My current pickup has a fiberglass cap with "pseudo-gutter" brackets that slide in tracks bolted thru the cap. No leaks or cracks in 8 years of hauling canoes and kayaks. Most of my boating friends with fg caps also have not had problems with cracks or leaks. However, most of us bought caps that appeared to have more beef (thicker fg, foam core roof or stiffing ribs, etc.) to avoid problems - there are some pretty flimsy caps put there. FTR, I got the rail system when I ordered the cap since the ability to adjust the rack spacing seemed potentially useful. The system has held up well but I have never changed the spacing. Next time I'll just get Yak brackets and bolt them on.

My previous pickup had an "cheap" aluminum-skinned cap with an aluminum frame. I bolted Yak 4-bolt side-mount brackets directly to the frame rails and hoops since the top has no real strength. This worked well structurally. Leak-wise, well, cheap alum caps are notorius leakers, especially after you drop boats on them. This one leaked when I got it but it was free . . .

I would bolt the brackets to the side of a Gem-top on account of the leak potential of top drilling, especially if the top-mount dimpled the sheet metal. You also avoid whamming your head into a bolt if you bump the ceiling inside the cap. Strength-wise, I would guess that either top or side would be fine, the Gem-tops seem fairly abuse-resistant in commercial usage.

: I've got one set of Yakima gutter mount adapters mounted on the side of my
: fiberglass shell. (The front rack bar is mounted on the roof of my
: pickup). I haven't had any problems. It doesn't leak. There is no
: movement...the mount is rock-solid. It sounds like your previous set-up
: could have been a little loose? (either the through bolts weren't
: sufficiently tight or the holes in the shell were little too big so it
: could wiggle?)

: Julie Kanarr