Attic floor sheathing?

[mike93lx]

Light duty storage and not even enough room to stand and walk around.

Supports are about 20" OC.

Is 15/32 SYP plywood enough or do I need to go up to 19/32? It will be screwed down so that it is removable for access to electrical

 

[wanderer]

20” on center you need 3/4 even for light use.

 

[mike93lx]

20” on center you need 3/4 even for light use.

Need? This is a space where I slide a bin up and maybe crawl around on my hands and knees. No engine blocks going up there

I should check my current stuff actually. It is 24 OC and feels like 1/2

 

[wanderer]

I have 1/2” on my roof with rafters 24” oc. I can feel it easily move underneath me. It’s your place, do what you want. I would go thicker especially if storing stuff also.

 

[danfromsyr]

what's your body weight pounds per sq inch on your hands and knees.. and you're not on all 4s all the time so calculate it for 2 points of contact.. all of your weight on 2 knee caps in mid span on 1/2" ply with say a weak spot will potentially crack it and buy a new ceiling ;-)

if you go 1/2" you can make it more rigid by screwing 2x3's along the edges.. but by then might as well pony up and go 3/4"

 

[The Cobbler]

I used 1/2 ply on 32" centers in my attic. a bit spongy but for occasional use, its just fine
ripped into 24" widths and scabbed 1x3 in the centers to screw to. I can lift any panel for access if needed

 

[mike93lx]

I have 1/2” on my roof with rafters 24” oc. I can feel it easily move underneath me. It’s your place, do what you want. I would go thicker especially if storing stuff also.

A little movement isn't a problem as long as it isn't unsafe, but I get where you are coming from.

 

[Falcon67]

7/16 OSB work fine over 24" OC joists, best in full sheets.

 

[mike93lx]

7/16 OSB work fine over 24" OC joists, best in full sheets.

No way full sheets can work... This has to go through my drop down attic stairs.

Other than cost, is there any reason you would use osb over plywood? I only need 5 or 6 sheets, so the cost isn't an issue and I would expect plywood of the same thickness to be stronger, right?

 

[rlitman]

Yes, plywood is MUCH stronger.

 

[theoldwizard1]

1/2" (nominal) plywood is fine. Yes, it will flex with 20" OC support (very odd number; typically 16" or 24" OC).

Of course, if you are north of 250 lbs (like I am), I would go with 5/8 !

 

[bad_idea]

Will the joists or trusses support the additional weight? Not the weight of the stored items, but the weight of the plywood. The ceiling in my last garage sagged a good inch in the middle of a 20' span. All of the sheetrock was cracked and can only imagine what that did to the roof (I don't go on roofs).

 

[mike93lx]

1/2" (nominal) plywood is fine. Yes, it will flex with 20" OC support (very odd number; typically 16" or 24" OC).

Of course, if you are north of 250 lbs (like I am), I would go with 5/8 !

The 20oc is my doing. Trusses are 24oc,but I need to add space for more insulation, so I am running 2x6's perpendicular to the trusses. Space is 7' ish wide, so the spacing won't be normal

 

[mike93lx]

Will the joists or trusses support the additional weight? Not the weight of the stored items, but the weight of the plywood. The ceiling in my last garage sagged a good inch in the middle of a 20' span. All of the sheetrock was cracked and can only imagine what that did to the roof (I don't go on roofs).

Good question, but not a concern here. Storage area is over interior walls and is already decked. I don't have any decking over my living room or kitchen where there are no supports underneath

 

[Jim greengo]

I vote for 3/4" also.

 

[rlitman]

The 20oc is my doing. Trusses are 24oc,but I need to add space for more insulation, so I am running 2x6's perpendicular to the trusses. Space is 7' ish wide, so the spacing won't be normal

Are the truss chords designed to hold the weight of these 2x6's, let alone the flooring above and insulation?

 

[mike93lx]

Are the truss chords designed to hold the weight of these 2x6's, let alone the flooring above and insulation?

The flooring is only above an area with interior walls.

No idea what the weight rating is for the bottom cords.

I do need to confirm that the drywall is hung off strapping as it likely can't handle the weight of the added insulation if it is 24OC

 

[finn]

I put 1/2” osb in two attics with trusses on 24” centers with no issues. Light storage is the key, although I have no issues walking up there, and I’m no lightweight.

One has been in use since 1986 or so, and the other since 2003.

Not sure what kind of plywood you guys have access to if it won’t support a man on 24” centers.

Many roofs are 1/2”, although 5/8” in now common.

3/4” is overkill for light storage use.

 

[PCMusicGuy]

3/4" is what I put in mine. North of 200 lbs and I'm glad I did because now I store all sorts of stuff up there. In hindsight, if I put thinner material I might not have so much clutter.

 

[1320stang]

I'm 299# and have had 7/16 OSB on my 24"oc trusses for 21 years with no failures, ripped to 16"x96"

 

[Bigbandguy]

My attic space has enough height for walk around so I added a framework of 2 x 4's going across the ceiling joists and then 5/8 (or whatever that is now) plywood ripped to fit through the attic stairs. Very solid floor and no issues.

 

[Marctrees]

I say 1/2" nominal plywood approved for use as roof sheathing... making sure to run the face grain 90 degrees to the support members.

Like when a house is roofed... 8 ft sheets are laid left to right across the rafters.

Huge difference in non sagging when loaded.

Marc

 

[Jim greengo]

I say 1/2" nominal plywood approved for use as roof sheathing... making sure to run the face grain 90 degrees to the support members.

Like when a house is roofed... 8 ft sheets are laid left to right across the rafters.

Huge difference in non sagging when loaded.

Marc

Dont you have to use clips with 1/2" roof sheathing?

 

[mike93lx]

Dont you have to use clips with 1/2" roof sheathing?

on 24" centers, yes.

in this application, I am not worried, although it would add strength for sure.

 

[KenC]

1/2" ply or even 7/16" OSB is approved for roofs with rafters at 24" centers. Clips are required of course to keep the edges aligned, not really to add overall strength. I always use two clips per space if any pieces are less that the full width. .

Nobody has ever fallen through a roof built that way to my knowledge, so it should be fine for attic flooring.

 

[esmith2039]

I used 7/16s OSB on ours. No flexing that I'm aware of. Granted we're all around 150-170.. but any weight should be ok as long as your not planning a barn dance.

 

[Chuckles]

Need? This is a space where I slide a bin up and maybe crawl around on my hands and knees. No engine blocks going up there

I should check my current stuff actually. It is 24 OC and feels like 1/2

Go with the 1/4" luan board

 

[spudley]

Go with the 1/4" luan board

I used 1/2" osb on 24"ctr. Works fine, just over 200lbs.

 

[Snip]

Now i'm sure that thicker would be better to an extent, but I did mine about 20 years ago with 7/16 OSB and have had no issues to date. Rafters are 24" oc with a 24' span (insulated and finished 5/8 drywall) 4/12 pitch, so hands and knees only. I have had SB chevy heads along with the manifold, heavy duty engine stand, 2 sets of Harley flywheel sets along with all the other **** i collected in those 20 years. Then again i may have swung a SB chevy in and out of my truck hanging from a 4x4 laid across 4 rafters.
Not saying what should or should not be done, just what has worked for me.

 

[mike93lx]

Go with the 1/4" luan board

Well that will be easier than 3/4 to haul home on the roof of my prius!

 

[NUTTSGT]

I used 1/2" OSB for my garage attic. I had to rip it down to 24" wide pieces to fit up there. There is some bowing but I'm not up there all the time so I'm not really worried about it. The ceiling joists are 2x6, 24" OC.