I have minor sagging of my roof plywood, 3/8" thick, placed on trusses 24" apart. Is there an amount of sag that is technically okay? I could not find anything on the APA website. I am not worried about cosmetic appearance of the sagging, but wondering what my limits are for structural integrity.

  • I would think any amount of sagging is probably bad, especially if there is no weight overhead. It is normal for it to sag a bit when you are walking on the roof but is it sagging by itself? Is the roof wet or does there appear to be any current or previous water damage? Do you see any evidence of termites or carpenter ants anywhere nearby? – maple_shaft Jan 23 '13 at 20:21
  • shingles are pretty heavy! – Steven Jan 23 '13 at 21:43

Courtesy of @lsiunsuex in chat: http://www.tecotested.com/techtips/pdf/tt_plywooddesigncapacities

Max Live Load Deflection = span/240, Max Total Deflection = span/180.

So for a 24 inch span, the deflection should be < ~1/8 of an inch. (2.1333 sixteenths)

  • The span/240 and span/180 numbers in that document are example requirements for an example calculation. They are not limits for the structure's safety. – ArgentoSapiens Jan 23 '13 at 21:35
  • Right, deflection is never a life-safety issue, that is the realm of bending and shear failures in respect to structural spans. Yet those ratios are generally accepted as reasonable limits from a serviceability standpoint. Then again, the ratios are usually applied to structural members, not sheathing elements. 24" span for 3/8" plywood is considered an acceptable application, but barely so. It will not fail under normal conditions, but significant deflection would be expected. – bcworkz Jan 23 '13 at 22:48
  • Thanks. I think my deflection is about 1/4 - 1/2" between each truss, which is about the same as all my neighbors in my 15 year old neighborhood. It can definately be seen from the ground. Just wondering at what point it goes from cosmetic to structural. – Peter Jan 24 '13 at 12:44

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