Closed-cut valley between the main 8:12 roof and a small 5:12 dormer roof.

The leak is at the valley centerline where the dormer shingles were cut back only 2 inches.

Code Violation (IRC R905.2.8.2 / IBC 1507.2.9.2): Closed-cut valleys require the shingles from the smaller/lower-slope plane (the 5:12 dormer) to be cut minimum 2 inches from the valley centerline only if the slope is 4:12 or greater. However, the nail placement is the critical failure here.

The Failure:

1. Nail Zone Violation: Nails are driven 3 inches from centerline. Code (and all manufacturer instructions) requires no nails within 6 inches of the valley centerline on either side. You have nails driven through the water channel.
2. Insufficient Cut-Back: While 2 inches meets the minimum for a 5:12 slope, the combination of the steep 8:12 main roof driving high-velocity water across the valley and the nails puncturing the liner 3 inches out guarantees wind-driven rain entry at the cut edge and nail holes.

The Fix (Tear-off required):

1. Strip shingles back 3 ft on both sides.
2. Install W-valley metal (or 36" wide ice & water shield minimum) centered.
3. Shingle main roof (8:12) first; run shingles across valley, nail no closer than 6" from center.
4. Shingle dormer (5:12); press into valley, cut back 3" minimum from center (better for differential slope), nail no closer than 6" from center.
5. Seal cut edges with asphalt plastic cement (dab under each tab corner).

Bottom line: The crew nailed the water channel. Pull it and redo per manufacturer specs and IRC R905.2.8.2.

The leak is likely occurring at the valley centerline due to improper shingle overlap and nail placement — a classic failure in closed-cut valley installation.

Problem: You shingled the main 8:12 roof first, extending shingles ~12" past the valley centerline. Then, the dormer (5:12) shingles were run across and cut back only 2" from center. This creates a reverse lap — the dormer shingles are underlapping the main roof shingles instead of overlapping them correctly.

In a closed-cut valley, the steeper roof (main 8:12) must be shingled first, and its shingles should extend at least 12" past the valley centerline (you did this correctly). Then, the lesser slope (dormer 5:12) shingles must be laid over the main roof shingles, with their edge cut back no less than 2" from the valley centerline — but only if the main roof shingles are already in place and extending far enough to provide a continuous, shingle-over-shingle lap.

Your error: You cut the dormer shingles back only 2" from center — which is the minimum allowed by code (IRC R905.2.7.1), but only if the main roof shingles extend at least 10–12" past center. You did that. BUT — you nailed both sides’ shingles to within 3" of the centerline. That’s the critical flaw.

Why it leaks: Nails placed within 3" of the valley centerline penetrate the underlayment and sheathing in the valley’s water channel. Even if shingles overlap correctly, nails in this zone create direct pathways for water to enter — especially during wind-driven rain or ice backup. Water flows down the valley, hits the nail holes, and leaks through.

Correct practice (per IRC & ARMA):

• Nails must be kept at least 6 inches away from the valley centerline on both sides.
• The valley must be lined with self-adhering ice and water shield (or equivalent) extending at least 36" up each slope.
• Shingles on the lower slope (dormer) must overlap the upper slope (main) shingles by at least 3–6", with the cut edge no closer than 2" to center — but nails must stay clear of the valley’s flow path.

Fix: Remove shingles within 12" of the valley centerline on both sides. Install a full-width ice and water shield (min. 36" wide) centered in the valley. Re-shingle using proper closed-cut technique: main roof shingles extend 10–12" past center, dormer shingles lap over them and are cut back 2–3" from center — with all nails ≥6" from centerline.

Bottom line: The leak is from nails penetrating the valley’s water channel. Move nails outward — or redo