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Old 11-21-2009, 07:45 PM  
GatorJoe
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Default Installing OSB subfloor in garage attic

I am in the process of installing OSB as a floor in my garage attic. I intend to use the area for storage. The garage is 24' x 24' and the joists are 2x12's installed 16 inches on center the full width of the garage (no blocking between them). While I am not even close to an engineer, this appears to be very sturdy and I would think could hold a moderate amount of weight (typical storage - nothing excessive). Is this a reasonable assumption based on what you see in the pictures provided?

The other important question involves a few random 2x4's that are nailed across the joists. I am guessing that these were put there just to add a little bit of stability. Since I am nailing down OBS throughout, do I need to leave these in place? They just seem too random to be anything required by code.

There is also one 2x4 that is installed vertically between the roof and one of these 2x4's. It is firm but in no means solid when I push on it.

Any thoughts would be appreciated.

Regards,
Joe



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Old 11-21-2009, 10:37 PM  
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Gator Joe:

I should preface by saying that I have a degree in mechanical engineering, but have virtually no knowledge or experience in construction carpentry or framing, so take what I say with a bit of salt. I've been renovating for 20 years, but it's all been in the same apartment block where every suite was constructed the same way with the same materials, so it's probably more correct to say that I renovated the same apartment 21 times than to say I renovated 21 apartments.

Why install OSB as a subfloor? I would be concerned that if you dropped something heavy up there, it would go through the OSB, or at least leave a major dent in the OSB.
If it wuz me, I woulda gone with T&G plywood, or fir 1X6's, just like a normal sub-floor for a working floor.

That 2X4 running the length of the attic and nailed to the joists effectively acts like blocking, but so would your OSB.

Welcome to Blocking 101:
If you have a joist holding up a floor, it has the most strength when it's vertical. As you put more and more weight on that joist, then the more it bends under the weight, the more it wants to start twisting under the weight. Once the joists twists, it looses very much of it's strength. Imagine a 2X12 on it's side; it's not nearly as rigid as a 2X12 that's straight up and down. In fact, you can use a 2X12 on it's side as a pretty springy diving board. So, the more a joist twists, the less strength it has to resist bending, and the more it starts to become easy to bend like a diving board.
The purpose of blocking is simply to keep the joists vertical so that they remain most capable of resisting bending. You can do that by putting a row of solid wooden blocks between the joists at mid-span, or two rows at 1/3 and 2/3 span for longer spans. You can also do it by putting in what they call "cross bracing" between the joists and they make steel cross braces that are hinged in the middle that you just nail in rather than make from scratch like wooden cross bracing. Or in a case like this, where the joists are supporting nothing more than a drywall ceiliing (if that) and the need for blocking is minimal cuz the weight supported by the joists is minimal, you can supply some "blocking effect" by simply nailing a 2X4 along the top of the joists. That 2X4 will help prevent any one (or several) joist(s) from twisting, but it don't help much if there's so much weight on the floor that all the joists are wanting to twist cuz if they all twist in the same direction, that 2X4 will just go along for the ride. Only solid blocking or cross bracing between the joists will prevent a collapse if all the joists are wanting to twist.

You can take out that 2X4 cuz the stuff you're nailing down will do the same job.

Considering this is gonna be used as a floor from now on, and if it was me, I would buy some steel cross braces made to be used between joists and put them in in two rows; at 1/3 span and 2/3 span. That's cuz 24 feet by 24 feet is a real big floor with real long joists. Also, if it wuz me, I'd use T&G plywood subflooring cuz you're prolly never going to take the flooring out, so you may as well install something more sturdy that can serve for any purpose throughout the life of your house, and possibly more than just light storage.

But, truth be told, as long as you're only using that space for storage, you'd be fine with OSB and using the OSB to do the same thing as the 2X4 you're wanting to remove.

Can you post a picture of how those 2X12 joists tie in to the walls on each side of the garage? I'm sure the carpenters in here would want to see that detail. I'm just concerned that the builder might have done sumfin stupid like just nail the joists to the sides of the studs in those walls. You wanna be confident that the joists are resting ON those side walls so that the walls properly support the joists.

I think that horizontal 2X6 in the first picture was installed just to hold the wall vertical, and that you can replace that strut with anything else that'd hold that wall vertical. No one is going to tell you to remove that strut without providing some other means of holding the wall vertical cuz the original builder obviously saw a need for it.



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Old 11-22-2009, 05:55 AM  
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Default hey gator

Having built this stuff for over 20 years, I'm pretty confident about what you have there.
The 2x4s going across the top of the joists are strongbacks. They keep the joists from movin around when they dry after installation, and help to hold things together for a little more strength.
The post to the ridge is usually a temp post fir installing the ridge as you install the rafters. guys leave them up for more support after it is done, you can remove it because you have collar ties...those 2x4 boards going to rafters on both sides.Keeps the roof from spreading, add more if you like...helps to keeep the spreading to a minimum.
OSB, is a poor choice for a floor, 1/2, five ply is what you should be using at the least. Go with 3/4 for a really good, engine block holding floor.

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Old 11-22-2009, 08:57 AM  
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Thank you both very much for your input on this. I don't have a picture of how the joists are resting at the wall, but I was sure to check that they are on top of the wall headers (double 2x4's). As there is a fairly steep pitch on the roof there is only a small notch out of the top end of the 2x12 joist to accomodate the roof.

I think adding blocking would be a wise choice to make sure everything is secure. I am quite bothered that OSB is a poor choice though. I went in my basement, looked up, and see OSB as a subfloor in my home which was built in 2002. In fact the panels I bought are labeled as tongue and groove OSB subfloor. Of course if I owned a truck and had a quick supply of helpers it would be an easy thing to switch out - unfortunately it would be quite a task. Hmmm. I would like to be able to store quite a bit of boxes up there as it is a large area, but of course would prefer the structure to be standing at the end of the day.

Thank you again for your assistance. You have reassured me that my issues with the 2x4's is likely correct...yet added something new to worry about. LOL.

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Old 11-22-2009, 11:29 AM  
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Maybe better to post in here before starting the work. I expect it was a fight getting those sheets of OSB up into the attic.

I think they use OSB as a subfloor just to save costs.

You don't have to use plywood. My building is built with 2X12's on 12 inch centers (!) and my subfloor is fir 1X6's nailed down at a 45 degree angle to the joists. Doing that helps keep the house construction more rigid. I expect it'd be a lot easier getting fir 1X6's up into your attic than plywood. And, fir 1X6's should be stronger than plywood cuz ALL of the wood fibers are oriented in the direction needed for maximum strength. And, with lumber (like 1X6's), you don't need to worry about leaving a gap cuz the stuff is likely to shrink rather than expand (like plywood does).

But, in future, if you ever want to install any kind of flooring in that space, be sure to put in an underlayment. That's cuz if you glue your flooring down to an underlayment, you can always remove the flooring by prying up the underlayment with the flooring still glued to it. If you glue your flooring directly down to the subfloor, then you're pretty well stuck having to remove the flooring from the subfloor (regardless of how difficult a task that might prove to be) or putting new flooring over the old. Lotsa times that old flooring can add a lot of weight (like ceramic tiles) and so you always want underlayment over a subfloor to always have the option of removing the old flooring by replacing the underlayment.



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