The felt under the shingles needs to go over the metal edge on the downside eave and on the rakes, the part of the roof edge that goes up the sides, the felt needs to go under the metal edge. If you use a leak protector like GAF StormGuard and WeatherWatch, Certainteed WinterGuard, Grace Ice and Water, Owens Corning WeatrherLoc, Tamko Metal and Tile and MoistureGuard, or Tarco MS300, you need to install it in the same fashion as the felt.
The metal edging has stiffening beads or ribs on the topside that goes under the shingles that act to keep water wicking from water tension between the starter shingle and the metal edge. The face of the metal edge has no beading because the customers would scream there was.
The next part is the starter shingle. Most roofers use a twenty year shingle turned upside down and backwards rather than use the professional starter shingle that have come out in the last decade. This cheap shingle used even with high end roofing can result in the ladder, or whatever attacking force encounters the roof edge, breaking the first row of roofing. Thus water enters the overhang, the paint on the eave goes bad, the wood rots, and water can end up on the floor having passed down through the outside wall. Plus the roofer needs to offset the starter at least six inches to keep the water from penetrating the eaves.
The starter also helps prevent "blow off" as it has a stronger tensile strength and has additional self sealant near the bottom of the roof 's first row. The three tab starter also causes a bulge further up the roof because it laps too much, lining up the top of the starter and first row. On roofs less than a 6/12 pitch this can help lead to water infiltration.
The soffit is lower than the ceiling and any water that reaches the top of the soffit will not make it to the ceiling. But it can run down the inside of the wall and come out at the baseboard or work its way under the floor to lurk and create primordial soup long strand amino acids waiting for the electrical short from the water logged wall socket.
There is a metal edge called "D" style, which by the way is not shaped like a "D." Whoever he was he had a corrupt sense of humor. The manufacturer seems to think that the extra overhang of the "D" style, which is the same as my index finger, means you don't need to extend the shingle past the edge of the metal. I don't like it because the tightness of the shingle laying in the metal allows water to wick just like water between two pieces of glass. There are extra stiffening beads that act as wicking inhibitors and I've never seen it to be a problem but that doesn't mean I have to like it.
Shake Roofs: We always left a one and a half two two inch over hang on wood roofs, which, by the way, do not use metal edge. At least that is the case hear in Dallas Texas because we don't, ad can't, put cedar over solid decking. The plywood would rot,. We use spaced sheathing, also called lathe. With both the thinner wood shingles, which lay together tightly, or the shakes, that are rough and loose and require a felt underlayment to keep the water out, we need a double course of wood shingles or tapersaw to keep the overhang dry. The shakes also use multiple layers of felt on the eave.
Tile, standing seam, and synthetic roofs use the 3/4 rule too but Gerard and Decra, the stone coated steel tiles and shakes, fit tightly over a fascia metal. This metal is rather wide, which adds protection to the eave that is lacking from having an overhang.
Today's lesson is that roofers are held responsible for the interior of the home in how their roofs function but rarely to property owners realize that the rotting wood and peeling paint on the exterior of their home is the result of poor roofing practices.
Then their is the old Jon Wright mantra of ventilation. Besides the high energy bills, house stink, sick kids, HVAC repairs, and radon gas, you can add the painter's and carpenter's bills to the list of costs that cheap roofer left you to pay.
Ask you roofer this: Does the felt go over or under the metal edge? There are two separate answers. They differ depending if it is the eave or rake and if he gives only answer one he is wrong. Maybe he knows a painter too? And maybe you know an attorney?
Jon Alan Wright
Jon Wright Roofing, Siding, and Windows
1915 Peters Rd., Suite 310
Irving, TX 75061
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