Rain shoots over the gutter at the roof valley: why it happens and how to fix it

In the post on r/Roofing, the owner described shingles at the bottom of the valleys that overhang the drip edge by a wide margin. A valley gathers water from two roof planes and drops it into one short stretch of gutter. The Roofing Brief, a trade publication, works an example: a valley draining 400 square feet in a 4-inch-per-hour storm sends about 16.6 gallons a minute into a spot 1 to 2 feet wide, 10 to 20 times what a straight eave delivers to each foot of gutter.

Overhang is the first thing to measure. GAF's technical bulletin TAB-R-141 calls for shingles to extend 3/8 to 3/4 inch past the drip edge and says anything beyond 3/4 inch is unsupported and can crack and break off. The same bulletin wants the shingles to reach past the inside edge of the gutter so water drops into it. A long tab at the foot of a valley gives fast water a ramp over the gutter lip, and trimming it back to the maker's limit costs little.

The valley cut is the second check. In a closed-cut valley, makers call for clipping the top corner of each cut shingle at about 45 degrees so water is turned back toward the valley center, then bedding the cut edge in roofing cement. CertainTeed specifies a 2-inch clip for Landmark, Owens Corning 1 inch, and GAF asks for a clip without giving a size.

If water still jumps the gutter, work on the gutter. A splash guard is a plate 3 to 5 inches tall fixed inside the gutter where the valley discharges, and retail guards run about $7 to $25 apiece, according to The Roofing Brief. It will not help if the whole run overflows, the gutter is clogged or pitched less than 1/4 inch per 10 feet, or a solid cover sits over it. In those cases the answer is a larger gutter or another downspout.