Space them four inches apart along the edges and six inches apart in the middle of the plywood sheathing.
Reinforce existing roof trusses.
Connect roof sheathing to the roof framing with ten penny weight common or eight penny weight ring shank nails.
Extra support for the trusses can be made by placing a 2 by 4 inch piece of lumber from each truss on one side to the truss on the other side high enough to provide head room but low enough to provide support for the trusses.
Simply take 2 diagonal pieces and place them between the trusses.
The first step to reinforcing the roof is to add an x to the frame.
Take 2 6x4 as long as your longest stored lumber and cut 2 inch deep grooves in them as wide as your 2x6 is thick every foot or so.
The plan is to tear out the drywall ceiling in the garage place 16 2 4 floor trusses 19 o c.
Then sister 2 12 s to the existing trusses and use plywood gussets with an engineered nail pattern in place of a ridge beam.
Use wood adhesive underneath the sheathing along the connecting joint with the roof trusses.
Throw in some collar ties and your done.
And build the decking on top of that.
Put on the adhesive as you would caulk.
Apply construction adhesive along the edge of the truss to strengthen the connection to the plywood roof deck.
The beam extends to the far end of the roof hanging over the trusses still sitting there.
Stiffen trusses by joining them with 2x4s running from one end of the house to the other.
In this situation the walls of a finished attic space or room would go with the roof line.
Install these over your existing horizontal 2x6s 2 feet from the roof edge inside and paralel to the roof ends crosswise from your existing 2x6.