Sheet Metal Screws vs. Wood Screws

I do a lot of repair work on items built of wood. And I frequently use sheet metal screws, especially if I want pan head screws. Easier to find pan head metal screws than pan head wood screws. I do drill a pilot hole thru both pieces and a clearance hole thru the outer piece.

Sheet metal screws will work just fine for that application. That is what I used (3/4" #8 screws) to secure my solar panels to the roof 7 years ago. And sealed with Dicor underneath and above the brackets.

Wood screws generally have a much coarser, deeper thread. I wouldn't recommend using them to hold panels to the roof while driving down the road.

Here is what most people don't realize about screws is that they are supposed to work like a bolt. A bolt slips through the item to be secured then into a nut that tightens things together. A screw should be applied the same way. The screw slips through the item to be secured then into the wood or other material that will act as a bolt.

Wood screws have a neck that facilitate the slipping through the item to be secured. A sheet metal screw needs a clearance hole in the item to be secured, which is usually thin a therefore does not need a neck.

Sheet metal or drywall screws are IMO the most misused screw, I see wo 2x4's attached with drywall screws that have a space between them. That's because the first 2x4 acts like a nut then spins trying to drill a hole in the second one (the gap) an then taps into the 2x4.

Sorry for the rant, this is a pet peeve of mine!

Yes filling a pretty drilled screw hole with a sealant is the best way on the exterior of a RV.

The prospect of drilling holes in the roof gives me the heebie-geebies. But, if'n it was me, I'd use a heavy sheet metal screw with a large pan head for secure contact with the metal bracket.

I would use Star head or Square head WOOD screws.

And more importantly, screws are a very effective way of channeling water into materials. To protect your roof do what is common on Sailboats and other yachts.

Drill a small pilot hole for the screw and fill it with silicone sealant. Then insert your screw. after the screws are in cover them with more sealant. I would also put sealant under the "feet" or mounting bracket of the panels.

This should seal the threads and under the head and not allow water ingress.

You definitely do not want water getting in to the plywood structure of your roof.

I had heard that metal screws can be used in place of wood screws, but not the other way around.

I prefer sheet metal screws to wood screws, but have no technical basis for that opinion other than they seem easier to screw in without splitting the wood. Otherwise, I mostly use deck screws for wood projects. I almost always pre-drill the holes.