Sheet metal screws
Spent most of the day putting up furring strips with sheet metal screws. Sill seal placed between the wood and metal as squeak deterrent. I know there are lots of different ways to do it, but this technique seems to work for me...
1. Mark on the wood where you intend to run the screw. i.e. safe places in the underlaying metal that don't already have holes. Don't use areas if you don't know what is behind the metal. Such as a gas tank, or the track for the sliding door, or the exterior skin of the van. Use a screw of the appropriate length.
2. Drill a hole thru the wood the same diameter as the outside edges of the screw threads. So the screw will spin freely in the hole and NOT push the wood away during installation.
3. If you are using flat head screws, countersink the hole just enough so the screw is flush or slightly submerged. Too deep and you weaken the wood. Not applicable if you use round / pan head screws.
4. Holding or clamping the wood in place, drill a pilot hole for the screw, the same or slightly smaller diameter than the full shank of the screw. USE A DRILL STOP! (You don't want to inadvertently go too far.) If you have more than one screw (and you likely will), do each separately, i.e. drill the hole, install the screw, drill the next hole, install the next screw, and so on. It is probable that you are attaching to a slightly curved surface, and as the wood conforms to the curve, the target hole will have moved.
5. I personally do not use self drilling sheet metal screws. Some have a drill bit tip, some have wings that cut wood but break off in metal. Nothing wrong with them, but I prefer to "feel" how the screw is doing. And I lose that ability with a power driver.
6. Here is where I attempt to describe how a typical sheet metal screw should feel. As mentioned, it should spin freely in the wood, so you will easily find the pilot hole with the tip of the screw. The tip is pointed, so the threads here are tiny and fragile, making this IMHO the most likely place it will fail. You need to keep strong pressure on the screwdriver to force the screw tip to bite into the sheet metal. You will definitely get a blister on your palm. After a few quarter turns, the screw should have cut threads into the sheet metal - the resistance to turning drops off notably and it will continue sinking into the wood, pulling the wood tightly against the metal. If not, the tip of the screw may have failed and it will just continue to spin and not cut threads. Throw that screw away and use a new one. Note - my experience is that stainless screws are weaker, and more likely to fail at this point during installation. So I prefer Zinc for interior / protected locations.
7. Once you have cut threads, just hand tighten to firmly snug. I doubt you will be able to strip the hole by hand, but with a power tool it becomes more likely. You may go back over them as you move along, as as the wood conforms it may affect this. If you leave access, check and retighten if required, in the event they back out or loosen during use. If buried, consider some way to ensure they don't back out. I have had good results filling the top of countersunk screws with JB Weld and scraping it flush, like a spackle. When cured it holds the screw head, kind of a poor man's version of Loctite for screws. YMMV.
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A few critical places are through-bolted, but IMHO sheet metal screws (almost 150 of them) are sufficient for the ceiling. The ceiling itself will be light, and a high G (front collision) scenario would test the combined shear strength of ALL the screws, not their individual pull-out strength. In a roll-over, all bets are off, as metal gets bent many things can break free and the inside of the van becomes a blender. Y'all be careful out there.