A few months ago I built a slab on grade foundation for our new house, so I thought it would be interesting to write an article comparing slab on grade to another common type of foundation: pier on beam. I considered several types of foundations for our new home, including pier on beam, so this comparison might give you some insight into why I ultimately chose slab on grade.
What Is a Slab on Grade?
A slab on grade is a monolithic concrete pad that sits directly on the surface of the ground. In other words, it’s one solid piece of concrete in direct contact with the soil. The slab is supported by a footing, sometimes called a “footer” or “footers,” that sit underneath the perimeter of the slab, buried underground. In climates with a frost line, the depth of the footing will be as deep as it needs to be to go beneath the frost line.
What is Pier and Beam?
Pier and beam, or post and beam, is a type of crawlspace foundation. Strong vertical piers are set in the ground, and a floor constructed of wooden beams is built on top of them. The piers can be made of concrete, brick, steel, or cedar, which has a high resistance to rot and decay.
The Differences Between Slab Foundations and Pier and Beam
There are many differences between these two foundations, so let’s try to run through them as succinctly as possible.
History
Before the 1960s, pier and beam was the most common type of foundation. After that, slab on grade became more popular, and is now the predominant type of foundation used across the southern United States.
Construction
Slab on grade is the easier of the two to build. A trench for the footing is dug along the perimeter, usually by heavy machinery, though sometimes by hand. It is then filled with a few inches of gravel or crushed rock, and one or two rows of suspended rebar is ran through the center for added tensile support. Lastly, the bare ground within the perimeter is covered with more gravel or a plastic vapor retarder, a rebar grid is built on top of it, and the concrete is poured into the form – some concrete contractors are now using synthetic fiber mesh mixed in to the concrete in lieu of rebar.
To build a pier and beam foundation, holes for the posts must be dug in the ground until bedrock is hit. In some areas, these holes may only need to be a few feet deep, in others they may need to dozens of feet below the ground before solid rock is hit. It’s just like drilling a well. Hollow, cylindrical tubes made of cardboard are inserted in to the holes, lined internally with a cage of rebar, and then filled with concrete. Sometimes these tubes have large plastic bases to allow for greater support and stability. While the concrete is still soft, supports for the floor beams are inserted, and then the beams are run horizontally across the piers.
Cost
One of the reasons slab on grade has become more popular than pier and beam is because it costs far less money to build. They are much easier and faster to build, and typically use less materials. Another reason slabs cost less is because you usually don’t have to dig very deep, and you – or your builder – already know how deep the footing needs to be.
With pier and beam, on the other hand, there’s typically no telling how deep the holes need to be until you start drilling. You may need to go 50′ down, and this is when costs start to add up significantly. Many people building a new home simply lack the budget for a pier and beam foundation.
Lifespan
A well constructed slab can last up to 100 years. On the other, a poorly constructed one could fail in as little as 10 years or less. Concrete, brick, and steel piers typically last 50 to 100 years, but that range still depends on the quality of construction.
Repair and Maintenance
Concrete slabs are much cheaper to build upfront, but repairing them can cost a fortune. There is no way to repair a slab, or the things that might be in it – pipes and wires – without heavy machinery or expensive power tools, such as a jack hammer. Note: I strongly recommend not putting your pipes and electrical wires in the slab for this reason. These repairs also need to be done by professionals, since it is unlikely you will have the skills or machinery to do it on your own.
Repairing slabs can also be dangerous, especially post-tensioned slabs. I’ll probably write an article in the future talking more about post-tensioned slabs, but for now, all you need to know is that they have steels beams in them that can violently whip up from the slab if they are accidentally severed during a repair. This has actually killed people.
As for maintenance, most slabs don’t require any maintenance; although, in some climates, depending on the soil conditions, it is recommended that you frequently water around the slab to maintain consistent moisture levels as a means of mitigating dramatic expansions and contractions, which can crack the foundation
Pier and beam foundations are much, much easier to repair, but require more maintenance. They are easier to repair for a number of reasons: Individual piers can be replaced if necessary; additional piers can also be added if extra support is needed; the crawlspace allows for easy access under the building… even the entire building can be lifted and moved, though this is not all that common.
As for maintenance, what you really need to be concerned about is water and moisture. It is recommended that you install and maintain a good drainage system to prevent too much moisture from accumulating under the building. You should also regularly inspect your piers for cracks and other defects.
Durability
Both types of foundations are highly durable, though I would say there are more things that can damage a slab on grade. The expansion and contraction of soil, frost upheave, and shifting/sliding soil can all cause problems. Soil, in particular clay, that wicks moisture out of the concrete can also do damage to the slab. Lastly, slabs can sink in ground that is too soft and muddy.
Everything I just listed can also be problematic with pier and beam, but to a lesser extent. A pier and beam foundation, however, should not be susceptible to sinking because of the bedrock supporting the posts – “should” being the operative word. If bedrock was not made contact with, then a pier and beam also has the potential to sink under bad conditions.
Thermal Efficiency
Slab on grade foundations are insulated by the ground, and since there is no air flow beneath the slab, they are, in most cases, intrinsically more thermal efficient. Slabs also have lots of thermal mass since they are basically large, solid rocks.
Pier and beam foundations, on the hand, need to be insulated within the crawlspace for optimal thermal performance, especially in cold climates. A ventilated pier and beam foundation in a cold climate will not be thermally efficient because cold air flowing under the building will draw heat away from the house.
Moisture Managment
As long as some type of vapor retarder has been installed under or over the slab, they are better at moisture management. If no vapor retarder or capillary break of any kind has been implemented, then they can wick a lot of moisture up to the building, which can cause failures within the assembly.
Buildings on pier and beam foundations are much more susceptible to moisture problems since there is a lot of room for moisture to accumulate under the structure. In particular, vented pier and beam foundations are practically guaranteed to have moisture problems because warm, humid air can flow through the crawlspace.
Acoustics
When it comes to acoustic qualities, slab on grade is the clear winner. Floors on pier and beam foundations can squeak and crack, and may even thud a bit, like a drum. That’s because they can bend and flex, and they are essentially hollow, except for the insulation, which can help reduce the boominess.
Slab on grade floors are perfectly quiet. They are rigid and solid, so it is virtually impossible for them to produce any noise.
How to Choose the One That Is Right for You
Assuming cost is not an issue, I think deciding which type of foundation is best for you is largely an arbitrary decision and more of a personal preference, with only a few exceptions.
- If you are building in an area prone to flooding, pier and beam is the way to go – which is somewhat ironic considering they’re more susceptible to moisture issues.
- If you live in an extremely hot climate, pier and beam may be a better choice since the building is – mostly – thermally decoupled from the ground, which can transfer heat up to the house – also somewhat ironic considering slab on grade is the most common in the hottest parts of the U.S.
- If you are building on a steep slope, pier and beam is probably the more practical solution.
It’s difficult to say one of these foundations is inarguably better than the other. That being said, for my money, I think a slab on grade constructed with the best methods and precision is the better choice in most situations. That’s because I don’t think it’s wise to build in flood zones, extreme climates, or on steep slopes; the only three scenarios in which I think pier and beam makes more sense.
