My '98 Chevy Astro didn't have a way to mount a roof rack because there are no gutters or factory installed mounts. This instructable will show you my approach to solving that problem. It does involve drilling holes into the roof of your vehicle. After a few months of testing in adverse conditions, mine has not leaked and has held up to wind and rain while supporting an awning. No guaranteed about carrying capacity etc.
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Step 1: Things You'll Need....
Drill, cordless or otherwise with a drill bit that is exactly the recommended size for the rivet nut you are using. the tolerances for the nuts are tight, and if you use a bit that is slightly over sized, the nut won't crimp properly and you'll be screwed. Remember, you're drilling holes in the roof of your vehicle. Don't try to be cheap by using the wrong bit!
Rivet Nuts and screws. Most hardware stores have a selection of rivet nuts although they are cheaper purchased in a larger package online. They are sometimes referred to as "nutserts". Basically a rivet with a threaded sleeve that crimps around a metal surface.
Rivet Gun. Many options for this one. I bought a cheap model from Harbor Freight (here) but there are much nicer versions and they probably work better as well. This one works okay, but it might not be strong enough for some types of hardened steel nutserts.
Silicone/Caulk: Once the rivet nuts are installed you won't see this, but it's a second line of defense beyond the gasket tape. Alternatively you can buy rivet nuts with a gasket material included. Mc Master Carr sells them.
Utility Knife: To cut stuff
Rubbing Alcohol: Surface cleaning
The Mounting Tower: I made my own out of aluminum. Could be prefab but I just wanted to mount a piece of wood as the rack base.
Step 2: Drill Holes, Caulk, Drop in the Rivet Nuts.
The first step is perhaps the most important. Measure, mark, check, measure again, check and be 100% sure you have located the holes in the right place. Assuming you figure this part out and are comfortable with the location, I would recommend using a punch or similar device to accurately dent the spot you want the drill bit to center on. Otherwise, it could wander slightly off and you want these holes to be accurately placed.
Once the holes are drilled, check first that the rivet nuts will fit through snugly. (I had to slightly open a couple holes with a chainsaw file.) If the nuts seem to fit well in your holes, file back any sharp metal that may be sticking above the surface and clean the area with rubbing alcohol to ensure a good bond with the caulk. Put a tiny bit of silicone or caulk around each hole and drop in the nuts. Use the rivet nut tool to crimp. Clean up any excess caulk.
Step 3: Add Gasket Tape to Seal the Bracket or Tower Around Your Rivet Nuts.
I used some 3M Tape from Ace hardware similar to this tape product on Amazon.com . Basically you want something UV, heat and weather resistant that has a high to permanent bond. I cut holes where the nuts are and stuck the tape to my mounting bracket.(In the photo, I have not yet removed the protective layer from the double sided tape.) Once you peel the tape to expose the second sticky side, carefully place the mounting bracket over the rivet nuts. You will only get one chance to get this right, so don't rush this step. Once your bracket is pressed firmly into place you should have a pretty good seal around the perimeter of the bracket.
Step 4: Add the Screws With Rubber Gaskets Around the Heads.
I used some rubber gaskets I found at ace which fit the screw heads snugly. I added a bit of thread locker to each screw before threading them on. You want to get the screws snug, but not over tight because you will smash the gasket and compromise the seal. The thread locker should hold them in there.
Step 5: Mount Your Towers, Bars and Test the Strength.
Once you've mounted everything, give it a test and make sure you can yank pretty hard on and at least rock the vehicle without the rack moving. I have no idea what the actual strength of this rack would be but I don't load it with anything. I just use it as an anchor for an awning. But I would be totally comfortable mounting a rack and carrying lighter loads on here.