35 years a go, the house was built with the roof having a 24 in span between truss. which was a little spongie to walk on.
it was reshingled this year.
what would you tell your client ?
when the roof was reshingled they should have used thicker plywood to strenghten the roof.
or they should have added bracing to the truse already there or added more truss, while the roof was off.
any other ideas?
Hard to say Eragorn, without more information and pictures.
35 years ago in Northern Maine, I was building duplexes and quadplexs, and the roofs were all stick built with 2x6 rafters or homemade trusses two feet apart with 5/8" CDX plywood on the roof for sheathing.
Was never an issue with spongyness.
Maybe they had used code minimum on the one your talking about, which is 3/8" :mrgreen:
Pictures would help. 
here are some pic o the attic.
Well, blowing up that first pic to 400%, the plywood through the hole sure don’t look like 5/8"
7/16" at best.
Ironically, I inspected a home built in 1975 today, and it happened to be a modular home and found the inspection sticker underneath the sink cabinets. Same as your picture and spongy when I walked the roof.
Plywood was very expensive in 1973-77. A lot of contractors cut down to the thinner plywood and that is what you got.
I could not find a grade stamp today either. But it definitly was not 5/8" plywood.
The use of h-clips had not been out yet either.
Should be fine. But would strongly recommend they not have double layer of shingles.
Hope this helps.
thats the funny thing, i think the roof was redone, there is only one layer of shingles. no water or ice sheild either.