Just finished redoing all the screws in the metal roof of the house. They periodically back out and cause leaks because they are in the valley of the metal sheet.... not on top of the rib. Unknown why they back out.. perhaps because the underlying material is 1x4 pine... not plywood. Yes, the screws are the kind that have a rubber washer to prevent leaks. They unscrew and allow leaks.
I have had to do the roof several times now....
First time, I took out EVERY screw in a valley, put down a tar material using a caulking gun and replaced the screw.
Some backed out again in about 2 years. So for those I screwed them back in and put more tar on them.
This has occured ever few years now for about 15 years... here is what I have learned... best I can recall...
the regular, cheap roof tar hardens, doesn't stick properly to the metal, lots of leaks
the latex based tar gets brittle, cracks, leaks
The silicone material I used once has been excellent... only ONE screw has backed out so far.... and, it is very tightly adhered to the roof metal.
Most recently, I have used HENRY brand elastometric material from Home Depot.. about $7 a tube... it is ruberized and has fiber in it... during application, it is clear that it is superior to any prior stuff for adhering to the roof with the possible exception of the silicone. Reading carefully, this was the only material that indicated it could be used for metal roof fasteners...
My net... use either the best silicone material you can find... read the label carefully... or the HENRY material... there are several types of HENRY material... be sure and get the one that says it is for metal roof fasteners.