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Structural Garage Floors

Posted by johnstaci (My Page) on
Mon, Jan 21, 08 at 17:53

A large number of new homes in my area are being constructed with structural floors (ie the full size of the garage is added to the basement below). I believe they are also called suspended garage floors. This is becoming very popular. Typically metal forms (looks similar to corregated sheet metal but much thicker)are set on the support beams and the concrete and steel placed above that. The cost per sqare foot is incredibly low - somewhere around $10 per square foot for added room to the basement and/or more garage room if you have a walkout. I have a walkout basement and will add a garage door opening to the back underneath the main garage. In essence, my 3 car garage will become a 6 car garage and I won't need to construct a costly out-building.

Question - I've heard pre-cast, pre-stressed concrete beams (with a thin concrete surfacing layer) are also being used instead of the corugated steel. Does anyone have any experience with either? What are the pros cons to each? I want the biggest span possible in the area beneath - just curious what others have.

Thanks, John


Follow-Up Postings:

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RE: Structural Garage Floors

I liked the idea years ago.

But when I asked the architect about it, he dissuaded me with a discussion of the difficulties of providing long term protection for any structural steel. Instead, I see a growing number of lift systems within an existing at-grade garage. A garage over a walkout basement would certainly seem attractive.

I don't know your costs. But here, the $10 sf. is not accurate. Just providing a proper floor in the under-garage space--5 in. gravel, wire mesh and 4 in. of concrete--would cost me more than that. And if the below-grade space is for a garage, you'd have to provide for a drain.

Besides an engineered plan for the supported deck, you should provide for waterproofing.

Instead of enlarging the basement under the garage, I've seen a few builder/renovators make the basement bigger than the upstairs by going under concrete back porches and even under the yard. The first time you walk in one of those homes, it's very disorienting.


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RE: Structural Garage Floors

Such a slab is called a "structural slab" instead of a "slab-on-grade".

The poured concrete on metal decking is called a composite system because steel studs are electrically welded to the top of the steel beams through the ribbed metal decking. When the concrete is poured it acts in a composite manner making the beams stronger. The ribbing of the metal decking allows the least amount of concrete to be used and it has deformations along the ribs to help the two materials act together structurally. There is no more efficient or lighter weight system possible that meets the code requirement for a concrete floor in a garage.

The metal decking usually doesn't span much more than about 10 feet so there would need to be at least one intermediate beam under a 2-car garage floor. However, that steel beam could be designed to be deep enough to span the entire garage width but at considerable additional cost.

The kind of pre-cast concrete planks that are extruded with round cores running lengthwise will also span across a similar steel frame but they have to be about twice as thick and add quite a bit to the weight of the structure. I can't think of a benefit to this option except perhaps achieving a fire-resistance rating without having to spray fire-protection on the bottom of the deck but the composite concrete and metal deck garage floors I have designed were thick enough to achieve the required rating without spray-on protection. Columns and beams are usually required to have two layers of fire-resistance rated gypsum board. Precast concrete that can span the entire garage would have a single or double T shape and take up a lot of headroom below.


Either system must be designed by a structural engineer and you could consult one for more about the systems and which is more appropriate for your situation.

Some jurisdictions restrict the use of a space below a garage because of the potential for combustibles collecting in a lower enclosed space. For code compliance it would be a good idea to involve an architect unless the builder or engineer have already worked these issues out with the local building officials.

I would not consider the space created to be cheap. It would cost considerably more than $10/s.f.


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RE: Structural Garage Floors

Around the Kansas City area, I'd say nearly half of the new homes I've been in over $250k have the structural garage which adds a ton of space to the basement. I just walked through a model home yesterday that has the same setup as me - a garage door leading out the walkout creating a 2nd full 3 car garage. Around here the cost for sure is less than $20/sf - remember this is unfinished. I'm pretty sure a couple people have told me around $10.

In any case, I would be curious to hear from anyone else that has opinions or experience on this.


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RE: Structural Garage Floors

John, I am in the midst of a project just as you describe. Mightyanvil described two options. A third is a product called "Lite Deck" which is similiar to ICF. The forms are foam and have to have to be braced for 21 days while the concrete topper cures. It was estimated at $20 per sqft by company out of Ohio.

I went the prestressed plank route. I didn't want to pay the litedeck price and I didn't want to deal with steel and posts in the basement. With concrete planks I could span the 24 ft depth of the garage. The planks themselves cost $10 per sq ft with tax and trucking. To that you have to add the extra cost of additional foundation work and the floor slab below as well as crane time to set them. The plans then have to be topped with 2 inches of concrete minumum. I haven't gotten that far yet. My walls needed to be 8 inch ICF to bear all the weight of the floor and the gargage loads. The planks are 6000 pounds each.

This was a non-trivial decision but made lot of sense in my case due to my site grade.

You mention three lower level garage doors. The one garage door I put in was the biggest issue for my floor system. There is a great deal of weight which needs to be carried across that header. We ended up with six #7 rebars and had to keep the door height to seven feet to allow two feet of concrete over the opening. When we frame the garage we are going to header over that opening in the wall above so that the roof loads are carried on the solid foundation and not that opening. A structural engineer is definitely a requirement.

You may also run into a some issues trying to deal with the prestressed concrete folks. Some don't respond to residential inquiries. I found my supplier via the Prestressed Concrete Institute website (www.pci.org). I used Nitterhouse Concrete Products in PA and they were an absolute joy to work with. There were closer companies but most didn't want to bother with my little 24x40 ft job.

Here are some pictures of the job:
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This one shows the planks from the side. There is one more course of ICF blocks that went on after the planks were set. It forms the curb and locks the planks to the walls. You can also see long rebar rods stick up from the wall. Those get bent into the joint between the planks and get grouted in to tie the floors to the walls.
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Here is an interior shot. Looks like a parking garage:
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I was planning on writing this up once I finished the entire garage as I hadn't seen anyone on Gardenweb discussing this before. At this point I'm in a weather hold waiting for a good chance to pour more concrete. Hopefully that will happen before my carpenter becomes available.

Klabio


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RE: Structural Garage Floors

mightyanvil- Thanks for the good info.

worthy- I don't believe any more gravel would be req'd.

klabio- Great pics/info. I'm new to the forum and am surprised there hasn't been much discussion on this topic. It is really becoming common in my area. I will likely price out the standard steel decking and the option you chose. Concerning the garage doors, I will only have one on the lower level. The lower level will be the size of a 3 car garage but will only have one garage door opening to the walkout. Please post more pics if you have them. I like your option best to avoid the 3/4 vertical beams. I will price out both - maybe along with the 3rd option you mentioned.

Thanks, John


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RE: Structural Garage Floors

Talked to a realtor in a model home office over the weekend. They put in spec homes 250-350k - all have 3 car garages. He said they add $10k for the costs for adding a structrual slab (a 2nd 3 car garage area below the main 3 car garage). Assuming around 700 sq ft for a 3 car garage, the added cost would be around $14 per sq foot.


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RE: Structural Garage Floors

Great info and pics!

worthy- I don't believe any more gravel would be req'd.

I'm referring to the basic slab in the underground space. If you add space for a garage/basement, you have to figure in the cost of finishing that space. It's not just the cost of the structural slab above your head. Even excavating deeper than usual under the garage and trucking that fill will cost me about $1,500.

As a spec builder, I still see more benefit upgrading the aboveground living space than adding 400 sf to the basement. An individual homeowner uses a different calculus.


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RE: Structural Garage Floors

I never heard of it refered to it as a "structural garage", but I have one. Our home is 22 years old and we purchased it 15 years ago. The "lowever level garage" was one of the things that we really liked about the house. (Still do!)

When we first moved in I called it The Bunker because it's where we'd seek shelter from tornados. Having storage below for snowmobiles & pool equipment, etc. is so handy and keeps the "upstairs garage" quite organized.

Our garage is like klaibo's...pre-stressed concrete. We also just have the smaller garage door opening, although the size is the same as our upper 2 1/2 car garage.

Here is a link that might be useful: our


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RE: Structural Garage Floors

Great thread. I was thinking of doing this on my house depending on cost.

Worthy - I see your point, but for those who aren't building in BFI, an extra 400 sq ft or so added to your living space is huge.


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RE: Structural Garage Floors

BFI? That sounds like shorthand for something an ex used to call me!


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RE: Structural Garage Floors

Since this thread has resurfaced I figured I'd update the pictures to show how things turned out. I'm 90% complete at this point which is code for I still have a long way to go before it is "finished" finished.
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RE: Structural Garage Floors

klaibo, looks wonderful! I'm dying to know what your view is like when you look out the windows on that side. And what is above your garage? Looks like you have lots of room.


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RE: Structural Garage Floors

Very nicely done!


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RE: Structural Garage Floors

Currently designing a 30 by 45 foot garage with 17 foot walls so I can put a basketball court below the garage. Cost for hollow core concrete floor plank is 13800 and 9000 to deliver and install. Currently working with engineer to design walls to support the weight and height. Thanks for the great info and pictures of your project.


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RE: Structural Garage Floors

Has anyone installed floor drains in the structural slab above? One contractor told me not to do it because cracks will form at the edge of the drain and leak though the concrete? I'm thinking about sloping everything to the garage doors.


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RE: Structural Garage Floors

Currently designing a 30 by 45 foot garage with 17 foot walls so I can put a basketball court below the garage. Take some pics. I'd love to see that!


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RE: Structural Garage Floors

I know this is an older post, but I'm new to the site and would like to share some pics of my build that has an upper/lower garage.











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RE: Structural Garage Floors

Looks good!

On steeply sloping lots the two levels make a lot of sense.


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RE: Structural Garage Floors

I am planning on building a 4 car garage with 22 foot walls for a basketball court below. Any advice? I will take detailed pictures and post everything I learn through the process. What is the best type of span deck to use? Is it an engineering nightmare to go so deep? How did that 17 foot deep garage turn out? -Ryan


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RE: Structural Garage Floors

I recently purchased a house with a suspended garage floor with basement underneath. i have a problem with snow melt water from the car running towards the kitchen access door in garage and water going to flo0r edge and dripping in the basement. Can I install floor drains in the garage floor to fix the problem?


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RE: Structural Garage Floors

Klabio,

Your garage looks very nice.

How much did you garage end up costing you to build? I am thinking about building either a two or three car garage next to my house with a finished basement. Also, I want to have an underground tunnel from one basement to the other.

Thanks,
Steve


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RE: Structural Garage Floors

I am also thinking of bulding a garage with another one below it. I am also going to put a 1 BR aprartment above the top garage.

I have contractor pricing for a single story, 1+ car garage (20' wode x 24' deep) with a 1 BR aprt above it for $50K.

Any clue as to what the premium woudl be to "insert" the second garage between these two? I can access the lower garage from main drive way. The to garage woudl be accessed by the road runnign behind my property which is elevated some 10' above my driveway. Without the secons "top" garage, the apartment woudl be about level with the rear elevated road.


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RE: Structural Garage Floors

I am a new member and while I have used the amazing information on these forums for years for reliable information on everything from recipes, gardening, animals, and remodeling, the information in this thread finally made me join.

I wonder if any of the previous posters in the thread have more pictures or updates about their projects. I am considering designing something like this as an addition to my house as I have a wonderful house, but would like a very modern, well-designed garage that would house my truck, my tools, toys - bikes, camping equipment - and also protect the firewood I use to heat the house and maybe be a space to shelter the ducks I want to add to our menagerie.

The structural garage floor sounds incredibly practical, and I have been pleasantly surprised by the cost information I have found here.

I've looked at products like the Studio-Shed, which offers a nice prefab garage in the style I like, but what you are talking about here might even be better.

I hope people are still following this thread, or that others will come along with similar interests or projects underway. I am especially interested right now in cost for the basic structure, how the steel and concrete is holding up, and seeing pictures.

Thank you for being such a great, informative community!


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RE: Structural Garage Floors

I am also interested in seeing how some of the other finished designs have held up. Any lessons learned here?

New pictures woudl be great addition.

Thanks for all of the info thus far.


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RE: Structural Garage Floors

Hello,
My name is Sean and I just found this forum while searching for my next BIG project.

I saw some numbers between $10 and $20 a square foot for the expansion under the garage. Is that still accurate?

I assume that is for "new construction".
Does anyone have an estimate for adding this on to an existing house?

We have a 20*22 garage and we will have to re-pour the garage foundation anyway because we are planning a second story expansion over the garage space.
I figure that while we have the crew present doing the expansion, we might as well expand the basement too?

For my build the garage is attached, so we will need to cut a hall way in between the two garages.
I say hall way because the existing foundation for the house is 1 foot, and the new foundation for the garage space will also need to be 1 foot. So I'm expecting a 2 foot deep hall way, maybe double door width?

If the entire project is closer to $50k than $100k it's a slam dunk!
The highest estimate I've seen on this thread so far would only be a $9000.00 adder to do the basement? It cant be that cheap can it?


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RE: Structural Garage Floors

We're grappling with the same question on our new build. The original plan was for the garage to be slab-on-grade, however due to site complications we had to bring the garage foundation walls (along with the rest of the house foundation) all the way down to sit on deeper ledge. So they wound up excavating out the entire garage, and now it's a massive glaring hole (opportunity?!). At it's deepest points the hole is 13-14' deep! Over 450 sq ft. Amazing storage potential but I'm really thinking future movie theater...

I'm in the process of getting a quote to do a structural floor (install steel beams, galvanized decking & reinforced slab) as well as cut out for access door and pockets to put the beams in. It's unfortunate this wasn't all planned from the start, but so far this project has been like the "Mr. Magoo" of house builds. Lots of mismangement and out-of-sequence steps. Like, I took the trouble to put up all this exterior waterproofing, and now (if I'm lucky), this space will all be on the inside. This happened because the garage foundation was dug out months after the rest of the house foundation was dug out & complete. So we just figured out the garage would be this deep now. Why? Don't ask me!!

I'll report back on added cost once I hear back from the builder. Keep your fingers crossed for me it's not astronomical... getting this added space into the house would make up for a lot of hurt we've felt througout this project.


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RE: Structural Garage Floors

Ok, so good news is we're going to do it. The builder came back with some really high quote ($17k++) for labor and material to do what's needed (including concrete cutting for door, hvac, and beam pockets). I spent a day making phone calls and lined up people and materials/steel to do it for $11k. Pays to be your own GC sometimes!! For 450 sq ft $11k is worth it. Hopefully it goes ok. I'll post pictures.


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RE: Structural Garage Floors

I only know 2 people who went the structural/precast concrete floors and both complain that in the winter the snow melts off their vehicles and drips to the room underneath. They both have tried sealing it many times but the leaks have always came back.

Most builds around here use concrete for the upper garage floor.


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RE: Structural Garage Floors

I owe some pictures of our progress, our beams and deck are in, and about to start rebar work. Then 5000psi concrete slab install next week.

However before the slab is poured I'm trying to figure out what, if anything, I should be doing to try and waterproof this assembly. Was thinking of sealing the edges where the pan meets the foundation wall with strips of roofing membrane (so that the concrete seals against it). I could maybe stick the strips in place with a spray adhesive.

Unless anyone has other ideas?!


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RE: Structural Garage Floors

I owe some pictures of our progress, our beams and deck are in, and about to start rebar work. Then 5000psi concrete slab install next week.

However before the slab is poured I'm trying to figure out what, if anything, I should be doing to try and waterproof this assembly. Was thinking of sealing the edges where the pan meets the foundation wall with strips of roofing membrane (so that the concrete seals against it). I could maybe stick the strips in place with a spray adhesive.

Unless anyone has other ideas?!


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