Monolithic slab foundations are the simplest, fastest, and cheapest types of foundations available. The fact that they are poured in one go means that they are efficient to make, and they also provide a solid foundation. The absence of seams and cold joints (plus the addition of some steel reinforcements) gives monolithic slab foundations a rigidity and stability that makes them extremely strong.
In some cases, you can pour a monolithic slab foundation yourself, meaning they are a great option for smaller structures such as sheds or mother-in-law construction (although if in doubt, hire a professional).
Monolithic slab foundations have other advantages over foundations like pier and beam or a crawl space foundation.
This guide will walk you through all of the key aspects of a monolithic slab foundation, distinguishing it from other types of slab-on-grade foundations as well as other types of structure.
There are a lot of confusing and interrelated terms when it comes to concrete foundations. At this stage, it’s worth clarifying exactly what each one means.
A slab-on-grade foundation refers to any type of foundation in which concrete (slab) is poured onto the ground (grade). Slab-on-grade is sometimes referred to as a concrete slab foundation. Both of these are umbrella terms. There are two main types of slab-on-grade foundation:
Monolithic Slab Foundation
Monolithic literally means ‘single pour,’ which highlights the key point of difference with this type of foundation – it’s poured all in one go. The fact that it is poured in one go makes it a quicker process, which helps to keep labor costs low.
The concrete slab that is poured is thicker on the outside to help with the load-bearing of walls, and there are no footers around the outside.
Monolithic slab foundations are best suited to flat ground. If the ground is not flat, then a lot of fill dirt is used to level off the ground; in time, this can lead to problems with cracking if the ground is not properly compacted.
Stem Wall Foundation
With a stem wall foundation, the concrete is poured in multiple stages. The first of these is to build a stem wall of concrete blocks up to the desired slab level. Then, the fill dirt is added and compacted. Finally, a slab is poured to the desired level.
The key difference in this build type is the use of stem walls. The compacted soil or fill also means that you don’t require a perfectly level grade in order to build.
For a full breakdown of costs of different types of foundations, see our guide Average Cost of Home Foundation, which will discuss all of the specific elements in more detail, from installation to repair.
The cost of a monolithic concrete slab is $5 per square foot, resulting in an average total cost of $4,600 to $20,000.
There are two beam types of monolithic slab foundations – one-way beam, and two-way beam. A one-way beam supports the foundation on two of its edges, whereas a two-way beam supports all four sides. The one-way beam costs $9.80 per square foot, and the two-way beams costs $13.52 per square foot.
PROS OF MONOLITHIC FOUNDATION
There are numerous reasons why monolithic slab foundations are preferable to other types of foundations – and not just based on simplicity. Some of the most common reasons are below:
SPEED OF BUILD
Monolithic slabs are extremely quick to build. Once you’ve compacted the soil (or removed the topsoil), added the perimeter trench, and spread the gravel, you can pour over the course of an afternoon.
The concrete needs to be 4 inches thick at its thinnest part (i.e. in the center of the home) and therefore will usually take a couple of days to dry fully. This is significantly faster than any other type of foundation construction.
Despite the simplicity, a monolithic slab foundation will last around 50 years if properly constructed. Because of the absence of complex elements, there’s little to go wrong with the slab itself.
There are no joins in the concrete and as long as the reinforcement and anchor bolts are successfully installed, you have a strong foundation, capable of enduring a great deal of weight.
Unlike a pier and beam system, which requires fairly regular maintenance, a monolithic slab doesn’t require you to do any upkeep.
As long as you give it an inspection at regular intervals to ensure there aren’t cracks in the foundation, there’s nothing you need to do on a month-by-month basis to keep it in good shape.
Monolithic slab foundations don’t have a space between the ground and the home, meaning that you don’t have to waste as much energy in your home heating the air below it. With a basement or crawl space, energy is being constantly wasted.
This means that a monolithic slab foundation is not only cheaper in the short term, but can save you money in the long term.
CONS OF MONOLITHIC FOUNDATION
Although there are undoubtedly many advantages to a monolithic slab foundation, there are crucial limitations that may make it unsuitable for your build.
A crawl space or basement gives access to the foundation, meaning that you are able to place plumbing and the electrical panel and wiring in this space.
If something goes wrong with a monolithic slab foundation, furthermore, you are unable to access it in order to address the problem.
Although things rarely do go wrong with monolithic slab foundations, when they do crack, it can be extremely expensive.
You generally need to use tactics like mudjacking or foundation jacking, which can cost thousands of dollars.
With concrete slab foundations, your home is only raised 6 inches or so from the ground, meaning that the rest of your house is susceptible to flooding. In some parts of the country, this is a major weakness.
RESALE VALUE OF THE HOUSE
In some instances, a monolithic slab may diminish the value of your home. If you have an older monolithic slab foundation, then a prospective buyer may figure that they will need to undertake the expensive repairs. If you installed it yourself, a buyer may be dubious as to its quality.
WHEN A MONOLITHIC
FOUNDATION IS THE BEST
In some climates of the United States, a monolithic slab foundation is absolutely the best option. These are, primarily, in wet climates and in temperate climates.
In wet climates, a monolithic slab will be able to resist moisture in two main ways. Firstly, it is a material that doesn’t corrode after being wet; by contrast, pier and beam foundations are made of wood, which handles moisture very poorly.
Secondly, because there is no space between the ground and home, moisture cannot enter. This not only reduces structural damage, but also damage from mold (which is common in crawl spaces).
According to a study by the National Association of Home Builders found that 84% of homes in the upper Midwest had basements, whereas less than 1% of homes in Texas, Louisiana, and Oklahoma did.
What this shows is that in very cold climates, where ground freezing is common, a basement is a useful feature of a house (indeed it is mandated by some building codes). However, if you do not live in an area where it freezes regularly, there is not the same imperative for a basement.
As mentioned above, in temperate climates, you can install a monolithic slab foundation to increase overall energy efficiency. In very hot or desert climates, a crawl space may be best, in freezing climates, it may be a basement, but in the climates that fall between the two (i.e. the majority of the United States), a monolithic slab foundation is a great option.
A monolithic slab foundation is really one of the simplest options available when it comes to a foundation. If you have a level area on which to build a home or a structure like a shed, then it’s difficult to envision any other type of foundation being the default choice.
The solidity it provides, combined with the low price means that you effectively have to have a strong reason for selecting another type of foundation.
Monolithic slab foundations are even simpler than other forms of concrete slab foundation, and in some cases, you can build the foundation yourself without the need for contractors – thus decreasing the price even further.
Although a monolithic slab may not be suitable for all circumstances, many contractors prefer it not just for the ease of installation, but also for the robust and durable foundation that it provides.
Where other types of foundation – like pier and beam – require more installation, they are also more prone to issues like subsidence. They may be easier to fix when something goes wrong, but that has to be weighed against the fact that things go wrong more frequently. A monolithic slab foundation is simple, cheap, and effective – what’s not to love?