Wood shingles, not always cedar because there are cypress and oak too, need only not to have their edges align. In fact, if you lay cedar shingles over felt here in Dallas they will rot.
Concrete tile roofs used to be laid directly over lathe and if one cracked the roof leak was disastrous. Later a therma-ply sheathing was used; we called it denny board. Remember that old aluminum colored cardboard put around homes before house wrap was invented?
Metal roofs leak into the roof and attic because the installers use caulk instead of gravity to keep the water out. If your Gerard or Decra stone coated steel roofing company turns the steel up outside your roof-to-wall nexus, caulks the top and places a covering over it to hide the nastiness... you will pay thousands to repair the damages later.
Flat roofs leak because they are flat and God didn't put it on. In fact God was not a roofer. He was the son of a carpenter. So roofers quit putting His logo on your stuff. The ground moves and the flat roof tears. The nails back out and the roof leaks bad. My favorite is when I went to check on my new flat built up roof that I personally installed in Preston Center for Mr. Lobello. I found a screw driver stuck in the roof. An HVAC guy came over and shouted down from the parking garage that was higher than my leaking roof, "Hey, that's my screwdriver!" I asked if he and I could go visit Mr. Lobello to verify that it was his. He was slightly slower than the rest and Mr. Lobello's office was one floor down. I was saved. The poor slob just wanted to drain the roof, while it was raining, so he wouldn't be electrocuted by the 1,000,000 watt A/C system that cooled the complex. I'm sure by now the government has zapped somebody with his mitochondrial DNA.
Ah, the composition roof leak. The song of drip, drip, drip, ker splat.
Read my previous blog on why composition roofs leak for more info.
Jon Alan Wright
Jon Wright Roofing, Siding, and Windows
1915 Peters Rd., Suite 310
Irving, TX 75061