The rafters can take the weight but the roof span inbetween them cannot hold you. Like Mack stated you need to spread out the load to the rafters to support you. You can feel them under the rubber. They are on 16" centers for the most part.
See here: Need a good sturdy ladder to get up there.
Then put something down to protect the rubber from abrasion. I use a canvas tarp but outers have used old carpet or anything cloth like to not have the plywood diretly on the rubber. I am servicing the AC unit in this pic.
I use 5/8" thick plywood pieces 2 foot x 4 foot as those are what I have laying around. Wide enough to span 2 rafters and light enough to lift going up the ladder. I would not go much thinner then 3/8" thick plywood. 1/2" and 5/8 works well. 3/4 is great but heavy to get up there.
Hope this helps
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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