Convert attic to usable space

[PALefty]

Another home project on my list for the year... is to make some useable storage space in my attic. With all my recent tool acquisitions, I am getting pretty overrun in my basement and looking to clean things up (esp those da#$ empty boxes). I am not planning anything fancy... but would like to at least lay down some plywood so I can throw some things up there.
I already have a standard attic latter installed.. which would require me to have the 4x8 sheets cut length wise to get them up there. This leads me to the following questions:
- How thick should the plywood be.. to offer enough support but not overly heavy? I believe I have 16'OC joists
- Is particle board a possible alternative?
- Should I lay it on the joists themselves.. or try to build up the load bearing walls so they support the weight of the floor and not the joists?
- Should I lay any additional runners (1x?) in an opposing direction for more support?

thoughts/ideas?

[LarryG]

For light storage, and assuming 16" joist spacing, 1/2" plywood will do it; 5/8" would be better.

Particleboard is suitable as an underlayment, over plywood sheathing, but not as sheathing itself. (Unless you use the industrial grade, which is mondo-heavy.)

Not sure I understand what you mean about "build up the load bearing walls." If the joists are adequately supported by the walls, the load will transfer just fine. There's no reason to extend the walls up between the joist spaces, if that's what you mean. (For a habitable floor you would normally add solid blocking between the joists, directly above the wall, but this isn't strictly necessary for light attic storage.)

No reason for any sleepers going the other direction if the joists are at 16" OC; all you'd be doing is adding weight and decreasing headroom.

[Tom Miller]

I put some OSB in my garage rafters for this purpose. I used the tongue-and-groove stuff, but it's not really necessary for this application. And, if you're cutting it into 2x8 pieces to get it up there, it kind of negates the t-n-g, anyway.

I don't recall if I used 5/8" or 3/4", but I guess I wouldn't go under 5/8". You never know what you might end up with up there.

Regards,
Tom

[PALefty]

For light storage, and assuming 16" joist spacing, 1/2" plywood will do it; 5/8" would be better.

Particleboard is suitable as an underlayment, over plywood sheathing, but not as sheathing itself. (Unless you use the industrial grade, which is mondo-heavy.)

Not sure I understand what you mean about "build up the load bearing walls." If the joists are adequately supported by the walls, the load will transfer just fine. There's no reason to extend the walls up between the joist spaces, if that's what you mean. (For a habitable floor you would normally add solid blocking between the joists, directly above the wall, but this isn't strictly necessary for light attic storage.)

No reason for any sleepers going the other direction if the joists are at 16" OC; all you'd be doing is adding weight and decreasing headroom.

Sounds good. Yes, that is what I meant by building up the lb walls. I didn't think it would be necessary... but I thought it might reduce the chance of nail pops (not that I plan on going up there more than once or twice a year).

[PALefty]

I put some OSB in my garage rafters for this purpose. I used the tongue-and-groove stuff, but it's not really necessary for this application. And, if you're cutting it into 2x8 pieces to get it up there, it kind of negates the t-n-g, anyway.

I don't recall if I used 5/8" or 3/4", but I guess I wouldn't go under 5/8". You never know what you might end up with up there.

Regards,
Tom

OSB is what I meant when I said Particleboard...if that makes a difference? I believe the 5/8 is going for $10-11. I'm not sure about the plywood at the moment...

[LarryG]

OSB never crossed my mind. That'd be perfect, and cheap.

[bigsteel15]

OSB never crossed my mind. That'd be perfect, and cheap.

That's what I did in my garage attic also. I used 1/2" thick OSB.
I believe it is stronger than plywood.

[crokett]

Find out what the load rating is on your joists. And are they true joists or just part of the roof trusses? Most attics are not designed to carry extremely heavy loads.

[JimD]

I put an extra 2x6 on top of the ceiling joists in my attic not for strength but so that I could have 11 inches of insulation below the storage floor in the attic (the extra 2x6 is on edge fastened to the first one with cleats on the side every couple feet). I used 7/16 waferboard for the floor. It flexs a little but my logic was that you can walk on the roof and it will support a significant snow load so why not for attic storage. It flexs a little but works OK. I put boxes of books or other heavy stuff directly over joists.

Jim

[PALefty]

Find out what the load rating is on your joists. And are they true joists or just part of the roof trusses? Most attics are not designed to carry extremely heavy loads.

I would venture to guess they are part of the roof trusses... but don't know what the technical difference is. Is there a formula to determine load rating?

I doubt it is designed to carry much weight... other than the roof itself.

[PALefty]

I put an extra 2x6 on top of the ceiling joists in my attic not for strength but so that I could have 11 inches of insulation below the storage floor in the attic (the extra 2x6 is on edge fastened to the first one with cleats on the side every couple feet). I used 7/16 waferboard for the floor. It flexs a little but my logic was that you can walk on the roof and it will support a significant snow load so why not for attic storage. It flexs a little but works OK. I put boxes of books or other heavy stuff directly over joists.

Jim

That brings up another point I meant to add. My attic has blown in insulation (pink stuff). If I put the plywood/osb up there, it will certainly compress the insulation to some degree unless I move it. Will the compression lower the R value?

[just4funsies]

Let me reinforce the above cautions... The bottom span on manufactured trusses is NOT engineered to support much weight, but only to keep the upper cantilevers from spreading where they meet the top of the perimeter walls. Intermediate stringers may be arranged to allow additional bearing on interior load walls, but this does little to increase the load bearing capacity of the BOTTOM span (the "joist"). Remember that, if you deck the bottom spans, they will ALREADY be supporting the weight of both the decking AND the ceiling drywall, and this is before you even start putting anything up there. I'm not saying that your ceiling will collapse (although it might), but any downward bowing of the trusses from the unanticipated additional weight will give your drywall joints cause to crack, and also might trigger nailhead pops, or other undesirable results. Be careful, and spread out any load you put up there toward the edges, and toward any internal bearing walls.

And just a word about insulation. DON'T compress it. It is not the insulating material, but the airspace within it that does the insulating. Compression forces the air out.

[dlminehart]

PALefty, if your area gets below freezing or very hot, you should keep in mind that an attic may not be the best storage location for most objects. You'll want to minimize air loss from house into attic (warm moist air hitting cold attic air in winter -> condensation). And to ensure really good attic ventilation, to avoid this humidity build up in winter and also overheating in summer.

BTW, it's handy to have your location listed in your user profile, so people don't make suggestions appropriate to Alaska when you live in Arizona.

[JimD]

The easiest way is to use tables in books about building houses. I at least used to be able to calculate this sort of thing but the limitation on floor joist sizing is deflection (not failure). The reference book on my lap under this laptop indicates southern yellow pine 2x6s spaced 16 inches apart can span 11'4" when spaced 16 inches apart to carry a 30 lb live load. This is for floor framing and would be very conservative for attic storage. There is a separate table for the same lumber (southern yellow pine 2x6s 16 inches on center) that says that when used in a limited storage attic the allowable span is 13 feet. What this means is you need a wall under each end of the joist with those walls spaced no more than 13 feet apart. A 2x10 on 16 inch centers could go 20 feet (limited attic storage).

Trusses are a whole different thing. The only completely safe thing to do is to ask the engineer who designed the truss (they never seem to be handy). In our old house, I added a member to the truss and it seemed to work fine but that was not something I would do in somebody elses house. Part of the reason I did it was the walkway down the attic was directly over the hall so there was lots of support for the trusses.

Jim

[crokett]

What just4funsies said. I stored stuff in the attic in our old house, but it was christmas decorations, etc. I kept the heavy stuff downstairs. Besides, it was easier than lugging it up a ladder.