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Here is my question folks, I have a single story home with a lot of room in the attic. I'm pretty sure my ceiling is capable of supporting a 2nd level, and I'm ready to start looking into it, but before I want to have a 2nd opinion. The roof of my garage (floor in the attic) is made of 2x12s spanning 25ft. and they are on 12" centers. On one end they sit on top of the wall coming into the house from the garage, on the other they sit above the garage door and walk in door from outside. I cant see it but surly there is a huge beam going across the door to support the floor upstairs. Here is a rough drawing that I did in paint to show you guys how it is laid out. Let me know if you think that the structure is sufficient enough to support a 2nd level.

 

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Also important to to know what you have for a foundation.
Adding your location to your profile may help with some local code issues.
 

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Framing Contractor
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Here is my question folks, I have a single story home with a lot of room in the attic. I'm pretty sure my ceiling is capable of supporting a 2nd level, and I'm ready to start looking into it, but before I want to have a 2nd opinion. The roof of my garage (floor in the attic) is made of 2x12s spanning 25ft. and they are on 12" centers. On one end they sit on top of the wall coming into the house from the garage, on the other they sit above the garage door and walk in door from outside. I cant see it but surly there is a huge beam going across the door to support the floor upstairs. Here is a rough drawing that I did in paint to show you guys how it is laid out. Let me know if you think that the structure is sufficient enough to support a 2nd level.
What makes you think your ceiling in the house can support a second floor?

2x12's @ 12" oc spanning 25' cannot be used as a floor. It doesn't matter what the header is above the garage door.
 

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while they cant span 25' they should have no trouble spanning 12.5ft (by adding an appropriate beam spanning perpendicular in the center of the span). you need basic engineering work that should run on the order of about 500 bucks in most places. they will spec out the beam (a wood beam may be very tall, taller than the 2x12's so you might have to drop it into the garage and do posts on the sides....not a big deal for a garage...steel would definitely be able to be hidden) they will also assess if you need to pour footers. the slab in your house is likely only about 4-6 inches thick, load bearing walls will have either footers (grade beams) or piers or who knows what.... can you find out if foundation plans were submitted when the house was built? in some cases its just quicker and easier to pour a footer for the two posts in the garage and call it a day and know its right. anyway, this is something you either want an engineer for, or a pro to spec out the beam/plans and have the inspector approve (if you dont go with the engineer stamping it).
 

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a bunch of times adding a second level to a structure requires examining the existing footing to determine footing strength. Keep in mind that the house was built to hold the size and weight that it is now so the footings should be sized accordingly. Another thing is,was the footings even built correctly when the house was built? Inspectors have been known to look the other way unfortunately, so even a submitted plan may not be what it should...
 

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ohh and you probably need a beam on the garage door side too, i doubt the beam for the opening can carry the live load. you can probably just beam it INSIDE the garage, on the garage door wall, and have the joists cantilever the extra 12-18 inches.....again, an engineer will spec all of that.
 

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room above garage

First of all the garage slab is irrelevant what you need to know is about the exterior foundation/footing 2X12 can be used for floor joists depending on species and grade of lumber This information is from the 2009 IRC span tables for floor joists.[ Chapter 5] If nothing else you can add TJI's [engineered wood structural floor joists] that would be designed for this application Remember the garage foundation will actually only be supporting the second floor.
 

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jermeyfX4 room above garage

I posted a reply but did not see the post so once again
2x12 floor joists are only acceptable with limited restrictions based on species of wood and grading Tables in floor section of 2009 IRC. In the garage there is no floor load at this stage as the slab means little The exterior foundation/footing is important. Your second floor will still only place one floor load on the project as said before the slab does not create a first story load. It could be dirt or gravel for that matter. Simply put you may want to consider TJI's which are wood engineered floor joists simply put wood I beams can Easily be purchased to meet your needs Go online As sail some 2X12 12 " on center will span see structural tables for your species and quality stamp on lumber Need a stamp on lumber or ok from structural engineer
 
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