Basement under the garage slab? Possible? Ideas?

[haugy]

I'm planning my next garage. This one will be 50x40x16.

I have a sloped grade that I'll be putting it on. Not huge, but sloped so a build up would be required for the pad, probably around 5-7'. At that point, I'm wondering if it would be possible or better to build into the slope some more and try to put a basement in that is accessible on the back.

My thoughts would be a drive in basement with 10' walls for a 8' drive in door. Support would come from the middle with a full wall running the 40 depth at 25' (center of the 50'). So 25x40 would be for storage, and the other 25x40 I could use as my hangout area/extra storage.

Doing this would give me some more parking for my **** and storage, as well as free up valuable work space up on the main shop.

So my questions are:
1) Anyone tried this?
2) Am I out of my gourd?
3) I know an engineer would be needed, but am I looking at ridiculous costs?
-I'm thinking 4 poured exterior walls, one poured center, then steel beams
to span the sections (that's where it gets expensive for sure)

I'm going to only get one shot at this shop, so I'm looking at all ideas, reasonable or insane to make sure I don't do "Damn I wish I had done that".

 

[ConCretin]

Not an uncommon design at all. In fact a little searching would unearth a few similar threads on here. It's more expensive than filling it in and placing a slab on grade but the cost per square foot for the added space would be cheap.

Rather than pour a wall down the middle, I'd install columns under the beams if the spans dictate. You can form the suspended slab with metal decking, precast plank or an ICF product such as LiteDeck.

I'd include some kind of waterproofing to keep water and salt(?) from infiltrating the floor system and rotting it out.

 

[haugy]

Not an uncommon design at all. In fact a little searching would unearth a few similar threads on here. It's more expensive than filling it in and placing a slab on grade but the cost per square foot for the added space would be cheap.

Rather than pour a wall down the middle, I'd install columns under the beams if the spans dictate. You can form the suspended slab with metal decking, precast plank or an ICF product such as LiteDeck.

I'd include some kind of waterproofing to keep water and salt(?) from infiltrating the floor system and rotting it out.

Thanks LLWillys, unfortunately searching Garage basement and various search terms pulls up almost every thread in here.

Thanks for the terms about metal decking, precast plank, and ICF. I'm already hunting.

Something like this: https://forterrabp.com/prestressed-hollow-core-plank/

 

[didit]

Just inquire what building codes or practical practices are required in your area for the building plan you have in mind. That will give you a better baseline to calculate cost.
There is a small injection mold shop a block away from me that has an awesome exotic car collection stored in an underground parking structure under the shop floor. A little foresight by the owner during construction gained a great space.

 

[Jeff95TA]

Here's one to check out:

https://www.garagejournal.com/forum/showthread.php?t=12234

 

[haugy]

Here's one to check out:

https://www.garagejournal.com/forum/showthread.php?t=12234

I remember this one when he did it. It was badass for sure. Unfortunately he deleted a ton of the pics.

 

[SGKent]

there was a "Heavy Rescue" episode a while back where they went to a house to pull a car out of the basement. In the middle of the night the garage floor collapsed, sending the car into the basement. I would for sure use an engineer to plan it, and a well known contractor to execute the engineer's plans.

https://fox5sandiego.com/news/mans-...through-garage-floor-minutes-after-he-parked/

another

 

[ConCretin]

Thanks LLWillys, unfortunately searching Garage basement and various search terms pulls up almost every thread in here.

Thanks for the terms about metal decking, precast plank, and ICF. I'm already hunting.

Something like this: https://forterrabp.com/prestressed-hollow-core-plank/

Yea, I guess the search is easier said than done. Maybe ‘structural slab’ would bring a couple up. I’ll poke around if I get a minute.

The precast planks you linked are one of the options I was thinking of. Determining the most cost effective system can be a challenge because of all the variables associated with beam sizes, spans, hoisting, contractor experience, distance to mfg, etc.

 

[sberry]

There are a couple reasons for engineer with this but one is to save cost. Its relatively easy to simply over build but it can drive the cost up a lot.

 

[nadogail]

I recall watching a basement dug under an existing 4 story wooden office building in my hometown. I have come to the conclusion that with enough time and money almost anything can be done.

As I understand your post you are still in the design phase, so adding a basement while the project is still on the drawingboard seems like a very reasonable idea.

Please Keep us posted.

 

[finn]

Isn’t the basement where the Corvette collection ended up at the Corvette museum in Bolling Green a couple of years ago?

Find out who did the structural engineering on that building, then pick someone else for your project.

 

[ycgoat]

Isn’t the basement where the Corvette collection ended up at the Corvette museum in Bolling Green a couple of years ago?

Find out who did the structural engineering on that building, then pick someone else for your project.

Lol, the vette museum had a giant sink for under it, which is impossible to predict.


Sent from my iPhone using Tapatalk

 

[firebirdparts]

I did it, and I have a lift on the second floor that makes it more interesting. That's a big building you are planning, but of course if you divide it in two, it's not very big span. I have a post right in the middle of my building which is very close to the post on my lift.

At the time, I paid about $3500 for steel; the building is 30 by 32, I used bar trusses, and as I mentioned, I have a post in the middle. The 2nd floor has performed wonderfully. The big expense in the building is really what is holding up the steel. The basement cost probably 3 times what the stick built box on top cost. However, it's temperate. That's worth a lot.

They offered to clear span it, but I just didn't want to. The bar trusses are cheap, really, and they take a bit of depth. If you have room for them I think you'd be happy with the cost.

This is my shop here:
http://www.firebirdtransamparts.com/hogheadgarage/joesgarage/joesgarage.htm

 

[finn]

Lol, the vette museum had a giant sink for under it, which is impossible to predict.


Sent from my iPhone using Tapatalk

Makes an interesting story, though!

 

[BombShelter]

Lot's of garages have rooms below them, they mostly use a product called Spancrete. You may have seen it if you've ever had underground parking in your apartment building, there're seams every three feet. You pour another few inches of concrete on top of it.

Under-garage rooms tend to have water issues, if you go this route make sure you seal the Spancrete/wall Seam very well, go overboard if possible any place where it's near or below grade. The front where the apron meets the garage floor tends to separate over time, this area will also let water flow through if it's not been waterproofed properly during construction and it can require some major work to fix after it's been built.

https://www.spancrete.com/

 

[The Cobbler]

When I lived in the country our house had the water cistern under the garage.it was walkable when empty standing upright. there were 2 concrete pillars in the center about 20" diameter if I recall. concrete was 6+ inches thick. I'm sure it was engineered.

 

[Chris705]

Haugy - As an alternative to the lower level situation you describe... I opted to fill in the grade difference. Then I created a full walk up ‘attic’. I have the 40’ depth and was able to span that with 2x12’s. I used a large center steel beam to support the attic floor joists & the rafters are 2x12 @ 16” oc. I struggled mightily with the basement decision and it proved too costly for me. That is when I figured rather than trusses I will use conventional rafters and get some additional space though not able to get a vehicle up there. I am slowly configuring to my wood shop. I set it up with very large dormers allowing for natural light and ventilation via windows. Difficult to get cross venting in the walkout setup. What ever way you go best of luck on your build- best decision I have made in a long time!

 

[ConCretin]

To estimate the cost of the various floor systems you almost need a design for each that maximizes it's efficiency. This obviously isn't practical and if you don't have construction and estimating experience it will be difficult to determine which is most cost effective. If you got lucky, you might find a contractor willing to do this exercise.

Given the size of your garage, you will need intermediate structural steel supports since none of the systems we are talking about will span the entire width. This will obviously require structural steel and equipment to set it.

Slab on metal deck will require the most structural steel due to the need for shorter spans but the decking itself is the least expensive option. You obviously have to place a slab on top that is usually reinforced with WWF.

An ICF product will require less steel and can accommodate surprisingly long spans. The product itself is more expensive but easy to install. The slab requires rebar in the integral beams and shoring during placement.

Precast plank will require the least steel but is probably the most expensive product. It requires more crane time to install and requires grouting between the planks and a topping slab to level the floor surface.

One advantage to plank the others don't have is you can install a membrane under the topping slab to provide a waterproof system. Slab on deck and ICF will need a surface applied traffic bearing type waterproofing.

All of this is complicated by the actual size of your garage which will limit your ability to maximize spans. If you have to divide a span in two, you need a beam regardless of whether your system can span farther than the bay that results. The only delta in that case is beam size.

These are obviously generalities but you get the idea. You might just have to ask around and rely on the opinions of others to come up with your best option and a budget. Hope that helps you get started.

 

[Quick240]

I have a garage like this using 8" precast hollow core slabs. My garage is a 36' wide x 24' deep, 3 car with about 15' of that width, extending all the way back another 24' to the back of the house (basically a big sideways L).

The extra costs involved:

- The slabs themselves
- Thickening the front and back walls from 8" to 12" (for extra strength and to create a 4" reverse brick ledge for the slabs to sit on)
- Changing the footings from 8" x 16" to 12" x 24"
- Extra rebar (probably about double the normal amount)
- A 12" tall steel beam to span that 15' wide area where the "L" is
- Crane service for placing the slabs
- Grouting between the slabs (5000 psi concrete)
- The liquid waterproofing membrane applied on top of the grouted slabs
- The 3" concrete topping layer poured on top of the membrane
- The extra single car garage door at the back

I'm not counting the 4" concrete floor in the newly formed "under garage" as you'd be buying one of those for the upper garage anyway, if you didn't do this.

 

[Quick240]

Oh, one other thing. You mentioned wanting an 8' back garage door. I have 10' tall foundation walls, and was hoping to get an 8' tall door at the back too, but I ended up with a 7' tall door instead. The reason being is because the "reverse brick ledge" I mentioned was set 12" down from the top of the foundation wall -- that's to give me 8" of space for the slab + 3" for the topping, leaving me with a baby 1" stem wall for the upper garage. Then just below that brick ledge, they form a sort of rebar skeleton for a lintel across the top of that back garage door. So with all that, there simply wasn't enough height left for an 8' tall door in the "under garage".

 

[KEH]

FIL did this for his one car garage years ago. He left a square hole in the floor so he could access the car's oil pan for oil changes.

KEH

 

[haugy]

Haugy - As an alternative to the lower level situation you describe... I opted to fill in the grade difference. Then I created a full walk up ‘attic’. I have the 40’ depth and was able to span that with 2x12’s. I used a large center steel beam to support the attic floor joists & the rafters are 2x12 @ 16” oc. I struggled mightily with the basement decision and it proved too costly for me. That is when I figured rather than trusses I will use conventional rafters and get some additional space though not able to get a vehicle up there. I am slowly configuring to my wood shop. I set it up with very large dormers allowing for natural light and ventilation via windows. Difficult to get cross venting in the walkout setup. What ever way you go best of luck on your build- best decision I have made in a long time!

How big is your shop? I'm running into issues with guys saying they don't want to backfill this much against a concrete wall. Thus the basement ideas.

To estimate the cost of the various floor systems you almost need a design for each that maximizes it's efficiency. This obviously isn't practical and if you don't have construction and estimating experience it will be difficult to determine which is most cost effective. If you got lucky, you might find a contractor willing to do this exercise.

Given the size of your garage, you will need intermediate structural steel supports since none of the systems we are talking about will span the entire width. This will obviously require structural steel and equipment to set it.

Slab on metal deck will require the most structural steel due to the need for shorter spans but the decking itself is the least expensive option. You obviously have to place a slab on top that is usually reinforced with WWF.

An ICF product will require less steel and can accommodate surprisingly long spans. The product itself is more expensive but easy to install. The slab requires rebar in the integral beams and shoring during placement.

Precast plank will require the least steel but is probably the most expensive product. It requires more crane time to install and requires grouting between the planks and a topping slab to level the floor surface.

One advantage to plank the others don't have is you can install a membrane under the topping slab to provide a waterproof system. Slab on deck and ICF will need a surface applied traffic bearing type waterproofing.

All of this is complicated by the actual size of your garage which will limit your ability to maximize spans. If you have to divide a span in two, you need a beam regardless of whether your system can span farther than the bay that results. The only delta in that case is beam size.

These are obviously generalities but you get the idea. You might just have to ask around and rely on the opinions of others to come up with your best option and a budget. Hope that helps you get started.

Good lord. I'm familiar with building and shops and such, but this is making my head spin. Plus I don't know where to even start.

Two excavators guys have said they don't want this project. Too much liability. I'm asking them to scrape, I then have a concrete retaining wall built, and they backfill with soil and gravel for a concrete pad. They both said that because it's over 6' in height, and it's sheer size that there is too much dirt load against the wall that would be there. They would have to be heavily engineered, placed deep, and be very thick before they would even consider taking the job and backfilling it. They both said it will fail, just a matter of time, and don't want the liability.

So that's why I'm looking at doing this, but holy ****, this is a lot to process. i've reached out to a few hollow core companies to see if I can get some details of what would be needed, but nothing yet.

I don't even know how to find an engineer for this kind of ****. Do I call the county or something for a recommendation?

 

[andyvh1959]

Buddy of mine years ago bought a house on auction, cheap, for like $8K. Tiny one car garage. He worked as an excavator operator so he brought one home, demo'd the garage and dug a extension for the basement. Got the walls poured and then capped it with Spancrete sections. Then he built the 2+ car garage above attached to the house. Above the garage he built a master bedroom suite. In part of the spancrete floor he left a hole through which he could winch a motorcycle down to the basement shop. He had a steel panel that covered the hole to park a car over the access hole. Worked great.

 

[Innovate1]

I have an 8 x 18' porch with a storm shelter under it. Used "spancrete" or hollow core or whatever you want to call it. Put a membrane over them and poured the porch. Have a precast company nearby and it was very reasonable 15 years ago. But looked into it recently and the prices have gotten crazy. Lots of garages here with rooms underneath with precast used for the floor. Some are on slopes so have access OHDs for both levels.

 

[pmiranda]

Sounds like spancrete is pre-engineered for a given weight load and horizontal load? If so they should be able to give you tables for what it can handle, although if you're not comfortable wading through the residential code and assuming all liability if something goes wrong, you should bite the bullet and hire an architect to do this for you. They will know how to translate your intended use into the engineering requirements, and they'll know engineers to crunch the numbers (unless they themselves happen to be an architectural engineer).
It can absolutely be done, but you don't want to depend on a guy that happens to own heavy equipment to do the math. Hire somebody that is qualified.

 

[firebirdparts]

You might be very entertained by this story about exactly what you're trying to do.
http://www.firebirdtransamparts.com/adventure/garage project/garage.htm

You're talking to smart people, and I admire that. They're right, it's an enormous engineering requirement. Based on the experience above, not everybody is afraid. They did it with blocks, which is insane. I told Brian that unless you use 8" blocks, there is no corner created in there, but they didn't care what I thought.

On the second try, they filled the wall and bent rebar over into the slab. It's pretty solid today.

It's pretty difficult to hold back earth. If you want to hold it back with enough stability for a building, that is especially hard.

 

[LS6 Tommy]

One of Mrs. LS6's old coworkers did that when they redid their house. The owner's "mancave", bar and indoor pistol range (you read it right) were under the new garage. IIRC, it was $50k just in reinforced concrete.

Tommy

 

[BD1]

Precast concrete panels as stated is best. Same as in a parking garage.


Sent from my iPhone using Tapatalk

 

[joe--h]

Knew a guy who had a basement under his, he's been in prison about 17 years now.

Joe H

 

[a6hux]

HollowCore!

 

[GooberMcNutly]

I've got a double-wide, double-deep garage built over a single bonus room on a walkout house. No columns or beams, just hollow core. The previous owners built it as a hurricane shelter, but I use it as my wood shop. 26'x54' and exactly 1 plug in it, attached to the light breaker... Sigh...

 

[peterlawl84]

We had this done when we built our house. Its been pretty common here, our builder called it a suspended slab garage. Our house has a 3 car garage on the front, then a side driveway that goes to the back side with a garage door to park additional vehicles in the "bottom" garage. It has the same floor space as the above garage, but the opening is the size of a single garage. Our neighbor did the same thing but turned part of his bottom garage into a theater room and left the 3rd bay for storage. He had to sign a waiver to the builder to promise not to sue if it was not water tight. I haven't had any issues with water in mine over the last 4 years.

 

[Fcvapor05]

Determining the most cost effective system can be a challenge because of all the variables associated with beam sizes, spans, hoisting, contractor experience, distance to mfg, etc.

Determining what system most efficiently meets the need is precisely what an engineer will do. They wiill also ensure that your design doesn't kill someone 1 year or 25 years down the road.

Designing efficient, safe, water tight multi-level structures is not a trivial endeavor. Pay a professional to do it for you.

 

[ConCretin]

Determining what system most efficiently meets the need is precisely what an engineer will do. They wiill also ensure that your design doesn't kill someone 1 year or 25 years down the road.

Designing efficient, safe, water tight multi-level structures is not a trivial endeavor. Pay a professional to do it for you.

The OP acknowledged that an engineer would be required in his opening post.

 

[pbon]

What about steel beams? You are essentially building a mini parking garage. You either need internal supports or steel for span the size the OP wants. Internal supports break up the space. Ideally you round minimize them.

 

[jmarkwolf]

The house I grew up in had a 2.5 car detached garage with a walk-out basement shop. It wasn't a slab, it was wood floor joists and planks, with a single big timber post in the middle of the lower level supporting the floor.

It worked fine and is still standing to this day. You might consider alternatives to the slab and save yourself some trouble and expense.

 

[Northislander]

I did a smaller suspended slab shop a couple years ago here's the build thread i started but unfortunately i got busy with work and a recreational cabin development and never got back to updating. https://www.garagejournal.com/forum/showthread.php?t=374836
PM me if you have any specific questions i might be able to help with. It was an expensive way to build a shop but i am still glad i did it it has made use of a sloping back yard and given me my shop and the wife her art studio. and a large flat patio between the house and studio. All from a sloping yard that all we could do before was mow.

 

[Stuart in MN]

I don't even know how to find an engineer for this kind of ****. Do I call the county or something for a recommendation?

Your state engineering board may have listings available to the public for structural engineers, or you can just google on 'structural engineer nashville' and a number of firms pop up. Finding one that does residential work may be difficult, a lot of companies just don't want to get into home projects - not enough money to be made to be worth it, too much liability - but if you make some calls they will let you know that. If they aren't, hopefully they can refer you to another local firm who would be interested.

 

[pbon]

Architect might have lead in structural engineer.

 

[ConCretin]

Most of the suppliers of the types of floor systems being discussed will have some level of in-house engineering. Maybe start with them and see how much more engineering, if any you need. Despite the dire warnings of impending collapse and widespread destruction, this stuff isn't rocket science. It's done all the time.

Northislander's ICF project is a good example. ICF manufacturers provide tables indicating load bearing capacity info for various spans and configurations. No additional engineering required. If you need stamped drawings for some reason, it's a much simpler job than starting from scratch. Btw, I love ICF floor systems. Nice project Northislander!