Fasteners for roof: self-piercing or self-drilling?

[Dr Paddle]

I'm about to remount the original 1" aluminum tubing roof rack that came with my 2001 Grandby. I had to repair it after the original owner apparently snagged a tree limb. (Embarrassingly, I am that original owner <sigh>.)

Is there a functional difference between using self-piercing (sharp piercing tip) and self-drilling (cutting blades on tip) screws on the roof? Does either one hold better? The factory used self-piercing screws, but that could have been for manufacturing efficiency. I intend to put butyl tape under the rack to prevent leakage that might lead to corrosion.

Many thanks for this help and for the help you collectively have provided before. With your help, I've installed new factory lift panels and lift cylinders, repaired broken frame members under the front window, fixed torn siding, fixed rotted plywood, and more!

Kevin in San Diego

 

[Wandering Sagebrush]

The only thing I can think of is possibly needing a slightly larger screw.

 

[Jon R]

Use galvanized - not stainless steel. Stainless steel and aluminum in contact causes corrosion of the aluminum.

 

[Lighthawk]

Butyl tape is the industry standard, although can be messy as it oozes under high temps.
I would shoot a little lap sealant into each screw hole to help glue them in and provide another level of waterproofing.

 

[PokyBro]

In my experience in doing a full rebuild, self-piercing screws offer a tighter, more secure hold than self-drilling screws, because the self-drilling screws by cutting, create a larger hole before the threads bite. I think you get slightly more thread formation on the receiving stock with the self-piercing screws, and that is my opinion why they’re slightly more secure, at least on the very thin aluminum tubing thickness used on the campers. Just my .02!

 

[Dr Paddle]

In my experience in doing a full rebuild, self-piercing screws offer a tighter, more secure hold than self-drilling screws, because the self-drilling screws by cutting, create a larger hole before the threads bite. I think you get slightly more thread formation on the receiving stock with the self-piercing screws, and that is my opinion why they’re slightly more secure, at least on the very thin aluminum tubing thickness used on the campers. Just my .02!

Thanks! That was is what I sort of suspected, but could find no supporting information on the web. Seems there would be more metal for the self-piercing screw threads to bite, since the metal is deformed but not removed by the screw.

 

[BlueSky]

I always drill a small pilot hole and then use anything but a self-drilling screw. I use a drill driver and then tighten the last few threads by hand with a screwdriver. I have also found that, in the roof, sealant squirted into the hole is a must.

 

[Beach]

Any fine thread self tapping will work well. As mentioned, squirting a dab of sealant in the hole not only acts as a water seal but will lock the screw in so it's unlikely to back out.

 

[Sleddog]

In my experience in doing a full rebuild, self-piercing screws offer a tighter, more secure hold than self-drilling screws, because the self-drilling screws by cutting, create a larger hole before the threads bite. I think you get slightly more thread formation on the receiving stock with the self-piercing screws, and that is my opinion why they’re slightly more secure, at least on the very thin aluminum tubing thickness used on the campers. Just my .02!

I completely agree here, self drilling screws did not hold well. I ended up drilling a small pilot hole then using self piercing screws.

 

[craig333]

You're not the first one to snag a tree limb. I like to use an adhesive sealant in the holes like 3m 4200.