It’s easy to think basements are built in American homes by default even though their prevalence varies between regions.
Later in this article, you’ll get to understand why they aren’t everywhere.
A garage in the basement is perfectly normal though not common.
Accordingly, Can You Put a Garage Over a Basement?
Absolutely! The garage-over-basement design carries more advantages than the weight above it. We’ll talk about benefits later but it’s probably going to be one of the hardest projects you will ever undertake.
Let’s put it into perspective – an average compact car the size of a Toyota Corolla weighs 2,000 – 3,000 lbs. Some types of cars are heavier.
The garage’s floor and basement walls should be designed and constructed with great precision to support the immense weight above them. Also, such external factors as winter frost and floods must be put into consideration for maximum safety.
1. But Why Would Anyone Take The “Garage Over Basement” Approach?
This approach comes with 4 benefits:
- The basement can be used as a shelter in storms or hurricanes
- The extra storage space in the basement can be used to hold all the tools and clutter that could’ve congested your garage
- Many homeowners wouldn’t hesitate to add a man/woman cave somewhere in the basement if given an opportunity. It can as well serve as a space for your home projects or a workstation for those working from home.
- If you live in an area where land is scarce, this approach ensures you have more space – underground.
2. What Are Some Hurdles?
The first thing you should check is whether your local building codes permit this garage design.
The second hurdle could be finding a qualified engineer that’s experienced in crunching the numbers and getting everything right so you don’t end up with a collapsed basement.
3. Planning Considerations?
Capital is not everything.
Consider these important factors in the planning phase:
- The state of drainage in your area especially in sections adjacent to the foundation of the home
- Whether your insurance company will even consider covering this sort of add-on
- How you will seal the floor
- The grade of your local area’s water table
- The position of the frost line
- The means and route of accessing the basement
- The purpose of the garage and basement
4. What Are The Key Factors That Must Be Addressed?
It’s actually easier to build a garage on top of your basement than you might think, just make sure you’ve addressed these factors:
This is where the engineer comes into the mix.
You will need their expertise to know the stability of the soil (whether the soil is loamy, clay, gravel, rocky, sandy, etc.).
The bearing capacity of your soil needs to be established as well.
Another important detail found in a Geo-technical report is information about the height of the local groundwater table.
If the elevation of the table is high but you want a basement so bad, then you have a lot of water-proofing/damp-proofing to do.
Moisture encroachment could render the basement unusable and even pose a safety risk (the garage may collapse on the soggy walls beneath it).
Grading is probably the hardest factor to get right.
You know you are out of the woods if you hack it. However, don’t fret, chances are your ground isn’t all that bad.
Ideally, the ground around your home should drop 1 in. for every 1 ft. that you walk away from the home for the first 5 – 10 ft.
Obviously that’s not always the case, but you can ensure that the ground isn’t sloping upward as you walk away from the home’s foundation.
Otherwise, the resultant trough between the upward slope and the house will collect water and direct it to your basement and garage.
You can fix poor grading by adding soil next to your foundation and sloping progressively away from the home. Ensure that the foundation (whether concrete, stone, or block) shows at least 4 ft. above the ground.
Here are other risks worth mitigating as far as grading is concerned:
- If there’s a significant risk to nearby footings, you can add shoring/underpinning to keep the walls stable during excavation.
- If your area is seismically active, you will need to consider another important factor – lateral earth pressures acting on your foundation walls. It would also be prudent to plan for quake-related soil issues like liquefaction.
Basement Structural Design
Lastly, you must have a clear structural plan for your basement which should have details on retaining walls, footing, and slab. F
ootings are typically made from concrete but there are several other alternatives.
Often, both the garage’s and basement’s floors are made of Flexicore slabs. You might as well add a perimeter drain to your basement.
5. Wait! What’s A Flexicore Slab? Is It A Must-Have?
It comes with a series of steel beams and tube-like channels in the core which is probably the most distinctive characteristic that separates it from regular wet concrete floors.
Don’t be fooled by the hollow parts, this slab is heavy enough (as a result of steel reinforcements) to require a crane to lift and lower it in the basement.
Must you use it for this type in your garage-over-basement project?
No, but it’s preferable. It’s recommended for 3 reasons:
- Firstly, it’s more durable than most other options.
- Secondly, because it is precast and manufactured away from the site, it removes the messy job of mixing your own concrete at the site just to make a floor.
- Thirdly and most importantly, the hollow ‘tubes’ within the structure provide ready channels for your plumbing and wiring. However, make sure you plan your plumbing early because you can’t cut this type of floor.
6. OK, So What Are The Drawbacks Of The Garage-Over-Basement Approach?
Start by pondering the question of how long it’ll take to complete the project.
Because of the amount of time spend on careful planning and sourcing the materials needed for the project, the project will take longer than you might want.
Unless you are skilled and experienced enough, forget about DIY.
If you want to get everything right, you must hire a certified engineer (not a general contractor as it’s usual with garages).
While it will cost more to hire an engineer, it’s worth it considering the risks involved.
Perhaps the biggest drawback is the likelihood that your home may not sell as easily as you might want. M
any homebuyers wouldn’t be at ease with the idea of a garage standing on hollow ground. Could it be the reason why the garage-over-basement design is rarely pursued?
Lastly, this design might put a limit on the kind of car you can bring into your garage.
All cars don’t weigh the same. The easiest way out of this is to rate the floor at twice the weight you intend it for, even more if you are planning to put the garage to multiple uses.
To summarize everything, it’s OK to build a garage on top of your basement.
Start by ensuring your local building code permits it. If it does, let your engineer do the rest of the tough job for you.