Hein has brackets that mount to plastic pads which are then secured to the roof with 3M VHB tape. Its what I recently used to put four solar panels on the top of our van. Three 80/20 15-series crossbars supporting two rows of panels. Once you get over the idea of them being only 'glued' to the roof, it works great.
By the way, we gave it one helluva first test. Heading out of town last week in the worst windstorm in 50 years. Driving 60mph into a 40mph headwind and crosswind. No problems with the panels or tower mounts.
The tower, mounted to my crossbar, and the panel mount tabs.
The 'pad'; 1/4" ABS plastic with full coverage of 3M VHB tape to secure it to the roof. The inserts are shouldered on the backside, will not pull through. I went 'proper overkill' when I did the final mount and used blue Loctite on the mounting screws.
Full setup mounted, lifted it up to get a 'paranoia' coverage of Dicor sealant to prevent and moisture messing up the VHB tape (which wasn't likely, and maybe not needed, but I had the Dicor, and didn't mind the work).
Finished setup, with crossbar and tower visible behind my combiner box.
Thanks Zyzzyx the pictures are great, now I understand "glued to the roof". Still a little faith is required for someone like me stuck on through bolting. It seems like strength depends on paint adhesion unless you sand to bare metal.
It does. But looking at the layout, I think the large majority of the forces are in shear load. Especially once I get a fairing built to cover the front crossbar. Having used small bits of VHB before on other projects, I'm confident in it. Especially after our recent drive in the winds. Though as you mention, faith in the paint adhesion is required.
For those people worried that 8020 is expensive, search on Craigslist for "8020 Aluminum", T-Slot" and "T-Rail". All the big technology companies use it and when they go out of business you get to buy all of their 8020 for pennies on the dollar. In the San Francisco Bay Area and Boston area there are always 80% off sales on used 8020. You want this stuff to build inside your cargo area for strength and to increase your MPG by using low weight materials. You don't need any expensive fittings, you just slide plain old carriage bolts into the T-Slots. Some of the surplus sellers, though, will give you all of the fittings for free, just to get rid of them. The used stuff is just as good as the new stuff. It lasts forever and can be reconfigured endlessly. It is your own personal adult erector set. Just check Craigslst weekly and EBAY under "8020 Alternative" for low prices.