I’ve been watching some of my neighbors recently pour small concrete slabs in their backyards for various projects. From what I can tell, it seems like everyone has gone about the job differently. One of the main differences is if you need gravel under the concrete patio, slab, or footing.
You do need gravel under a concrete slab, footing, or patio. Gravel provides a solid foundation for your concrete as it can be compacted. It also improves drainage, preventing water from pooling beneath the concrete.
While some may argue that very solid soils such as clay provide just as good a base as gravel, they still do not provide adequate drainage. This leads to pooling moisture and erosion, which causes a slab to sink and crack.
In this article, we’ll go over all the reasons to add gravel beneath your concrete slab. We’ll also briefly go over the process of what making a concrete slab entails and some instances when pouring concrete right onto bare earth might be appropriate.
- Why Do You Need Gravel Under a Concrete?
- What Type of Gravel To Put Under Concrete?
- How Much Gravel Do You Put Under a Concrete Slab?
- Should You Put a Vapor Barrier Under Concrete Slabs?
- Can You Pour Concrete Over Dirt?
- How To Compact Gravel for Concrete
- Do I Need Foam Beneath a Concrete Slab, Patio, or Footing?
Why Do You Need Gravel Under a Concrete?
Gravel, or crushed stone, provides a stable surface for your concrete slab. Even the most solid, compacted soil can shift dramatically with the weather changes, which will undermine your slab and cause cracks.
It is important to note that not all gravel is the same. There are many different sizes of gravel that are crushed to sizes that are specific to certain tasks. In this article, when I reference gravel, I’m referring to crushed rock between ¾” and 1” in diameter – roughly.
Gravel can be compacted. This is beneficial because it allows you to create a level surface. A level surface allows you to determine exactly how much concrete you’ll need.
The most major benefit, though, is drainage. Water that seeps beneath a slab can erode soil, causing gaps between the earth and slab. Those voids cause cracks, destabilizing your concrete slab. Compacted gravel allows water to escape and travel into the earth without pooling.
Another major advantage of using gravel is that it acts as a moisture barrier. Water seeks the path of least resistance, so water that pools beneath a slab can often end up seeping through the tiny capillaries in the slab and onto the top surface of your slab.
Gravel serves as a capillary break – the spaces between the gravel are too large for water to travel “up”. Slabs set directly onto earth will often experience damp spots on the top surface as water wicks up from the dirt into the slab and then onto the surface.
The stability and drainage properties of gravel make it an excellent base for a slab, and it also reduces the amount of concrete you’ll need to pour. A level gravel surface will make it easy to calculate how many yards of concrete you’ll need.
Pouring onto earth would force you to take into account the dips and rises of the terrain, resulting in a more general estimate for the amount of concrete you’ll need.
What Type of Gravel To Put Under Concrete?
The ideal type of gravel is ¾” to 1” washed and screened gravel. It is known by many different names throughout North America, from driveway gravel to “57”, so we’ll stick with the specific size instead of a nickname for accuracy.
Gravel that is larger than 1” can be difficult to compact and could be too porous for some types of concrete. Therefore, it can also be tough to level larger gravel, making it tough to estimate the amount of concrete needed. If you are putting rigid foam on top of the gravel, large rocks may impede the installation or break the foam.
The crushed rock used for concrete slab foundations, between ¾” and 1”, is still quite rough. Many find the pieces very jagged and wonder if that is still acceptable. It is because the jags and sharp edges, when compacted, act to hold one another together. Thus, the rough shape of these rocks acts as a sort of “glue” when leveled and compacted.
It is important to get washed and screened gravel. As mentioned above, we don’t want any “wicking” of water in our gravel. Wicking is when water travels up through the gravel into the slab. If you purchase unwashed gravel, you risk the gravel including excessive amounts of dust, which water can wick upwards.
Screening removes smaller rocks mixed in with the ¾” gravel. This also helps prevent moisture wicking up through the “fines” in the rock. The fines are smaller rocks and rock particles that allow tiny water passages to travel.
Getting a smaller diameter of gravel, say ½” or pea gravel, is also common and adequate as long as it is washed and screened. The smaller the gravel, the more expensive it can be as it takes more energy to crush rock into smaller bits than larger.
Some claim that any crushed rock is just fine, even mixed gravel with old concrete or other solid material. While these “mixed” materials can work, too, you’ll have to work harder to compact and level it, and you run the risk of having too many fines in the rock.
How Much Gravel Do You Put Under a Concrete Slab?
You will need 3 inches of gravel under a concrete slab that is 4 inches thick. More gravel is better, but 3 inches is the minimum amount of gravel you should have with a 4” slab. Use ¾” washed and screened gravel, then compact it to level.
If you choose to add more gravel, which is never a bad idea, you’ll have to compact every 5” or so if you are using a plate compactor or other compacting machine.
Also, if you plan on using rigid foam boards beneath your concrete slab, the foam’s thickness does not take the place of gravel. Thus if you have 2” rigid foam, you’ll still need 3” of gravel beneath the foam to support a 4” thick slab.
Should You Put a Vapor Barrier Under Concrete Slabs?
Yes, you should put a vapor barrier under concrete slabs to prevent moisture from wicking up into the slab and onto the surface of your concrete slab, patio, or driveway. Ensure the vapor barrier is above both the gravel and rigid foam and immediately below the slab.
Moisture wicking up into a slab can cause unsightly wet spots on top of your slab, and the efflorescence can be hard to remove. It is also a safety hazard, as wet spots can become slick and have an odor.
If moisture wicks through your slab and you have flooring on top of your slab, then your flooring could mold, rot, or mildew, which would result in having to remove it.
A plastic vapor barrier ensures water will not enter up through the slab. Putting it above the gravel allows water to travel beneath it and out through the gravel. If you put the barrier below the gravel, water gets trapped and looks for a way out – through your slab.
Lastly, if you have rigid foam under your slab, you’ll want to put the vapor barrier between the slab and the foam. This ensures the foam stays moisture-free as the water can travel away through the gravel. Putting plastic under the foam could risk the saturation of the insulation and compromise its ability to insulate.
Can You Pour Concrete Over Dirt?
Yes, you can pour concrete over dirt. If you have a very compact soil-type, such as solid clay, then you may have ground that is solid enough to support a slab over a period of time.
One of the big risks for pouring directly onto the earth is water pooling beneath the slab and traveling up into the slab. You can mitigate this issue by putting a vapor barrier between the slab and the dirt.
Water will still get trapped under the plastic and soil, but a hard soil like clay will resist erosion much better than sandy soil.
Even with a vapor barrier, you may still experience moisture in your slab if it is on top of dirt since there is no gravel to drain the area properly. In this case, be sure your slab is for an unfinished building like a shed. That way if you do get moisture in your floor, it won’t be a huge problem.
There is no question that pouring concrete directly onto the earth saves time and money, but you are much more likely to get cracks in your slab as the earth shifts. Even if you live in a warm climate, the shifting of the earth from warm to cool or from dry to wet is enough to cause cracks in a slab.
How To Compact Gravel for Concrete
When compacting gravel designated for a foundation of a concrete slab, it is best to use a machine. A plate compactor or jumping jack will compact the gravel in a fraction of the time that it would use to use a hand tamper.
If you have more than 5” of gravel, you’ll have to compact gravel more than once. It is recommended to compact every 5” to ensure even compaction throughout the bed of crushed rock.
Compactors are widely available at any rental store. Big box stores that do rentals will almost certainly have one – or many – to rent by the hour. While heavy, they are small enough to be loaded into the back of a family van or truck without much hassle.
The job takes about an afternoon, depending on the size of your slab, and you’ll find the cost justified considering the time you’ll save versus hand tamping. Compacting manually takes much longer, as you’ll need to tamp every inch or so to achieve, or come close to achieving, compaction anywhere near what the machine compactors can accomplish.
Do I Need Foam Beneath a Concrete Slab, Patio, or Footing?
No, but it’s a good idea because it will seriously negate any cracking or shifting between seasons. The flexibility of the rigid insulation has “give” allowing it to absorb movement as the ground shifts and moves between seasons.
Rigid foam is different from the insulating foam you would use in your walls. Foam for subfloors is much sturdier and can be walked on without risking cracks or breaking. It is ideal for placing on top of gravel foundations and withstanding concrete pours.
The foam will have a tremendous impact in terms of the insulation of your floor. If you intend to heat the space above your concrete slab, it is imperative you use foam beneath the slab. As a barrier between the cold of the earth and the slab, it will help your slab retain heat much, much longer in the winter.
When using rigid foam, make sure the vapor barrier goes above rigid insulation so that concrete does not get beneath insulation when pouring. If that occurs, it could lift part of the insulation and make it protrude through the top of the slab, causing cracks as the concrete settles and compromising the ability of the board to insulate the slab completely.
Pouring a slab is a lot of hard work and very time-consuming. But anything that is supposed to last for years and years should be hard work. Therefore, if you are going to take the time to pour a slab, invest a little more money and add some gravel. While you’re at it, throw some rigid foam and vapor barrier on top for added protection of your investment.
The addition of foam on top of your compacted rock is just another layer to act as a preventative measure against cracking and shifting. Plus, a properly built slab can have a lifetime of uses. Even if you don’t plan on heating the space, you might change your mind in the future.
Thanks for taking a moment to look at this article. Hopefully, it answered all your questions about what should go under a concrete slab. If you have any other suggestions or noticed anything I left out, please drop me a line below. Best of luck with your next concrete slab project!