Garage Slab Slope Code That You Need To Know (Important!)

You may have noticed that your garage floor is slightly sloped instead of being perfectly level. This is not poor workmanship or a design error by the contractor. On the contrary the contractor has followed the garage slab slope code and given you the perfect garage floor.

This sloping of the garage floor raises two questions.

  1. Why are garages sloped?  And
  2. What slope should a garage slab have?

In this post, not only do I answer the above two questions but also answer some more questions that you may have regarding the sloping of your garage floor.

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Why are Garages Sloped?

A garage slab must have a slope as prescribed in the International Residential Code. The 2021 International Residential Code (IRC) Section 309.1 stipulates

Garage floor surfaces shall be of approved noncombustible material. The area of floor used for parking of automobiles or other vehicles shall be sloped to facilitate the movement of liquids to a drain or toward the main vehicle entry doorway.

Garage Slab Slope Code

It is commonly believed that the garage slab slope is to allow any water on the garage floor to flow easily towards the garage door or to a drain. The slope is certainly useful in clearing the car wash water, rain water or snowmelt.

But the folks at IRC have a far more important reason for writing Section 309.1 in the Code; Safety. The giveaway are “noncombustible material” and “liquids”, not water, used in the wording.

Your garage is primarily designed to park automobiles. Leaks and spills of “combustible liquids” such as gasoline and motor oil are a distinct possibility. 

A garage floor that slopes towards the garage door or drain will carry the inflammable liquids away from your house. 

In essence the garage slab slope prescribed in the code is a safety feature.

What Slope should a Garage Slab Have?

The Code as per IRC does not prescribe a specific slope for the garage slab.

However, a pitch of ¼” for every foot is a commonly accepted norm. In %age terms this is around 2%. More specifically, for a 24’ deep attached garage, the door end will be 6” lower than the house end.

This garage slope ensures smooth flow of water towards the garage door or the garage floor drain as the case may be. Many contractors will insist that a slope of ⅛” for every foot (or 1%) is good enough.

Though IRC does not prescribe a specific slope. State and Municipal Building Code generally follows the IRC or IBC but can make modifications & additions to the garage slab slope code. So always check with them before placing the concrete for a garage slab floor.

You can also go down the rabbit hole by trying to find the relevant code for your municipality through https://library.municode.com

The good news is that a garage floor slope that allows for smooth flow of water, will also allow for smooth flow of gasoline and petroleum oil. The flow rate of a liquid, in relation to the flow rate of water, is inversely proportional to the specific gravity of that liquid.

A liquid, lighter than water, such as gasoline or petroleum oil, will flow faster than water. The Liquid Flow Conversions PDF from RegO puts the flow rate of gasoline at 1.17 and that of petroleum oil at 1.07 times that of water, respectively.

What is the Minimum Slope for a Garage Slab?

This question does not have a very clear-cut answer. I hope that the following advice is helpful.

  1. The IRC states that there must be a slope, but it does not give a number.
  2. Many State & Municipal Building Codes just follow IRC. They also do not give a number.
  3. In any case, check with your local building authorities, in person if possible, and get a number. Comply with it, if they give you one.
  4. Ask your local architect. They will be aware of the local code or the best practice followed in your area. Insist on them putting it in the specifications / drawings. Else you will not be able to enforce it on the contractor.
  5. If none of the above work or you are placing the garage floor slab yourself, use the table below as a rough guide.
Slope
(Inch per Foot)
Slope
(%)
Floor Level Difference
for a 24’ Deep Garage
Comment
¼”2%6”Recommended for good drainage.
⅛”1%3”Acceptable
1/16”0.5%1 ½”Not recommended.
Difficult to achieve in practice.

How do you Determine the Slope of a Garage Floor?

The simplest and the cheapest way to check the leveling of a garage floor is to use a six foot level. Place the level on the garage floor, lengthwise on an imaginary line running from the attached wall to the garage door.

If the garage floor is sloping, as it should be, then the air bubble in the six foot level will be off center. Next insert a 1 ½” thick plywood or wood block under the level on the garage door end. If the air bubble centers, then you know your garage floor is sloping towards the door at ¼” per foot (that is 2%).

Repeat this process at several locations to ensure you are getting consistent readings and your garage floor has a uniform slope.

If you do not have a six foot level already, you could order Johnson Level & Tool 3772 72″ Machined Top-Read Aluminum Level from Amazon.

If you want to go hi-tech or want a more accurate reading you can always use a Self Leveling Laser Level also available at Amazon.

To learn more about Concrete Garage Floors, check out my earlier blog post 7 Important Things To Know About A Concrete Garage Floor.

How do you Fix the Slope on a Garage Floor?

The best way to ensure that the garage slab slopes at the desired %age is to ensure it right from when you dig the soil, compact the grade and prepare the base for the concrete mix placement.

If for any reason, that is no longer possible, then you must determine the garage floor slope using either a six foot level or a laser level. If the slope is within 1-2% range, do not worry about it too much. 

There is no need for fixing the garage floor slope.

However, if the slope is less than 1% and the garage floor gets flooded quite often, a fix may be required. The nature of the fix will depend on the reason for the fix.

Is it because the local building authorities are insisting on it? If so, you have a costly fix on your hands!

However, if you just want to make sure that the garage floor is not constantly wet or flooded, several solutions are available. I have written extensively on this topic. Please check out my earlier blog posts listed below:

Simple and Easy Guide to Best Garage Floor Drain Solutions

How To Keep Water Out? Garage Floor Water Diverter Is Best

How To Dry Out Garage Floor Quickly After A Leak?

Bottom Line

The bottom line is that:

  1. The 2021 International Residential Code (IRC) Section 309.1 stipulates that a garage floor must slope towards the garage door or a drain.
  2. There is no code on how much the slope should be. Between 1-2% is considered as the best practice and will ensure good drainage.
  3. A slope less than 1% is not recommended as it is extremely difficult to implement it in practice. You may end up with frequent water puddles on your garage floor.
  4. You can determine the slope of your garage floor using a six foot level or a laser level.
  5. The slope of your garage floor can be fixed. The nature of the fix depends on the exact nature of the problem.

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Thank you very much for reading the post. I do hope you found it informative and useful.

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