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Old 08-22-2014, 02:42 PM
382 posts, read 507,112 times
Reputation: 230


I am looking to lay plywood, OSB or other material, largely for safety purposes as we expect to be doing some network, coax wiring via the attic, and don't want to rely on correctly stepping on each rafter (partially obscured by insulation). We don't plan on using for storage, but who knows the future (e.g. another homeowner).

One concern is weight support. Lots of info on web about calculating this stuff, but unless you know which measures are needed, it could be way off.

Hopefully, someone can provide some guidance here.

  • The attic looks similar in structure to this, with web roof bracing and collar tie beams: http://www.summerville-home-inspecto...bracing%20.jpg
  • What is different is there are also "center beams" from the peak about every nine feet, but they don't fall perpendicular to the peak. Instead, they connect to a load bearing wall (hallway wall below) that is between 1 and 2 feet from center. The roof bracing on one side is tied to that same load bearing wall, and the other side is tied to the other load wall in that hallway.
  • The peak is over 10 feet from the joist tops (plenty of clearance for walking).
  • All beams are 2 x 6 lumber. Joists beams are 24 inches on center from each other.
  • The span from where each joist meets the roof on each side is about 30 feet.
  • The roof line is more complicated than a simple rectangle, but for this purpose, I presume we can assume a rectangle for the main area I want to address - it is 50 feet, with joists use beams that are 24 inches on center apart from each other.
  • There is a pull down ladder for access.
  • House was built in the 2000s.


The confusing part is that in calculating load, what is the "span" to use? Is it the space between each roof bracing along the joist? Then, how does the center beam factor in? Or, is it the full 30 feet?

Maybe it is simpler to determine if any of these should be ruled out, given the above description...

Cheapest seems to be OSB, but I understand it is heavy:
19/32 in. x 24 in. x 4 ft. OSB Attic Decking Board-14876 at The Home Depot

Plywood is a bit lighter (as I understand) than OSB, but is pricier. No tongue and groove in the same size, thickness as the OSB. Will have to cut to size to fit through attic access. So a little more effort.

Another alternative is this lightweight product (much easier install, but quite a bit more expensive):
Attic Dek 16 in. x 24 in. Attic Flooring Panels (10-Pack)-3509 at The Home Depot

I anticipate the plactic Attic Dek will not be an issue for weight. Would thinner OSB or Plywood work, if they are too heavy at 19/32 thickness?

Hope this is enough info to go on. If not, let me know.

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Old 08-22-2014, 07:32 PM
Location: Johns Creek, GA
13,162 posts, read 50,627,040 times
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Calculating load would be based on the size of the ceiling joist, the clear span (from wall-wall), and the spacing between them- but for right now; forget about that.

The biggest mistake that most H/O's make is the one you're getting ready to do- add more decking to an attic area that is not designed for it AND reducing the R-value of your insulation by reducing its thickness and/or compressing it. The only decking that should be there is for an attic furnace (if so equipped) or a platform for minimal storage and standing to make visual inspections of the attic.
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Old 08-22-2014, 08:33 PM
Location: Knoxville
4,333 posts, read 21,354,324 times
Reputation: 5201
If you are just looking for something to put down while you install wiring, then 3/4" plywood or 1x lumber should be just fine. Keep in mind that whatever you use will compress the insulation, and you will lose R value in those areas. However, for temporary walk boards while you do work, its not really an issue.
Unless you are really heavy (over 250 lbs) 3/4" should be fine. If you need more support, use 2x10's for walk boards.
If you remove them after you have done the work, you will have little disturbance with the insulation.
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Old 08-22-2014, 10:22 PM
Location: Somewhere in northern Alabama
18,760 posts, read 55,897,290 times
Reputation: 33052
For use setting wiring 1/2" ply ripped into 2' x 8' strips would be fine. It'll bend but unless you intentionally jump up and down won't break. Remove the walkways after use and don't even think about using the space for storage.
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Old 04-03-2015, 10:10 AM
382 posts, read 507,112 times
Reputation: 230
Thanks! Forgot to say it to y'all for providing a response.
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