Garage Foundation 101: Your Guide For Rock Solid Garage Foundation

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Building a garage is a multi-step project that can take several weeks to complete. But before you construct walls or install a garage roof, you first need to lay a foundation. You’ll also need to choose what type of foundation you’d like for your garage.

The two primary types of garage foundations are concrete slab and basement. Concrete slab garage foundations tend to be the most popular option. You can build concrete slab garage foundations far more quickly than basement foundations.

This guide will explore different types of garage foundations and outline the steps you’ll need to take to build a typical garage foundation. We also take a look at building codes and other requirements for foundations.

What Is Garage Foundation?

A garage foundation is the structural base upon which a garage is built, providing stability and support to the structure. Like any foundation, it ensures:

the building remains level,
distributes the weight evenly, and p
revents sinking or shifting due to soil movement or other environmental factors.

There are several types of foundations for garages, including slab-on-grade, floating slab, and full basement foundations.

The type of garage foundation chosen depends on factors such as soil conditions, climate, and intended garage usage. A well-constructed foundation is crucial for the longevity and safety of the garage, ensuring it can withstand various loads and weather conditions.

Types Of Foundation For Garage

When you’re thinking about the types of foundation for your garage, the options come down to what works best for your budget, site prep, and the size of your garage. The most common garage foundation types include:

Slab foundations,
Basement foundations,
Pier foundations, and
Crawl space options.

Concrete Slab Foundation

A concrete slab foundation is the most popular choice, particularly if you want a concrete garage or metal garage. This foundation is affordable, fast to install, and can support the entire weight of your garage.

A slab foundation typically consists of poured concrete spread over a metal bar or wire mesh to strengthen the structure.

To add strength, you’ll often need to install footings around the perimeter to support the walls and distribute the load evenly. It’s crucial to consider the frost depth in your area.

If your garage is in a cold climate, digging deep enough to avoid frost heave will save you from garage foundation problems down the road.

The average cost to install a concrete slab garage foundation is approximately $4,000. Installing this foundation without outside assistance may lower costs, while adding heated flooring and electrical components can increase costs.

Pier Foundation

If you’re dealing with expansive soil, which swells and contracts with moisture, a pier foundation might be a better option. In this foundation design, stronger concrete piers are drilled deep into the ground to create a stable foundation that won’t shift with the soil.

Piers are also ideal for sloped sites, and they’re common for metal carports or smaller garages, especially in areas where excavation is challenging.

This foundation type works well for homeowners wanting a durable solution without the expense of a full concrete pad.

Crawl Space Foundation

A crawl space foundation is a bit of a hybrid option. It elevates the garage floor off the ground, allowing you to run utilities like plumbing or heating under the garage.

This foundation typically involves building footings and a short wall around the perimeter of the garage. If you plan to build a garage in an area prone to flooding, a crawl space may help prevent water damage by lifting the structure above potential water levels.

Basement Foundation

A basement foundation requires extensive excavation, as you’ll need to dig deeply into the soil to create a basement space beneath your garage floor.

You’ll likely need a permit to build a basement, as well as assistance from contractors and electricians.

Consequently, building a basement foundation is costlier than laying a concrete slab foundation.

The average cost to install a basement foundation is approximately $40,000. However, prices may be higher depending on the size and complexity of the basement.

Different Types of Concrete Foundations for Garages

The 4 different concrete foundation types are floating concrete pad, floating concrete foundation, monolithic concrete foundation, and block foundation. Your options include a gravel pad or a concrete foundation.

Gravel Pad

Is cost-effective, fast to build, and usually done with gravel surrounded by a treated wood perimeter. They are suitable for carports, small prefab kits, or small modular garages.

Concrete Foundation

Is required for larger garages and proper support for heavier vehicles. There are 4 different kinds of options for concrete foundations:

Floating Concrete Pad – Simple and cheap. Usually 4” of concrete over 4” to 5” of clean stone.
Floating Concrete Foundation – 4” thick floor with moisture barrier under the concrete and mesh reinforcement and footers up to 12” deep around the perimeter. With optional saw cuts to minimize cracking. Over 4” to 5” of clean stone.
Monolithic Foundation – Similar to the floating foundation with the addition of a frost wall about three or more feet under the pad. And 10″ wide around the pad.
Block Foundation – at least 2 “wide footers are poured at the required frost depth around the perimeter. Concrete blocks are built on top and around the footer. The foundation is completed with a 4” concrete floor.

Bigger foundations for garages are usually best left to professionals.

Building Code Requirements For Garage Foundation

Here are summarized code specifications derived from the International Building Code (IBC) for 1 and 2 story residences.

The International Residential Code (IRC) also includes provisions for residential and commercial buildings. The IBC encompasses the IRC.

The IBC and IRC are model codes for building structures that communities can adopt and adapt in part as needed. Code rules are enforced at the local level. So always check everything from the local building department or similar entity.

Site Grading

The exterior of the foundation (the ground immediately adjacent to the foundation footings )should slope down at a 5 % minimum slope for at least 10 feet.

Load Bearing Value of Soil

Foundations must rest on undisturbed soil. Undisturbed soil means soil that has never been turned over, compacted, dug, scraped, tilled, graded, amended, or anything similar of that nature by machine or man.

Undisturbed soil is much stronger than soil that has been disturbed.

A disturbed soul can be classified as undisturbed after enough time has passed. But only a soil test can determine that.

The code refers to load-bearing values (LBV), so a soil test is needed to know the LBV of the soil for the site.

Soil Type	LBV Per Square Foot
Bedrock	12,000
Sedimentary Rock	4,000
Sandy Gravel or Gravel	3,000
Sand, Silty Sand, Clayey Sand, Silty Gravel, Clayey Gravel	2,000
Clay, Sandy Clay, Silty Clay, Clayey Silt	1,500

Depth, Width, and Thickness of Footings

Footings should extend to at least 12 inches below previously undisturbed soil. It needs to extend at least 12 inches below the frost line. This may not apply to accessory buildings like sheds.

The width of footings will vary according to the site, soil, local conditions, and structure size. Generally, one-story buildings should have a minimum width of 12 inches under the soil with LBV between 1,500-4,000

Levelness requirements of the footings are different for the top and bottom.

Top & Bottom

The top of the footings must be level in all cases. Preference is level for the bottom of the footing. Still, if you are building on a sloped grade, you can step the footings 1 unit vertical per 10 units horizontal.

Spread

The load carried by the footings distributes over to a broader area with spread footings. The spread should be no less than 6 inches thick and project at least 2 inches on both sides. Preferably more.

How To Lay Garage Foundation

When you’re ready to lay a garage foundation, you need to consider a few key factors, like:

the type of foundation,
soil conditions, and
local building codes.

Whether you’re building a metal garage, a concrete garage, or something more unique, the process can seem overwhelming.

But with careful planning and execution, you can ensure the structural integrity of your garage and avoid potential foundation problems down the road.

Here’s a detailed, step-by-step guide to help you lay a durable and long-lasting garage foundation.

Concrete Foundation Preparation

The first step in laying a foundation for a garage is site prep. You need to clear the area of any debris, rocks, or vegetation, creating a level surface.

A smooth, even surface is essential because it reduces the risk of your concrete slab foundation settling unevenly, which can lead to cracks and other issues.

If the soil is expansive, meaning it swells or contracts due to moisture changes, you’ll want to compact it or consult with a structural engineer to ensure stability.

This is especially important if you live in an area with a high frost line, where frost heave could shift your foundation.

Excavate The Area

Next, you’ll need to excavate the area where your foundation will sit. The depth of the garage foundation depends on the type of structure and local regulations.

For instance, footings should extend below the frost depth in cold regions. Most footings are around 12 to 18 inches deep, but you might need to go deeper depending on the size and weight of your garage or if your soil conditions are less stable.

Set Up Form Boards

Once you’ve dug the foundation trenches, it’s time to set up form boards. These are essential for shaping the concrete slab and holding it in place during the curing process.

Use plywood or metal forms for larger structures to ensure straight, clean lines. It’s a good idea to install a vapor barrier beneath the slab to prevent moisture from seeping up through the concrete surface, which could weaken the slab over time.

A 6 mil vapor barrier is often recommended for garage floors, as it prevents water damage that could lead to cracks or warping.

Reinforce Concrete Slab Foundation

Now, let’s talk about reinforcement. To prevent cracking and garage foundation problems down the line, you’ll want to strengthen your slab with rebar or wire mesh.

Reinforcing bar adds structural strength to the slab, particularly in areas that will bear heavy loads, like where you’ll park vehicles.

You should place the reinforcing bar about halfway into the slab thickness—typically 4 to 6 inches for most garages.

For larger garages, or if you’re installing a second story or heavy machinery, consult with a professional about additional reinforcing steel or thicker slabs.

Before pouring the concrete, make sure the form boards are perfectly level. Use a laser or a traditional level to check, because even a slight slope can affect how the slab cures and ultimately lead to uneven settling.

The standard slab foundation is about 4 inches thick, though thicker slabs (up to 6 or 8 inches) are recommended for garages that will house heavy equipment like trucks or RVs.

Pour Concrete

With everything in place, it’s time to pour the concrete. Mix it according to the specifications from your concrete supplier, typically aiming for a strength of around 3,500 PSI for a garage floor.

Start pouring at one corner and work your way across the area. Use a screed board to level the concrete, and don’t forget to add expansion joints.

These joints will allow the concrete to expand and contract with temperature changes, preventing cracks from forming. The joints are usually cut into the concrete surface every 10 to 12 feet. For larger garages, you may want to use a power trowel to get a smooth finish.

Once the slab is poured and leveled, you should cover it with a curing blanket or plastic sheeting to keep moisture in.

This ensures the concrete cures slowly and evenly, which increases its structural strength. You can also mist the slab with water periodically, especially in dry climates.

Add Features

While the concrete is curing, you may want to start planning for additional features like:

drains,
radiant heating, or
plumbing lines.

It’s much easier to add these elements now, rather than cutting into the slab after it has hardened.

If you’re building a metal garage or carport, be sure to anchor the structure securely into the foundation. You can embed steel anchors into the slab during the pour for added strength.

When Should You Seek Professional Help To Lay Garage Foundation?

You should consider seeking professional help to lay your garage foundation when the project involves complex factors like:

expansive soil,
large structures, or
tricky local building codes.

Soil Conditions

Soil conditions are a significant factor. If you’re dealing with expansive soil, which can shift and crack the slab, hiring a structural engineer to evaluate and design the right foundation type is essential.

The wrong foundation in such conditions can lead to costly garage foundation problems down the line, such as cracking and uneven settling. You may also need professional advice on how to reinforce the slab with rebar or wire mesh to prevent damage.

If your area has a high frost line, a professional can ensure that the foundation design includes footings that reach below the frost depth to prevent frost heave, which can compromise the structural integrity of your garage.

In cold climates, hiring a pro for the installation process can make sure that both the footings and the concrete slab foundation are deep enough to withstand seasonal ground movement.

You should also seek professional help if your garage is built on a slope or if the site prep is complicated by the perimeter of the foundation being uneven.

A professional can use precision tools to ensure the foundation for your garage is level, distributing the load evenly and preventing future cracks.

Larger Garages

For larger garages or those with special features like drains, radiant heating, or a vapor barrier, pros are invaluable.

They can make sure these elements are installed before pouring the concrete, preventing issues later that would require cutting into the slab.

Tricky Building Codes

Finally, if you’re working with local building officials or navigating local building codes, a professional can handle the permits and ensure the foundation type meets all regulations.

Complex pier foundations, masonry work, or even heavy prefab kits generally require a licensed contractor to ensure proper construction. When problems arise, you’ll be glad you consulted a pro sooner rather than later.

Garage Foundation and Concrete Slab Cost

If you want the job done by professionals, Here is up-to-date info at Homeadvisor.com and Homeguide.com

Garage Foundation: Do It Properly

Building a rock-solid garage foundation is essential for the longevity and stability of your garage.

Whether you opt for a simple concrete slab or a more reinforced foundation with footings, proper planning, site prep, and understanding of your soil conditions are key.

Consulting local building codes and working with professionals ensures you choose the right foundation type for your garage’s size and weight, preventing future issues like cracks or settling. A strong foundation is the first step to a well-built garage.

FAQ

How Many Bags Of Concrete Do I Need For A Concrete Slab?

To calculate how many bags of concrete you need for a concrete slab, multiply the slab’s length, width, and thickness to get the volume in cubic feet.

Divide this by 0.6 (for a standard 60-lb bag) or 0.45 (for an 80-lb bag) to find the number of bags.

Size	Thickness	Cubic Yards	#Bags	Bag Weight
10X10	4	1.25	56	80 LB
10X10	5	1.6	72	80 LB
10X10	6	1.85	84	80 LB
10X10	7	2.2	99	80 LB
10X10	8	2.5	113	80 LB

The amount of concrete you need for a 10 x 10 slab that’s 3 inches thick is 1 cubic yard.

Here are some calculators, so you get a good estimation and don’t buy too many bags.

What Is The Best Type Of Foundation For A Garage?

The best foundation depends on factors like soil conditions, climate, and the size and weight of your garage.

A concrete slab foundation is a popular choice for most residential garages, but for expansive soils or larger garages, you may need footings or a pier foundation for added support.

How Thick Should The Concrete Slab Be For A Garage?

For most garages, a concrete slab should be at least 4 to 6 inches thick. If you’re building a heavier structure, like a metal garage, or have expansive soil conditions, it’s recommended to strengthen the slab with rebar or wire mesh and make it thicker around the perimeter.

The concrete slab’s minimum thickness is 4 inches usually. The minimum strength of the concrete should be 3,500 PSI for garages. Vertical walls exposed to the weather may be required to have 2,500 PSI minimum strength for concrete or up to 3,000 PSI for severe local weather conditions.

What Are Footings, And When Are They Necessary For A Garage Foundation?

Footings are deeper concrete elements that provide extra support for your garage foundation. They’re typically needed in areas with frost depth concerns or when building larger, heavier structures to ensure the garage doesn’t sink or shift over time.

Do You Need a Rebar in a Garage Slab?

Yes, you typically need rebar in a garage slab to enhance its strength and durability. While concrete is strong under compression, it has weak tensile strength, making it prone to cracking over time.

Rebar, or wire mesh, helps reinforce the slab, preventing shifts or cracks, especially under heavy loads like vehicles or equipment.

For larger garages or those housing heavier vehicles, reinforced concrete is crucial to maintaining the structural integrity of the garage foundation.

When Should You Add Garage Foundation Drains and Heating?

You should add garage foundation drains and heating during the initial foundation installation, before pouring the concrete slab.

This is because it’s much easier to integrate drainage systems and radiant heating into the foundation at this stage rather than cutting into the concrete later.

Properly placed drains help manage moisture, keeping the garage dry and preventing water damage, while radiant heating offers an efficient, dust-free heating solution, ideal for workshops or garages in colder climates.