Thanks everyone.....but you're talking just a bit over my head. I've seen deflection calculators on other sites, but it's for joists, not trusses. Where can I find a way to calculate the "L-factor (?)" for the open web trusses I have and if the number is within the safety range can I go ahead with just putting down the DitraXL; what are dead and live loads; will I find the exterior grade ply at HD or Lowe's?