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Old 04-28-2013, 12:55 AM   #1
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adding second floor to garage

Pardon me for not using the proper terminology.

I have a 21'x21' 1.5 story garage with a basic two-hip roof that needs to be replaced. It has an older wood shingle roof under a comp, so I have to tear off both of those all the way down to the rafters (or the 1x4 slats) and install sheathing.

Since I'm that far, I'm considering converting its attic into a garage apt but the floor is not strong enough (I think it's 2x8s floor joists and I think it has a "breakback" (?) - a beam running perpendicular to the joist across the top of the floor boards. My thought is to cut out almost all the center rafters except a couple of feet at each end so I don't have to mess with the second floor exterior siding. I would then build a rectangular center section that rises a little higher than the original roof line and extends another 15' or so out the front over the driveway. The resulting floorplan would be like a cross with a thick center and short arms.

Wondering how to support all that second floor. Should I a) work from underneath, in the garage, to build supporting beams under the existing attic floor so that I don't have to mess with the floor joists or floor boards (I'd lay plywood on top to smooth it all out and add strength); or b) tear up the floorboards and install new, wider floor joists alongside the existing joists that meet code for that span.

Since I already have to set columns to support the 15' extended section, I'm thinking that using columns inside the garage as well and tying them all together might be better.

what does IBC say about this? What size joists would I have to install for that span if option b is better? Would engineered joist assemblies be better? Could I just go alongside the existing 2x8s with more 2x8s?

Thanks for any help you can offer.


I already have


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Old 04-28-2013, 08:29 AM   #2
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First thing you need to check is the foundation for the whole thing. You most likely cannot just build on what you have as your thin slab garage floor is probably not sufficient to hold the weight of a finished second story apartment.

You will no doubt need sign-off for new structure so you might as well consult with an architect or home designer or addition specialist sooner rather than later. As I have said many times before, they are not just for the rich and you will find working with them saves you lots of headaches, speeds the permit and inspection processes, and should even save you a few $$$.


Last edited by user1007; 04-28-2013 at 08:32 AM.
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Old 04-28-2013, 08:31 AM   #3
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Pictures and location would help us in answering the question.
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Old 04-28-2013, 08:55 AM   #4
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In addition to what the guys above said....I would avoid will destroy your 'apparent' room downstairs.

You really need an engineer to look at this.

If you want an idea of a 2-story on the garage link in my signature.
Even if you are on the right track, you will still get run over if you just sit there.

My 2-Story Addition Build in Progress Link ... My Garage Build Link and My Jeep Build Link
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Old 04-29-2013, 06:05 PM   #5
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Well, I'm out of town and can't take pics. But I attached a couple of drawings I made.

I understood that the slab would have to be tested for that weight, which it would almost certainly fail. Which means I'd have to pour a whole new slab, which basically means tearing down the entire garage and starting from scratch, which is what you did, Dawg. B I don't want to spend that kind of money, and the garage is actually in very nice shape.

Hoping I can just cut squares out of the slab and my driveway and pour new footings for each column to save the existing building. As I said, I would have to do this anyway to support the new addition that extends out over the drive, and I want to build a 2nd story deck connecting this to the house, so that would have to have columns supporting it. Seems easiest and cheapest to just throw some more columns in the garage to support the existing second floor and add stronger floor joists/more joists/ engineered joists -whatever it takes.
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adding second floor to garage-lot-plan.jpg   adding second floor to garage-garage-side-view.jpg  
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Old 04-29-2013, 06:14 PM   #6
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Don't think you are going to get away with pilings for the foundation for a two story structure (maybe a deck) but I could be wrong. Seems like even if they let you, you will end up losing all useable garage space with a bunch of posts in the way?

Your floor support for the second story is not your first concern. Gotta see what your foundation has to be first before planning further? Planning from the roof down seldom works out. You could build the strongest and best floor supported second story on the planet but ultimately if there is not adequate foundation under it the weight has to fall somewhere and with gravity, that usually is down as I remember.

As mentioned before, you are going to need stamps and sign-offs from somebody licensed for structure for this to get permits. Why not involve a designer or architect sooner rather than later so you do not beat yourself up heading down paths to nowhere? You will enjoy the experience and it will save you, I promise!

Just a nit picky suggestion. You might want to modify language like "just throw some more columns in the garage" before talking with the powers that be and that handle permits and inspections. I am guessing you really do want to think this through and not build so casually? You would hate for someone to get hurt up in the new addition?

Last edited by user1007; 04-29-2013 at 06:57 PM.
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Old 04-29-2013, 06:14 PM   #7
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oops forgot to label the red Xs and lines as the columns.
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Old 04-29-2013, 06:24 PM   #8
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Retrofitting an existing structure to add a second floor can be surprisingly tricky, especially if you want to comply with code (you do want to comply with code, right?) I have worked on several projects involving adding a second story on to an existing structure. Some of the issues that needed to be addressed included:

1. Foundation capacity. As noted previously, a garage slab is typically not suitable, and it can be difficult to install a proper column footing at a reasonable cost, not to mention the columns get in the way of your project.

2. Zoning issues, and planning board issues. You typically need a permit, unless you live in an uncontrolled county. Usually need architectural plans before you can pull your permit, sometimes engineered plans with a stamp as well.

3. Egress issues and room space. For an apartment, you need to have minimum floor area, windows, doors, fire escape. In a garage, there are often special regulations due to carbon monoxide etc.

4. Framing details. Often less obvious than you would like. You want the addition to look like it fits the house, not some kind of scab. That is what architects and home designers are trained to do.

There are other issues, but you get the point. Asking an internet chat room forum to design your project sight unseen is two bus stops beyond the end of the line. As others have suggested, I recommend hiring a designer to assist in the process, a forum like this is good for specific questions regarding means, methods and techniques, but not really much of a substitute for hands on assistance from a design professional.


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