How to Determine Concrete Slope for Exterior Slabs

Believe it or not, the best concrete slabs aren’t perfectly flat and level. A well-designed exterior slab — whether it’s a driveway, patio or sidewalk — is engineered to slope to some degree to allow for the runoff of water. Not only does the slab need to slope by the right amount, but also in the right direction.

So how do you build slope into a concrete slab, and how do you determine the amount of slope you need for your exterior concrete project? Read on to learn more.

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Why Is Slope Important?

The most important reason for including slope, or concrete fall, in a slab rather than making the surface level is to promote drainage and prevent ponding water. Standing water can seep into the concrete, leading to cracks, spalling, and other forms of deterioration. It can also freeze in the winter, making surfaces unsafe for foot and vehicle traffic.

In addition to concrete slope for drainage, it is also important for directing water away from a home or another adjacent structure so the water doesn’t pool up next to it and cause serious and potentially costly damage. One of the major causes of leaks in residential basements are concrete slabs that are pitched toward rather than away from the home.

What Is the Minimum Slope for Concrete?

The key to concrete slope is to make it as gradual as possible to ensure adequate drainage without affecting the usability of the slab. A good rule is to use a minimum slope of 2%, which is equivalent to a 1/4-inch drop for every foot of length.

As an example, if you plan to install a concrete patio that extends 10 feet from your home, a drop of 2 ½ inches from the starting point by the house to the opposite end of the slab will give you the desired slope. Similarly, a 4-foot-wide concrete walkway that runs parallel to a house or other structure should drop 1 inch facing away from the structure to avoid water damage.

What Is the Formula for Calculating Concrete Slope?

The slope of a concrete slab is calculated as the ratio of the rise (or vertical change in height of the slab) to the run (the horizontal distance over which the rise occurs) expressed as a percentage.

To calculate the percentage of slope for your slab, first convert the rise and the run to the same units and then use the following formula: Slope = Rise/Run x 100. For example, a rise of 2.5 inches (which is equivalent to about 0.2 feet) divided by a run of 10 feet and multiplied by 100 is equal to a slope of 2%.

Can the Slope Be Too Steep?

Yes, the slope of a concrete slab can be too steep when it negatively impacts the slab’s safety and functionality.

For example, many municipalities have regulations that mandate the maximum slope of a concrete driveway as a safeguard to prevent vehicles from losing traction. Generally, the recommended maximum slope is 15%, since anything steeper than that can be hazardous to drive on during snowy or icy weather. It can also lead to rapid water runoff, which may cause flooding and erosion.

Thanks to gravity, it can also be more difficult to pour a concrete slab on a steep slope because the wet mixture will tend to flow downward. Typically, you’ll need to use a stiffer, low-slump concrete, which can be difficult to work with and will require more effort to spread and finish.

How Much Slope for Exterior Concrete?!
Time: 04:14
Jeff of Deco-Crete TV dives into the crucial topic of how much slope to incorporate into exterior concrete projects.

What Other Factors Affect Slope?

Although a 1/4-inch drop per foot of slab length is the commonly accepted standard for concrete slope, there is often some wiggle room, depending on exposure conditions and the expected amount of precipitation. In a hot, dry climate, for example, a slope of 1% (1/8 inch per foot of slab length) is often sufficient, while wet climates that routinely receive a lot of rain or snowfall may need a steeper slope of 3/8 inch per foot to ensure good drainage.

The texture of the concrete surface can also impact how efficiently water drains. A smoother surface will allow water to flow more freely, while textured or stamped concrete can slow down the drainage process because the water will tend to collect in the pattern indentations.

The direction in which your slab slopes will usually be dictated by existing conditions, like the landscaping and neighboring structures. But what if you have two structures that you want to protect from water runoff and a concrete slab in between them? In these circumstances, you may need to slope the slab inward slightly from both sides and place a channel drain in the middle to carry water away to another location. This is a common solution for concrete pool decks, where you want the water that accumulates on the deck to drain away from an adjacent house on one side and the pool on the other.

How Do You Build Slope Into a Concrete Slab?

To create slope in a concrete slab, the forms need to be set using a level and a stringline to achieve the desired ratio of rise to run. As the concrete is placed and finished, the angle of the forms serves as a guideline to help maintain the correct slope. Proper grading and compaction of the subbase before the pour is also important for preventing differential settlement of the concrete and maintaining the slope’s integrity.

To ensure optimal drainage of your slab, it’s always wise to use a professional concrete contractor to install the concrete rather than trying to do the work yourself. A professional will not only know how to form and place the concrete to achieve the desired slope, he will also have the expertise to calculate the right slope for your project based on the intended use of the slab, your climate, and site conditions.

Can You Add Slope to an Existing Concrete Slab?

If you have an existing concrete slab that doesn’t slope enough to remove water or is pitched in the wrong direction, there is not much you can do other than remove and replace the concrete. However, that may be a small price to pay if poor drainage is causing safety issues or damage to your property.

If your existing concrete is sinking due to erosion of the subbase, it may be possible to raise it back to its original slope by slabjacking. This is often a good solution for smaller slabs, such as patios and sidewalks.

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