Garage attic

1172 Views 12 Replies 4 Participants Last post by Mingledtrash, May 20, 2016

vujer98

Discussion Starter · May 19, 2016

I have an attached 2 car garage with a drop ceiling installed. There is a door installed with a folding ladder to access the rafters above the drop ceiling. I've never used this space before and found the previous owners of my home laid down some 1/2" OSB in parts to create a small storage space immediately surrounding the ladder door in the attic space. I also discovered they left some junk which I had to clear out...

I had decided to extend the attic floor to gain more storage space and purchased the necessary 1/2" OSB sheets based upon what I saw briefly in the attic. Trouble I've run into though is after clearing out the previous owner's junk I noticed the rafters holding up the drop ceiling (which the original OSB was resting on) are 32" OC, not the 16" I had assumed. After further investigation, I discovered the previous owner "scabbed" some 2x4's between the rafters to take the bow out of the OSB sheets.

I know this isn't a "living space", but I'm concerned 1/2" OSB will not be strong enough to create a safe attic floor given the 32" rafter spacing. Without installing additional rafters (which is NOT an option at this point) to be 16" OC, is there a way I can reinforce the space between the rafters to SAFELY accept the 1/2" OSB sheets I have? I can not return the OSB so if I don't use them for this project, I'm out of luck.

Any suggestions would be greatly appreciated. Thanks for reading.

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craig11152

· #2 · May 19, 2016

I think you may be using the term "rafter" where you should be using "joist".
A rafter would hold up the roof while a joist would hold up a floor.

In any case if they are 32" apart they may be rafter ties rather than joists. A joist in this case often doubles as a rafter tie but a rafter tie does not often double as a joist.

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craig11152

· #3 · May 19, 2016

To clarify a rafter tie might be a 2x4 and they could easily be spread 32" or even 48" apart. They are designed to keep the outer walls from bowing out from downward pressure of the roof.

If they are also joists they would be bigger than a 2x4, depending on the width of the garage maybe 2x6 or 2x8 or even 2x10 and they would be spaced 16" or 24" apart most likely

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vujer98

Discussion Starter · #4 · May 19, 2016

Thanks for your response... The rafters are 16" OC holding up the roof, but the rafter ties which are larger than 2x4s are 32" OC. Based upon your definition, I believe this would make them "joists doubling as rafter ties", right?

In any case, 32" between joists I believe is too much space to create an attic floor for storage using 1/2" OSB. Would you agree?

I'm less concerned about the strength of the joist spans as I don't anticipate large loads up there at all, but considering I already have the stock of 1/2" OSB, is there a safe and economical way I can add bracing between joists to properly support the floor?

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craig11152

· #5 · May 19, 2016

vujer98 said:
Based upon your definition, I believe this would make them "joists doubling as rafter ties", right?

That would depend on how big they are and how big the span is they cover. The wider your garage the wider the 2x needs to be. I would defer to more expert folks on specifics. but a 24 foot wide garage needs a bigger joist than a 16 foot garage would. So "we" need the particulars to tell for sure.
So how wide is the span between supporting walls and how big are the rafter ties?

If they meet the criteria then I would cut equal size spacers between the joists. Enough so the edges fall on one and one in the middle. Maybe two in the middle.

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craig11152

· #6 · May 20, 2016

assuming your rafter tie/joists are sufficiently joists something like this should give you enough support for light loads

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vujer98

Discussion Starter · #7 · May 20, 2016

I was thinking the exact same thing but was looking for a sanity check from someone with more experience. The span between load bearing walls is about 21'-9" and utilizes 2x6's as the rafter ties.

I figure I could add some vertical 2x4s to connect the rafter ties to the rafters to stiffen them up a bit to be safe. Would you agree?

As far as the cross bracing between rafter ties, would I need joist hangers or just simply screw them into the 2x6's? Also, should I use 2x6's for the cross bracing to match the rafter ties or would 2x4's suffice in this instance? I'm thinking 2x4's would be fine just to take the flex out of the OSB but I'm curious your thoughts.

Thanks for your time. I appreciate it.

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craig11152

· #8 · May 20, 2016

all bets are off. Those are just rafter ties. At that span you need 2x12, maybe 2x10 but no way 2x6 or even 2x8.

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vujer98

Discussion Starter · #9 · May 20, 2016

So adding multiple sets of stiffeners to the rafters wouldn't make a difference?

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craig11152

· #10 · May 20, 2016

vujer98 said:
So adding multiple sets of stiffeners to the rafters wouldn't make a difference?

I would differ to an expert on whether transferring some of that load to the rafters is OK or not. There is at least one maybe a couple engineers who frequent the forum. Maybe one will pop in but that is beyond my pay grade.

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craig11152

· #11 · May 20, 2016

Do you mean fastening to the rafters and the rafter ties so the rafters bear some of the load?

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Daniel Holzman

· #12 · May 20, 2016

2x6 joists spanning approximately 22 feet, spaced 32 inches on center, are barely able to support 10 pounds per square foot. And the 1/2 inch OSB is not capable of supporting a 200 lb individual walking on the OSB midspan between the joists.

Conclusion: You need to reframe the garage if you want to support any kind of storage load, and/or be able to walk on the floor. Adding vertical framing members to support the joists may be possible, but you have to carefully analyze if the rafters can handle the additional load.

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Mingledtrash

· #13 · May 20, 2016

Daniel Holzman said:
2x6 joists spanning approximately 22 feet, spaced 32 inches on center, are barely able to support 10 pounds per square foot. And the 1/2 inch OSB is not capable of supporting a 200 lb individual walking on the OSB midspan between the joists.

Conclusion: You need to reframe the garage if you want to support any kind of storage load, and/or be able to walk on the floor. Adding vertical framing members to support the joists may be possible, but you have to carefully analyze if the rafters can handle the additional load.

Daniel is right, i would not recommend storing anything up there unless its a small box of feathers. I would even go so far as to say you should tear out the existing plywood except maybe a small 18" wide walkway/catwalk for accessibility that way no one will be tempted to store stuff up there.

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