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Old 01-16-2008, 02:16 AM   #1
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Attic floor joist design

This is a cool site and my first time here. I need your help.

I have a three car garage that has a huge attic space above the garage with a 9.5 foot span between the wall and the beam (4X16") on the one car door side and 17.5 ft. span over the two car side of the garage. The beam runs from the house behind the garage out to the front of the garage and is posted between the one and two car garage doors. My goal is to not permit this or have it as actually occupied space but do want full load bearing capabilities for storage. My neighbor has the same house and constructed an office and a bedroom in the space. He used 2x8" joists on the 9.5 foot span to the beam. I can easily do the same - no problem on the one-car side of the garage.

My question is in a design for the attic floor joists that will span 17.5 feet and give me the necessary load capacity to load the attic. I plan on sheeting with 3/4" T&G ply and will need to post the main beam in the center of the garage as the beam is not designed for the addtional load. (learned that from my neighbor doing the remodel (he is an engineer). He ran two additional parralm beams along the 4X16" and posted at each end so that he would not need to post the center of the garage. His plan also used two paralam cross beams and then smaller paralams on 12 inch centers running parallel to the large 4X16 beam back into a ledger on the house. His use of the 2X6 parralams as joists had alot to do with giving him the necessary headroom in the attic bedroom to meet code so he needed the stongest joists with the least amount of depth.

I cannot afford to use all of those paralams and was looking for ideas that would meet my needs but not break my bank. I was thinking maybe two 4x8" parralams as beams across the 17.5' span and then standard doug fir 2X8" joists in three equal length segments from the back ledger of the garage to the first beam - between the beams and then out to the garage door headers. Each span would be about 8 feet.

Any ideas would be a great help. One other idea was to use 2-2X12" joined as cross beams with 2X8 joists. Let me know if this is clear as I am not used to writing in these terms. Look forward to your responses. Looking for best value design or is that the cheapest way I can do it almost right.

Regards, Mark B.


Last edited by Mark Bierei; 01-16-2008 at 02:20 AM. Reason: typo's
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Old 01-16-2008, 02:15 PM   #2
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Originally Posted by Mark Bierei View Post
... (learned that from my neighbor doing the remodel (he is an engineer)...

Regards, Mark B.
Has your neighbor offered any ideas?


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Old 01-18-2008, 05:15 PM   #3
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Attic Joist Sizing

Redline: Thanks for your response. I have not asked him since he is out of town for much of the time. I have his design plans but they are complex because of the need to obtain minimum heights in both the garage and the attic bedroom. I am not going to use attic space for occupancy so just need a floor that is level with the top of the large 4x16" beam through the center of the garage. Hope to use standard non-engineered beams and floor joist to keep cost down.

Looking for basic opinion on layout of cross beams and joists that would accomplish this. Basic area is 17.5 X 20' with load bearing walls on all three sides and the large beam on one of the 20'sides. As mentioned I will post the 4X16' beam in the center of the grarage to handle the addtional load and will do the same along wall where cross beams tie in. Would appreciate any ideas. Thank You. MB
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