Advice on garage attic storage

[thammel]

Hi Guys,

Got a couple of questions re using my garage attic for light storage. It's an attached 2 car garage with 2 x 4 trusses spaced at 24" centers. The ceiling is drywalled and there is currently an access panel. I plan to install folding stairs, insulate the attic floor and install some plywood for light storage. The key word is light, I know. What do you all recommend for plywood thickness for the flooring? I will be using about four 4 x 8 sheets cut into 2' x 4' pieces to give me about a 6' x 20' floor section for storage. I want to go as thin as possible to minimize flooring weight but not crack when I crawl/crouch walk on it. I'm thinking 1/2" or 5/8". I've already installed 2 lights up there and will be installing some switched fluorescents in the garage itself. Then I plan to blow insulation into the already drywalled exterior walls so the place will be tolerable in the winter.

Thanks,
Tom

 

[boiler7904]

My trusses are 24" o.c. The builder of my house used 7/16" OSB. I added two more sheets to match what they did. If I was starting from scratch, I'd use 5/8". The 7/16 flexes when you walk on it... a lot.

 

[kyeakel]

When I did this in my garage attic, I sistered 2x6's to the 2x4 bottom plate of the truss. I did this to a point just past the 1st truss junction on either side. This not only stiffened up the floor of the attic, but gave me enough room to get in R-19. Which helps in the summer and winter!

Kipp

 

[twostory]

I plan on laying a 2x6 grid over my current truss cord in the attic. This will allow me to install 10 inches of insulation under the attic floor.

I currently have 5/8 plywood for the attic floor, and that is fine. Thicker would be nicer, but the 5/8 is fairly solid. I would not use 7/16 OSB, that is way to thin.

 

[1320stang]

I used 7/16" OSB on the 2x4 cords of my 24" o.c. trusses on my attached garage. I'm 275# and it flexes, but I've yet to fall thru. I cut my sheets in half lengthwise as I also had to install a drop down stair. My trusses run from front to back, so there is 24' of unsupported area, but I have about another 8' of house at the front of the garage (utility/pantry).

This being said, my drywall is cracking at the seams. I wished I'd sistered 2x6's along the bottom cord of my trusses before putting the OSB down. I decked the width of my garage about 6' wide. Up to the purlin/rat run that keeps the bottom cord spaced right to the center of the truss. My stair top is right at the center of the truss.

I have gone out above my house and put scrap pieces of 2x4 in the webs about 2' above the bottom cord, then I ran scrap 2x material for a rat run.

 

[thammel]

Lots of good thoughts! The 2x6 sistering concept is a good one, but I wonder how much it helps unless you run all the way to the load bearing walls? I'm not driven as much by the added insulation thickness as by the added rigidity. I can not run all the way to the load bearing walls - I do know that the greatest flex is at the middle of the truss, so I guess it makes sense to rigidize the center section even if the 2x6 doesn't run all the way to the ends. I"m only 175 lbs so maybe 1/2" stuff is good enough. I don't think I can get 2'x8' strips of plywood up into the attic so would probably cut them to 2'x4'.

THanks!
Tom

 

[1320stang]

I would think that if you sistered a 1/3 of the bottom chord length and centered it on the chord, you'd be good.

Once you put your attic stair in, the 2'x8' isn't bad, you just have to lay them down to get them spun around the right way. I slid mine up the stairs on their sides, then let them swing down between the trusses, then slid them back and forth until they were oriented right.

Cool thing with that is, Lowe's or HD will give you one cut free with their panel saw, you can get them to cut them by length for you. Once you get home, you can always rip them the other way if you find you can't get them up there. Then you're only dealing with a 2' cut which is easier to keep straight with a circular saw.

 

[JMURiz]

This article might help for ideas on getting the spring/flex out of a floor:

http://www.thisoldhouse.com/toh/print/0,17071,451111,00.html

I was looking at it the other day, sistering is the best, but to get the best results you'll have to go the whole length. If you can take down the ceiling drywall the 2x4 on the underside of the truss is the next best (kind of like making your own wooden I-beam)...then again you'll have to redo the ceiling. Best of luck to you.

Another idea is to do a grid the other way with 1x3 strips...this will help spread the load.

disclaimer: I'm not a builder or an architect so these are just ideas.

 

[Junkman]

I would suggest that you gather up as much information about the trusses that you can, length, hight, spacing, etc. and take that information to your local lumber yard and ask them what the load capacity of the trusses is the way that they are presently installed. Tell them what you want to do, and they will give you the proper information that you will need to make the changes to the truss system to be used as an attic. Many times, the trusses are not designed to be load carrying beyond the capacity of the roof structure. If you put a plywood floor over the trusses, you might be stiffening them, but you might also be pushing them beyond there load capacity, and that can lead to "catastrophic failure". This means that any additional weight might be sufficient to cause the to suddenly fail, or what is more commonly known as "the straw that broke the camels back". While we all might feel comfortable making suggestions you are ultimately responsible for the work, and to make sure that it is reasonably safe. I was going to do the same thing with the attic space over my garage, until I spoke to the lumber yard, and learned that if there was a snow load on the roof, and I also put additional loading on the floor, that under a heavy snow load, that the roof structure could fail. In short, what would be safe to do in the summer, might cause it to fail in the winter with 12" of snow on the roof. In my case, it can be done, if I were to add additional floor joists to the attic, to increase the load carrying capacity. The people that do this at the lumber yard will know how to do the calculations so you can have a safe attic floor.

 

[thammel]

More info on the trusses in my garage attic. They are mono-b trusses which means they are right triangles with the base 14' long and the side 7' high. (do your math and find that the hypotenuse is 15.7' long). These are essentially comprised of 4 smaller triangles with 2 supports up to the hypotenuse (one from the corner and two up from the center of the base) so the hypotenuse is divided up into thirds. Anyway, the point is that the floor of the attic has a 14' span with a 2x4 but is supported (from above) at the middle (suspended?) by means of the truss structure. All I can say is that if really feels very rigid when walking on the bare trusses. I would be using the 7' section of the truss for storage obviously that is nearest the 7" vertical side of the truss. I'm wondering if this affects your recommendation to do the 2x6sistering trick. I also wonder about the extra weight the sistering with 2x6's adds. Actually, if there wer some way to get 14' long 2x6's up there I could do more than sister - I could run them from end to end, load bearing to load bearing.

Thanks,
Tom