From the factory, all sheet laps happen over a rib. We try to make our roof raises look as factory-made as possible, so we lap the new panels in the same places and use the factory rivet spacing. Our lap joints at ribs generally cover both sides of the rib channel, so that overlap is 4 inches or so. We seal the joint, and add strength, with 3M 5200 adhesive sealant.
If you need to lap the panels and make a joint which is not supported by a rib, you want to make sure that your panel structure has enough strength to hold a rivet securely. On thin sheet metal, especially with beefy rivets, the rivet may not act as it was designed...it can expand all the way to the head, which doesn't really hold the joint well. So, if your sheets are thin it's a good idea to add a washer on the inside of the rivet. The rivet then pulls the washer against the inside skin and pulls the joint together. That, combined with a good sealant, can make a strong lap joint.
EDIT to add: I just remembered that we showed the use of washers in one of our roof raise videos. And, in places like the new horizontal joint over the windshield, I add a strip of 1/8-inch thick steel across the seam and rivet through that. That adds strength to the lap joint and also stops "oil canning" while you drive. That strip is easy to place over the flat part of the seam, but for the curved front corners we use washers behind those rivets.
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