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EngineerJet

Looking to add flooring to attic for storage. Any structural engineers/architects here?

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I think i saw UCC requires 5/8" for 16" supports (plywood or OSB) but I would think 3/4" is easier to find and gives you that much more strength

 

15 minutes ago, EngineerJet said:

Appreciate the speedy response. Is the 3/4 more for supporting walking on top of or for spreading the load across the joists?

 

What do you see as the difference between those two things?

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3 minutes ago, Shocker said:

I think i saw UCC requires 5/8" for 16" supports (plywood or OSB) but I would think 3/4" is easier to find and gives you that much more strength

 

 

What do you see as the difference between those two things?

One side im looking from a foot falling through the floor aspect, the other is stressing the joists and possibly causing sagging aspect.

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4 minutes ago, Shocker said:

I think i saw UCC requires 5/8" for 16" supports (plywood or OSB) but I would think 3/4" is easier to find and gives you that much more strength

 

 

What do you see as the difference between those two things? 

I think the issue is 1/2 will flex when you walk on it in between the joists... 3/4 will not.... TG is best for that reason.
 

 

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3 minutes ago, EngineerJet said:

One side im looking from a foot falling through the floor aspect, the other is stressing the joists and possibly causing sagging aspect.

1/2 will flex.. we had 1/2" in our attic from the prior owner and it flexed... we changed over to 3/4 tg and the issue was resolved.

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9 minutes ago, EngineerJet said:

One side im looking from a foot falling through the floor aspect, the other is stressing the joists and possibly causing sagging aspect.

A normal human wouldn't puncture even 1/4" luan, if it was attached properly....the web thickness is for sagging/flex of the floor

 

A 2x6 can support a crap-ton of weight if the span isn't too big. If you're causing the ceilings below to sag you have bigger problems

 

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6 minutes ago, Shocker said:

A normal human wouldn't puncture even 1/4" luan, if it was attached properly....the web thickness is for sagging/flex of the floor

 

A 2x6 can support a crap-ton of weight if the span isn't too big. If you're causing the ceilings below to sag you have bigger problems

 

maybe i just overestimate my fatness. i've packed on a few pounds since EAS, lol

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29 minutes ago, EngineerJet said:

Appreciate the speedy response. Is the 3/4 more for supporting walking on top of or for spreading the load across the joists?

Yes.

Also when loading your attic with crap. Try to load directly over walls and not mid span. 

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I put 19/32" OSB in my attic 18 years ago.  we had to rip it down to 2x8 sizing to get it up into the attic though, so keep in mind how big your opening is. a 10# box of screws later and it's been in place with no issues so far.

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13 hours ago, EngineerJet said:

My attic does not have flooring and I'm looking to use it for some light storage. The joists are 16" on center and are 2x6. Can I just add some 1/2 inch plywood and call it a day? Not concerned about insulation.

@EngineerJet:

I think that there may be a more important concern for you in order to achieve your goal of creating storage space up in your attic.
That concern is whether or not your 2x6 joists that are set 16" o.c. will be adequate enough to support the added weight (live load), of whatever you ultimately put up in your attic.  Do you have any idea of what wood species your 2x6 joists are, (such as Douglas Fir or SPF...), since that also is a contributing factor in the strength of the wood and its load-bearing capacity?  If your house was constructed using traditional wood balloon framing then the original  purpose of your 2x6 ceiling joists were to tie the perimeter walls together and neutralize the tendency of the roof load to put pressure on, or thrust the walls outward.   Since they are 2x6's, they may not have been sized for also being able to support an additional live load that would come with occupied or storage use.  The critical dimension in determining that is the clear, (unsupported) span of the 2x6 joists.  If the span is too long then the added load (weight) of whatever you plan to store in your attic may cause those joists to severely bend and deflect which would then noticeably deform your drywall ceiling in the rooms below, maybe even creating a safety concern.  You may want to consult with an Architect or Structural Engineer to determine whether your existing 2x6 ceiling joists are adequate to support the intended load in your "new' attic space, or if you should add a deeper wood joists, (i.e. 2x8 or 2x10, etc...), at the same spacing, if your clear span warrants that.  Also, I would use a minimum of 5/8" plywood for the floor sheathing, but would agree that 3/4" is preferable to avoid deflection.

AVB-AMG

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3/4 plywood with T&G edges will prevent sagging between joists and also add structural capacity, especially if you add construction adhesive on top of the joists, then screw it down. If you can't use T&G there, the 3/4 is stronger anyway, and will be better in the long run.

The other issue is that 2x6 at 16" o.c. is not all that great, so do not overload the attic. Place heavier stuff near support walls, etc.

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1/4 inch plywood is fine.  As long as the beams are what you say they are and your not storing super heavy boxes.  Plus it's cheaper, lighter and easier to install.

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1 hour ago, AVB-AMG said:

@EngineerJet:

I think that there may be a more important concern for you in order to achieve your goal of creating storage space up in your attic.
That concern is whether or not your 2x6 joists that are set 16" o.c. will be adequate enough to support the added dead weight load of whatever you ultimately put up in your attic.  Do you have any idea of what wood species your 2x6 joists are, (such as Douglas Fir or SPF...), since that also is a determining factor in the strength of the wood?  If your house was constructed using traditional wood balloon framing then the original  purpose of your 2x6 ceiling joists were to tie the perimeter walls together and neutralize the tendency of the roof load to put pressure on or thrust the walls outward.   Since they are 2x6's, they may not have been sized for also being able to support an additional dead load that would come with occupied or storage use.  The critical dimension in determining that is the clear, (unsupported) span of the 2x6 joists.  If the span is too long then the added load (weight) of whatever you plan to store in your attic may cause those joists to severely bend and deflect which would then noticeably deform your drywall ceiling in the rooms below.  You may want to consult with an Architect or Structural Engineer to determine whether your existing 2x6 ceiling joists are adequate to support the intended load in your "new' attic space, or if you should add a deeper wood joists, (i.e. 2x8 or 2x10, etc...), at the same spacing, if your clear span warrants that.  Also, I would use a minimum of 5/8" plywood for the floor sheathing, but would agree that 3/4" is preferable to avoid deflection.

AVB-AMG

Wow we agree.

 

24ae51a74dc129549d1c2b69b6415c1f.jpg

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Are the same 2x6” used as floor joists for your downstairs living spaces?  Are the spans similar?  If so then the additional load of storage shouldn’t be a problem as long as you don’t go nuts. 

Technically still live load though. 

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6 hours ago, diamondd817 said:

My attic is 1/2" ply over 2×6. Been there for almost 50yrs. No problems yet. Lots of junk stored up there.

I agree,  1/2" is normally fine for standard loads over 2x6's. The important factor is how many of those joists have walls underneath? If a bunch are supported by walls, then the overall load will be spread over a wider area. If those 2x6's span wide rooms, keep heavy weights off the centers. Also, unless he weights 300 lbs., a "normal human can walk on the 1/2' plywood.

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7 hours ago, AVB-AMG said:

Also, I would use a minimum of 5/8" plywood for the floor sheathing, but would agree that 3/4" is preferable to avoid deflection.

Another thing every needs to keep in mind, this is an attic, that only gets accessed very seldom. It's not a main living floor that has people and furniture using it every day. Don't over think the obvious. Holiday decorations don't weigh all that much.

Now, if he's storing ammo up there....

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4 hours ago, Sniper said:

I agree,  1/2" is normally fine for standard loads over 2x6's. The important factor is how many of those joists have walls underneath? If a bunch are supported by walls, then the overall load will be spread over a wider area. If those 2x6's span wide rooms, keep heavy weights off the centers. Also, unless he weights 300 lbs., a "normal human can walk on the 1/2' plywood.

 

4 hours ago, Sniper said:

Another thing every needs to keep in mind, this is an attic, that only gets accessed very seldom. It's not a main living floor that has people and furniture using it every day. Don't over think the obvious. Holiday decorations don't weigh all that much.

Now, if he's storing ammo up there....

@Sniper:

When calculating loads, any "stuff" stored in the newly created attic space is considered live load.  That, in addition to whomever is accessing that space to store and retrieve that stuff is also live load.  So keep in mind that whatever is being stored up there will be "using it everyday"....
The concern is not really the thickness of the plywood floor sheathing but the load capacity of the floor joists.  The load capacity of the wood floor joists is determined by their vertical dimension and the clear span between supports, whether those supports are a load-bearing end wall, or wood girder or a doubled wood stud column, etc.  Also, who knows who the next owner of that house may be and what they may decide to store up in that attic space....? 
None of this is "over thinking", but is a prudent exercise in safety to provide reassurance and peace of mind that he, (the O.P.),  is not inadvertently creating a potential safety hazard.

AVB-AMG

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10 hours ago, remixer said:

Wow we agree.

24ae51a74dc129549d1c2b69b6415c1f.jpg

@remixer:
Wow....  we actually do agree on something....  Surprise, surprise....   But no, I would not go so far to say we are friends.... :p
AVB-AMG
Image result for MaCaulay in Home alone

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4 hours ago, AVB-AMG said:

When calculating loads, any "stuff" stored in the newly created attic space is considered live load.  That, in addition to whomever is accessing that space to store and retrieve that stuff is also live load.  So keep in mind that whatever is being stored up there will be "using it everyday"....
The concern is not really the thickness of the plywood floor sheathing but the load capacity of the floor joists.  The load capacity of the wood floor joists is determined by their vertical dimension and the clear span between supports, whether those supports are a load-bearing end wall, or wood girder or a doubled wood stud column, etc.

That is what I said, without writing a short novel. The open span is a bigger issue then the load, if the ceiling joist isn't being supported by a wall below. And the "live load" only uses a small portion of the total available support of the attic floor joists, in most cases, unless someone is storing a library of books or some other uncommon heavy items.

Yes, an attic has "live load" with the stored items in it, but it doesn't compare to the "live load" on the main floor like I mentioned above. Not even close. The main floor is carrying the weight of all the walls/sheetrock, cabinets, flooring/tile, plumbing fixtures, doors, all types of heavy furniture, personal belongings, and finally people. Plus add in the weight of the attic and what's stored up there. There's a hell of a lot more "live load" on the main floor joists than the attic, and there's usually very few walls under that main floor to help support it.

So, back to the OP, unless he has some really long open spans that the ceiling joists run across, 2x6's with 1/2" plywood would be fine for the average attic storage. The only weight on those joists now is the ceiling sheetrock on the bottom of the joists.16" OC is fine for that application.

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The difference in price between 1/2 and 3/4 is minimal unless this attic is 10000 sf.

Point is why bother with a lighter thickness for a few bucks more you will have a sub floor that is over engineered and able to hold plenty of weight in case the attics needed as a safe location  during the Zombie Apocalypse.

7 hours ago, AVB-AMG said:

@remixer:
Wow....  we actually do agree on something....  Surprise, surprise....   But no, I would not go so far to say we are friends.... :p
AVB-AMG
Image result for MaCaulay in Home alone

I'm hurt :)

I would say take AVB and My advice... We are both in the building trades.

 

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On 4/10/2019 at 12:58 AM, Sniper said:

That is what I said, without writing a short novel. The open span is a bigger issue then the load, if the ceiling joist isn't being supported by a wall below. And the "live load" only uses a small portion of the total available support of the attic floor joists, in most cases, unless someone is storing a library of books or some other uncommon heavy items.

Yes, an attic has "live load" with the stored items in it, but it doesn't compare to the "live load" on the main floor like I mentioned above. Not even close. The main floor is carrying the weight of all the walls/sheetrock, cabinets, flooring/tile, plumbing fixtures, doors, all types of heavy furniture, personal belongings, and finally people. Plus add in the weight of the attic and what's stored up there. There's a hell of a lot more "live load" on the main floor joists than the attic, and there's usually very few walls under that main floor to help support it.

So, back to the OP, unless he has some really long open spans that the ceiling joists run across, 2x6's with 1/2" plywood would be fine for the average attic storage. The only weight on those joists now is the ceiling sheetrock on the bottom of the joists.16" OC is fine for that application.

@Sniper:

From what you have written in your post above, it is clear that you do not understand the basics of how residential balloon frame wood structural design works.  Not to worry, many people do not understand it and that is why they hire Architects, Structural Engineers and General Contractors to determine the best solution to safely meet their residential structural goals.  

What is reckless and potentially dangerous is for you as a lay-person, who does not know how and where the variables of structural loads are determined, carried and dispersed, is stating to the O.P., with apparent absolute certainty, that you know that his existing situation will work for his intention.  Of course, who cares if you are wrong.....   you are just some guy giving his silly uneducated, as well as unlicensed opinion on something you know nothing about.  

We Architects and Structural Engineers have studied theses issues and are tested and licensed by both state and national registration organizations for a reason.  Unlike you, we have an obligation to gain an education from both formal schooling, apprenticeship to other licensed professionals and from our work experience.  We must be knowledgeable of and to adhere to accepted means of design and construction in order to, at the very least, meet the minimal established standards for safety and durability, as well as adhering to all applicable building codes.

You may be knowledgeable about many things, but this topic is clearly not one of them.

AVB-AMG

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FYI,

Plenty of home improvement websites state it might be safer to use 1/2 plywood over 3/4 due to the weight and not to overload the joists.

Weight per 4 x 8 sheet

1/2”40.6 lbs

5/8”48 lbs

3/4”60.8 lbs

"The joist spacing in some attics is 24 inches, and 1/2-inch sheets will sag and possibly break when spanning that distance. You need 3/4-inch plywood. The thinner plywood is acceptable when the joist spacing is 16 inches. If you plan to finish the floor in the attic and turn the room into a living space, however, you should always use 3/4-inch plywood. Thinner plywood can flex when you walk on it -- even with 16-inch spacing -- and this will damage the flooring."

IMO If 20lbs over a span 32 sf makes it a danger or less safe then you might other structural issues.

1/2 plywood might be strong enough but when you are walking on a floor carrying items for storage it makes it very uncomfortable mentally that the floor is flexing.. also pinpoint loads IE a table with legs that you place heavy items on will just seem to sink in the 1/2.

FYI: I'm not an Architects and Structural Engineers My family is mainly in the steel industry but we have built homes and just how we do it.

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