Of the hundreds of shingle roofs put on each day, how many are put on with metal drip edges? Almost all shingle roof manufacturers show it on their installation instructions and the Asphalt Roofing Manufacturers Association "Residential Asphalt Roofing Manual" shows it in its recommended application procedures. The International One-and-Two-Family Dwelling Code's Chapter 9, "Roof Coverings," states that "Asphalt shingles shall be applied according to the manufacturer's printed instructions…"
So why do so few residential shingle roof installations lack metal drip edges? For one reason, the metal edging is the first item to get omitted from a bid whenever a price is given to install a shingle roof, whether on a new roofing project or a reroof. Unless the specifications or the scope of work expressly calls it out, this item will be omitted in both the submittals and the installation. Even if drip edge is called for, are such particulars as the type of metal, the gauge, and the dimensions ever given? Or is a thin aluminum drip strip from the local home center adequate?
One "old timer" actually tried to justify the absence of drip edge material on a project by saying that he had been putting on shingles for almost 15 years and did not think drip edge was necessary-whether it was called for or not. His mindset was that if you extend the shingles far enough over the edge of the deck into the gutters, you shouldn't need any edge metal.
Most Critical Drip Edge Location
Obviously, the most critical location for drip metal is exactly at the location of the majority of the drips-the eaves! Rake edges should also get metal edging, but it is simply not as critical. And the installation sequence of the edge metal with the felt underlayment is optional in most manufacturers' printed instructions. You can install the felt either on top of or below the edge metal. And since water travels somewhat parallel to the rakes, this does minimize the need for coverage.
However, the edge receiving the most water on a steep-sloped roof needs the best protection affordable. If the shingles are extended much more than 3/4-1-inch over the edge, they tend to bend, eventually fracturing along the edge of the roof deck below. If the metal edging is left off, this decreases the chance for all of the water cascading over the eaves to make it into the gutters, if indeed there are any. Otherwise, shingles breaking along the line of the roof deck allow the possibility of water getting into the substrate by turning back up under the bottom of the shingle. The deleterious effects of this condition are exacerbated when the underlayment is not fully extending over the edge and/or if the fascia board is not flush with the lower edge of the wood roof deck.
Many times the roof deck is installed early on in the project and covered with felt in order to dry the house in and speed up the interior work below. Roofers will hurriedly run a cutter along the edge of the wood deck, but rarely do it in a straight line. This leaves the edge of the felt somewhat short of the edge, thereby failing to overhang the roof deck. It only gets worse when the (usually 3/4-1" thick) fascia board is added later when the finish work is being done. This makes the felt edge that much farther from the true drip edge.
Water Under Shingles is Damaging
If water now gets under the shingles, it can possibly cause short-term staining and long-term deterioration of the lowest edge of the roof deck and along the top of the fascia board. Prolonged existence of this condition can also affect the ends of the roof joists or trusses used to attach the fascia board.
This should point out to builders to specify, install, and insist on adequate metal drip edge to give residential clients the most value for a relatively low-cost item. It is ironic that contractors who leave out the metal drip edge in an effort to save money lessen the long-term value of a home. It is an issue whose absence and consequential side effects may take years to discover. But, in all fairness to the consumer, it is an item that should be included to proved a quality installation.