking to purchase or build a home.
Typically, there is also a layer of gravel between four and six inches thick underneath the final layer of concrete, often with a sheet of plastic a few millimeters thick to seal the moisture out. This is accompanied by mesh and steel reinforcement bars.
This is known as a floating slab because it “floats” on top of the soil while the deep concrete on the edges of the slab keeps it in place. If you live in a colder climate, these edges have to be deep enough to remain below the frost line during the winter months.
As of 2020, a monolithic slab foundation costs on average about $4–$5 per square foot. By contrast, a crawl space costs about $7 per square foot and a basement about $18 per square foot. Because of this, you can easily save 30% to 70% on the cost of your foundation build.
In addition to being relatively inexpensive to install, a slab foundation also saves you money by lowering your energy bills long term. This is because there is no space between the ground and the home, so you don’t have to pay to warm or cool “wasted space” – as long as the slab is insulated from the ground below.
If you're trying to save money on the cost of builing your home, you might consider a small house like this 1-story, 3-bedroom, 2-bath, 1,260-sq.-ft. Country style home, the building plan of which comes standard with a slab foundation – the least expensive option (Plan #178-1175).
2. Little Maintenance Required
Because slab foundations require little to no maintenance, builders often see this as even more savings in the long term.
As their design is simple, not much can go wrong. So if constructed correctly (which is typically the case), concrete slabs will last at least 50 years and likely much longer. In the meantime, homeowners don’t really have to do much, apart from occasional inspections to ensure there are no cracks present.
There are a couple of reasons a foundation might be weak or compromised, but a slab foundation manages to avoid them. For example:
Cold Joint. A cold joint is a weakness in concrete caused by too long of a pause between pouring layers of concrete.
Seam. A seam is where two separate parts of a foundation join together.
Because slab foundations are one element, poured all at once, neither of these imperfections is present, making the foundation rigid, stable, and strong.
Something to consider when selecting a foundation type is the accessibility of a home. Because there is no basement or crawl space, which raises the front door a minimum of 18 inches above ground level, homes with slab foundations are closer to or level with the ground, which reduces or eliminates the number of steps needed to gain access to the home.
This 3-bedroom, 2-bath, 1693-sq.-ft. home designed to be built on a slab is level with ground, with a minimal "weather step," which prevents snow or water from coming into the house, at the front door (Plan #204-1000).
While slab foundations are a popular and economically friendly choice, they do come with a few drawbacks to consider.
1. More Floor Space Required for HVAC Systems
As there is no space under the house to hide water heaters, space heating systems, and air handlers for air conditioning units, these appliances often then find a home on the ground floor of a building with a slab foundation.
While not too large in size, they do take up space that might have been used for something else. In the grand scheme of things, however, this may be a fairly minor drawback.
This 3-bedroom, 2-bath Acadian style home (top) is designed for a slab foundation. As its 1,600-sq.-ft. floor plan (bottom) shows, there is no space designated for a water heater or heating/cooling system(s). You could put the water heater and a small heating system in the storage area at the back of the garage, sacrificing storage space. If the heating system is too big for that space, you could also put a forced-air system in the attic. Otherwise, you'd have to carve out additional space from the floor plan (Plan #142-1063).
2. Expensive Repairs
This is the main disadvantage of a concrete pad: they are not flexible to work with. There are at least two instances in which you would be stuck with a costly repair.
Something Goes Wrong with the Utilities
As mentioned, when building a slab foundation the plumbing and electrical conduit are underneath (or within) the slab. So if anything goes wrong, you’re looking at a larger bill because the repairman needs to break through the slab to reach the problem.
In recent years, however, improvements have been made with the introduction of things like cross-linked polyethylene plumbing supply lines (Pex plumbing), so the need for such drastic repairs isn’t as high as it’s been in the past.
Something Goes Wrong with the Foundation Itself
Slab foundations can crack. This is a rare occurrence, but it does still happen. And when it does happen, it will likely cost thousands of dollars – and a lot of disruption – to fix.
As mentioned, a slab foundation can last at least 50 years with little to no maintenance or repairs; however, if you are selling a home with an older monolithic slab foundation – i.e., one that doesn’t have newer Pex plumbing and the like – then buyers might view the property in a more negative light because they assume costly repairs may be just around the corner.
Is a Slab Foundation for You?
After considering the pros and cons, there are a few more factors to take into consideration about slab foundations.
The main one is the climate. Especially in the United States, there is a difference in the popularity of foundation types based on the climate of a region.
For example, in more temperate climates, installing a monolithic slab foundation can increase overall energy efficiency. In desert climates, a crawl space may be best for this purpose, while in freezing climates, it’s probably a basement.
Another consideration is topography and soil conditions. In an area with a high water table, for example, slab and crawl space foundations are preferabale. The same is true in areas with sandy soils, such as desert locales, or areas where bedrock is close to the ground surface, such as mountainous regions.
Be sure to talk to your Realtor and contractor about what best practices are for your area, particularly the local climate and the topography, and always consult with local engineers regarding the best foundations for your specific building lot.
Monolithic slab foundations are an inexpensive, sturdy, long-lasting, and easy-to-install foundation choice. But if something goes wrong, it’s not at all an easy fix. The rewards of a slab foundation tend to greatly outweigh the risks, however, which is why it is such a popular choice.