How to beef up 2x6 ceiling joist to use attic for storage
I have a rancher style home with 2x6 ceiling joist rafters and 2x6 roof joists rafters. I would like to use my attic for storage of normal house hold goods such as seasonal decorations, clothing, books, files, paperwork, etc., not sure of exact weight of items to be stored. I have purchased 3/4 inch plywood to apply to top of the ceiling joist rafters to use for the attic flooring, but I am concerned about the span of the 2x6 rafters as they span 12 foot in one section and 16 foot in another section, all ceiling rafters are on 16 inch centers. My concern is that once I install the 3/4 inch plywood flooring and start using the attic for normal household storage I may start to see the ceiling plaster start to crack due to the weight of the flooring and storage. I am wondering if I can get away with either sistering a ripped down piece of 5 inch x 8 foot plywood to the center of each joist to prevent sagging, or a 2x6 sistered along the entire length, or a 2x8 along the entire lenght would be enough. My budget is tight so I want to use the least expensive method to beef up the ceiling rafters. If someone could please advise me what they think would be the best and least expensive way to go I would greatly appreciated it.