[Mr Chips]

I have a house with a hip roof. my garage is a 20x20 2 car attached. there is a bedroom and half bath behind it, and even though it is attached it kinda has it's own roof.

there is a HUGE space above the garage that i really want to use. it looks to my novice eye that the builder meant for it to be used, since directly above the garage they used 2 x 10's set 16"OC, and stepped it down to 2 x 6's above the back room

See picts below. The first pict is looking toward the side exterior wall ( the rear of the house is to the left, the front of garage is toward right)

the distance from the knee-wall to were the joists step down to 2 x6 is about 15'. There is about 77-1/2" inches of height between the top of the 2 x 10's and the bottom of the parralel rafters

Second pict shows the unisulated floor area is over garage, the area with the loose cellulose insulation on the right is the back bedroom. The area behind the fiberglass insulated wall is the master beedroom

what I'd really like to do is put a door in from the master bedroom, and frame in a a walk-in closet that would go about 7' deep and 15' wide and then a seperate "rough" storage area in the remaining 13'x15' feet

any thoughts? concerns? Suggestions

The opening appears to be have a 54" roughed-in opening for a pull down stairs

 

[Scuba_Dave]

A knee wall one one side too - seems like it was prepped for finishing. Depending upon the wood a 2x10 will span around 16'

What is the total span of the 2x10 without support below?

It looks like the joists run perpindicular to the garage doors & span 20' ?

 

[Mr Chips]

What is the total span of the 2x10 without support below?

It looks like the joists run perpindicular to the garage doors & span 20' ?

Yes, the 2x10's do run perp to the garage door. It's a 20' run from the back wall of the garage to the front, and there are no columns or any type of additional support in the garage below. That's what had me a little nervous

 

[Scuba_Dave]

Yeah, even 12" on center w/structural 2x10's you won't get 20'
I built a walk-up attic floor (possible future bedroom) & I was unsure of 2x12's - even tho they would "work" at just under 19'
Instead I had I-beams sized, & then went to the next stronger size just to make sure

I think you would need some sort of support under this to use it
Possibly a beam under the joists could support it
My local lumber company sized beams for me
I ended up with 16" LVL's in my garage to support the floor above

 

[Mr Chips]

That's what I was afraid you were going to say!! My original thought was to put a beam across the center of the garage, parallel to the door with a post on each side. looks like i'm heading to the lumber yard.... I was hoping this one was going to be an easy one

 

[jogr]

As you can see by the attached span chart your 2x10s (depending on species, grade and actual span) are correct for supporting the garage ceiling drywall, limited storage and no future rooms (20 psf live load section) but wouldn't be correct for adding a walk-in closet and more extensive storage. Since you want to only use this for closet and storage you could use the 30psf live/10 psf dead load chart unless your local codes require more. If 30 psf live load is ok, and if you select the right species and grade of lumber, you could also sister in 2x12s if that would be easier than a beam. Another possibility is adding and additional 2x10's between each existing one to get 8" oc spacing. Depending on the grade/species and actual span that might get you the load rating you need.

http://www.orgsites.com/ct/strober/LumberSpansJan31-2003.pdf (this chart is for canadian species, you can find many other span charts online that should cover whatever your lumber is)

 

[Mr Chips]

i thought about sistering, but unless i am missing something it seems to me that a beam across the center with a post at either end sounds a lot easier. my garage ceiling is 10' 4" high, so i don't mind giving up a foot or so of height across the top.

do you agree that a beam is the better choice? what are the pros and cons of sistering vs single 20' beam running perpendicular across garage ceiling with a post at either end

 

[jogr]

If you've got the headroom then the beam might be easier if you are comfortable working with the weight. A beam will be pretty heavy (a steel beam of the right size might weigh as much as 800 lbs for your application) but since you are working in an open garage it could be handled pretty well with a couple genie lifts. You'll want to be sure the footing where the posts are going is adequate.

The pros for sistering are lighter weight components and you can do only the area you want to use. The cons are trying to get the 20' joists in the attic and placed, a lot more components, dealing with wires, moving insulation and in the end the floor will probably have more flex (bounce) because of the long span (but for closet/storage you probably won't notice the flex)

You've got such a nice big open space up there it might be worthwhile seeing if you can raise those collar ties another foot and a half higher so you can have 8' ceilings up there. Your going to need to talk to an engineer to size the beam anyway so bring him some good detailed drawings and ask him to detail out how you can raise the collar ties to get 8' ceilings.

 

[Scuba_Dave]

The LVL's I used to span 24' were just under 200 lbs each
It was sized to use (3) 14" beams to support the floor above, the attic wall above that, then the roof load above that
I went with 16" LVL's for added strength
Once in place the (3) beams are screwed together
I was building the garage & installed these myself 1 by 1

Since you have less distance & less support needed 14" beams may work or even 12" beams. I would have the beams sized so that you could use the whole area.
My local lumber company sized the beams for free
They then sent the sizing plans out for an engineer stamp - also for free

The beam would run straight down the middle of the garage
If you can stand having a post around the 1/2 way point then I imagine 12" LVL's would be all that you need

 

[Mr Chips]

You'll want to be sure the footing where the posts are going is adequate.

So the obvious question is " How do I know if the footings are adequate?"

It's a poured concrete garage floor. I have no idea how thick the actual floor is, but the concrete does come up about a foot high all around the perimeter of the garage. I have no idea if that's important to know, but my hope is that will work into my favor some how!!

I was wondering if , rather than cut the concrete and dig down a foot or two and pour concrete back up to the floor ( which is what I am guessing is involved), Could I just set the posts on top of a thick steel plate, about 18" wide. My thinking is this would spread the load across more surface area.

Another thing that I am wondering about regarding the columns, is the dead center of the garage is the most convienent ( and probably best place) to put the beam, but that's also where the floor expansion joint is. will setting columns on top of ( or cutting away and pouring footings) where the expansion joint is be problematic?