plans call for plywood on inside of roof - why?

[atomicskier]

Hey all,
   I'm making my materials list and this is the first time I've closely looked at the plans. I purchased online but not from countryplans (found the site too late ) - anyway, I've got a 20x30 building with a 2/12 roof that's a single slope. the roof decking is to be 1/2" plywood which makes sense, but it also calls for 1/2" plywood on the underside of the joists - essentially the deiling of the living space - why? why not just deck, insulation, drywall? I'm not sure what the point of plywoood on the interior ceiling is, any clues?

[MountainDon]

No.   

[hpinson]

Probably just as an inexpensive ceiling?   Finished, sanded plywood can be made to look fairly nice, and different veneers are available (maple, oak, birch, etc). A thinner plywood might ripple a bit.  Assuming you want it there in the first place, maybe to hide insulation? I can't think of any reason a more attractive material could not be substituted.

[CjAl]

i have found most all 1/2" plywood to be in miserable condition. i would use 5/8

[PEG688]

 Specifically what type of "1/2" plywood" does it call for? 

I'd guess like some others have suggested it would be considered a finished ceiling , to cover insulation, wiring , plumbing vent pipes etc.

I some instances,  IF you use a wood ceiling,  you need to provide a fire resistant barrier paper beneath the wood to slow fire spread.   FRK paper,  IIRC is the acronym. 

I say you could substitute drywall  / sheet-rock / Gypsum wall board / instead of the plywood to create your finished ceiling.