I think the more common term for "rat run" is Strong back. I suppose you can do whatever you want to with your ceiling but I think the first question you should ask is 'is the ceiling built or designed for a floor for living space or storage.
If living space the joists should be designed for the type of loading that you intend for up there. Is it a bedroom that will need 30# live loading or other living space that requires 40# live loading?
Light storage for about 10# live loading, etc.
Will there be a permanent stairway access or just a pull down staircase?
There are many things to be considered before just jumping in and tacking plywood for a "floor".
Also if you plan to use this area for storage and keep it unheated by adding plywood your going to be cutting way down on the avalible area for proper inshttp://www.energystar.gov/index.cfm?c=home_sealing.hm_improvement_insulation_tableulation.
Thanks for the responses. I have checked with my Truss manufacturer and although the space is not designed for living, they say I can load up to 25 lbs psf in the attic with the trusses installed. I plan to add 1/2" plywood and 2x4 supports between each truss bottom cord at 2 foot intervals. The weight for this I will add will equal about 3.0 psf. Then I plan on storing my wifes christmas and other holiday decorations, suitcases, cat cages, etc. I also will install a pull down stair.
What do you think can I remove the Strong Back that was nailed in the center of the space?
A “strong back” and a “rat run” are two different things where I come from.
A strong back consist of two 2x’s nailed in a L shape to stiffen up ceiling joist. A rat run is a single 2X nailed flat on top of the bottom cored of the trusses and have no other purpose other than keeping the bottom of the trusses on layout. You nail the rat run on layout prier to nailing the bottom cord of the trusses to the top of interior walls.
If what you have is a rat run (cat walk) then it could be removed imo. Whether or not your trusses can support storage weight is another story.
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