Blazer top sag / repair

[mcampbell]

My friend did the foam trick with bad results. He called one day all excited about how well the foam had lifted the sags out of the top. about 2 hours later the tune changed. As the foam starrted to cure the sags returned to the top layer and started to push the bottom (inside) down. He got the bottom back in place by propping it up with 2 x 4's but didn't discover a way to pull the sags out of the top layer before the foam cured. Maybe if wood screws were put in the sags and wired to the ceiling and the bottom layer were propped up first then the foam would fill and hold it all in place but with out some bracing the foam won't