Inside the van, we need more support and anchoring surface for the nuts than just thin van metal. Per Renogy “A minimum roof thickness of 3/8″ is recommended,” which means we need to build up our van roof. Our components that the nut is compressing up to the van metal are, from top to bottom:
- Neoprene washer (0.065” thick) – to help the rest of the components conform to the roof curve
- Strap tie (20 gauge .0359” thick) – only on the bolts in the center near a roof rib. The metal at the center of the van has less support and flexes easier than metal nearer the sides. After everything was bolted, we used spray foam to “tie in” these straps to a horizontal rib, which we think added structural support to the bracketed area (less flexing/shaking of the roof on bumpy roads).
- 3/8″ Super Strut Square Washer (12 gauge .1046” thick) – adds thickness
- 3/8″ flat washer (0.078”) – more thickness, to meet minimum thickness for expansion nut
This is why we used a 1-1/16″ inch expansion nut, which is made for a material thickness 0.312″ to 0.625″. The van’s sheet metal is about 18 gauge or 0.05”. Total thickness of all components was 0.33”, plus some sections had sound deadening on the sheet metal which adds another ⅛ inch or so.
The other size expansion nut available is ⅝” (which is actually what Renogy specified in the bracket manual) is for material thickness 0.05” to 0.22”. Since we had more than just thin van metal, we needed a longer expansion nut.
After experimentation, we found 1-¼” screws were the shortest that could be used with a 1-1/16” expansion nut. Those were rarely in stock though, so we sized up and used 1-½ inch Machine Screws #10-32. #10-32 is the same threading as the expansion nuts, and stands for a #10 gauge screw, with 32 threads per inch. The screws and washers are stainless steel because they are outside the van and we don’t want them rusting. On top of the van, with the screws we used #10 stainless steel flat washers and #10 stainless steel split lock washers. Split lock washers are common in automotive use and prevent the loosening of a screw from vibration.