How to build floor in garage attic?

I want to add storage space above our 2-car garage by building a floor in the garage's attic and adding some drop-down stairs, so it's easy to get to the garage's attic.
Problem is the attic has 2x4 engineered trusses spaced every 2 feet apart, so I would have to add joists to support the weight of the floor, the weight of anyone walking up there, as well as the weight of the boxes and things that I'll be storing up there. I'm planning on using 3/4 or 1 inch plywood for the flooring in the garage attic.
The garage is 22 feet wide (the trusses go this way, so the trusses span 22 feet) and 20 feet deep. Would I need some kind of beam going down the middle to support the additional joists, so they wouldn't have to span the full 22 feet?
What's the easiest way to add and support joists and a floor in a 2-car garage attic? The 2x4 trusses are, of course, resting on the walls of the garage, and there doesn't really appear to be a way to put any taller boards for joists (i.e., 2x8s or 2x10s) between the roof and the top of the garage walls since the only thing that will fit there now is the 2x4 ends of the trusses. It would be nice if I could just add 2x4s to the bottom of the trusses to make those thicker and stronger to support a floor, since they would fit perfectly between the tops of the walls and the roof, but since the trusses are spaced every 24 inches, rather than every 16 inches, as normal joists would be spaced, I'm guessing that won't work.
Has anyone done a similar project in the attic of a 2-car garage, and if so, how did you do it? Any advice or suggestions would be appreciated!!
--
DK



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you need an engineer to design replacements for trusses. it'd be easier to rip the entire roof off and start over.
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Spitzer has a point. What occurs to me is the truss configuration cannot be changed on a whim. That probably doesn't leave much room for storage. TB
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I'm not disagreeing with anyone, just pointing out an experience. I just sold a workshop, concrete block, 28 X 50, with a 4/12 pitch, 1/2" plywood deck, asphalt shingle roof, span on 28' width. Trusses were 2 X 4 on 24" centers, metal splice plates, with the "W" format, using hurricane clips on top plate at attachment. The center chord exposed clear area was 8 feet wide in the middle of the truss. I floored the entire 50' length using 1/2" plywood, and stored everything from books to domestic items to auto parts up there for 23 years. No snow load to talk about here, though, I'm in the South. By the way, there was a 6" monorail I beam mounted under the ceiling by bolting it up through the trusses every 2 feet, using a small timber across the lower chords of the truses, and running 32 feet down the shop. It was supported at both ends by pipe posts, and it was located 8 feet or so out from the wall at the bottom of one of the V location of the trusses, and was used to move equipment and pull auto engines and such. I KNOW that 1,000 pounds would sag the beam 1/16" by measuring before and after. My brother in law has an identical workshop, except with 5/12 pitch, with the same setup. It is always best to be safe, but my experiences have shown properly installed truss roof systems to be very strong. This is presented only as a real world experience. Use your own judgement.
RJ

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What is not known is the design of the system. Yours may have been designed to take a fairly large load. Others may not. If you know the design capability, no problem.
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My shop was built by me when I was 28 years old, and didn't know know what I think I know now. :-) I simply ordered the roof trusses, built the roof, and later added the hoist beam. The truss suppliers only knew the roof pitch, roofing type, and that they would need to supply trusses for a clear span underneath. There's a splice plate in the middle of the lower chord. I contacted them recently about building a new shop, asked about spanning 34 feet, and the engineer told me they are now supplying clear span wood trusses spanning up to 60 feet. Damn! that's a long way! I didn't ask wood dimensions on this, as I'm not going that wide.
RJ

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using 2x4s running perpendicular to the bottom chords of the trusses (with the 2x4s screwed into the bottom chords)? You could probably even get by using widely spaced (say, the width of a 2x4) decking boards, since you just want to have something to walk on and stack boxes on.
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You don't want to mess with or add weight to the trusses, unless you can do the math to figure out what they're going to do in response. You can add a floor anyway, but figuring out how requires knowing things like: How much headroom is there in the garage for beams and joists sticking down? Which way do the existing trusses go relative to where the garage door(s) are, and to the roof-slope?
Remember that you don't actually have to have a load-supporting floor over the entire area, either. If you can find a place to plant 3 or more posts supporting 2 or more beams either under, over, or between the trusses, then you can put a platform in.
It might even be cheaper or simpler *NOT* to put a regular floor in, but just put up scaffolding through the trusses with planks across them.
--Goedjn
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The trusses are designed to hold up a roof, not a floor. You should not build a floor on them or hang much weight on them.
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If this were possible I can't visualize you having any head room, that being said is this a barn style roof where you can stand up?
I was thinking of suporting the 2x4 roof trusses with 2x10's or 12's from underneath the 2x4's like another poster mentioned building a deck under the trusses.re-enforceing the walls where the 2x10's would rest.
Tom
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You can use a much narrower steel bean running under the ceiling joists if the 2x10 are not feasible.

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wrote:

2x4's then place 1x6's on top to provide a walkway & provide a "floor" to store boxes etc on... ? The sistered truss should be able to hold the extra weight....for storing boxes etc..
Bob G
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I disagree with the idea that 2x4s sistered to the trusses will work. Look at posts by Goedjn & Pawlowski & Spitzer. Trusses are designed to support particular loads. To add a new load to them is to ask for trouble.
TB
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