Roofing — Shingle Replacement, Flashing, and Leak Tracing

Overview

A residential asphalt shingle roof is a layered system: decking (plywood or OSB sheathing), underlayment (synthetic felt or ice-and-water shield), drip edge metal at the eaves and rakes, and shingles installed from the eave upward in overlapping courses. Each layer sheds water to the layer below it. The system works only when every layer is intact and properly lapped. One reversed lap or lifted shingle and water gets behind the system.

Roof work is performed at height on a sloped surface. Falls from residential roofs are the leading cause of fatality in construction. Do not begin work without appropriate fall protection, a stable ladder, and favorable weather — no rain, no frost, no wind above 15 mph.

Prerequisites

• Extension ladder: extend at least 3 feet above the eave. Set the base at a 75° angle (1 foot out for every 4 feet up). Secure the top against lateral movement.
• Fall protection: harness, lanyard, rope grab, and a ridge anchor or temporary roof anchor rated for 5,000 lbs. Anchor into a rafter, not just through the sheathing.
• Roofing nailer (pneumatic) loaded with 1-1/4" galvanized coil nails, or a hammer and roofing nails if working by hand
• Flat pry bar for lifting shingles and removing nails
• Utility knife with hook blades for cutting shingles
• Chalk line for snapping course lines
• Matching shingles. Asphalt shingles fade and vary by lot. An exact color match on a repair is not always possible.
• Roofing cement (also called mastic or plastic cement) and a caulk gun
• Bucket and rope for hauling materials up. Do not carry materials while climbing a ladder.

Leak Tracing

Roof leaks rarely appear directly below the breach. Water enters at a penetration or lifted shingle and runs along the underside of the sheathing, rafters, or ceiling joists before dripping onto the drywall. The stain on the ceiling may be several feet from the actual roof penetration.

Start from the attic if accessible. During or immediately after rain, go into the attic with a flashlight and trace the water. Follow the wet trail uphill along the rafters to the point where it first appears on the underside of the sheathing. Mark this spot by pushing a nail or wire up through the sheathing so you can find it from the roof surface.

If no attic access, work from the exterior. Common leak sources in order of likelihood: pipe boots (the rubber flanges around plumbing vents), chimney flashing, step flashing where the roof meets a wall, valleys, and damaged or missing shingles. Inspect each systematically.

Replacing Damaged Shingles

Each shingle has two nail lines: one concealed by the shingle above it, and the visible exposure below the tab line. To replace a shingle, you must access both its nails and the nails of the shingle above it that pass through the top edge of the damaged shingle.

Step 1: Break the sealant strip

Asphalt shingles have a thermally-activated adhesive strip that bonds each course to the one below it. On a warm day, carefully slide the flat pry bar under the shingle above the damaged one and pry upward to break this seal. Work slowly — aggressive prying will crack the shingle above, creating a second problem.

In cold weather (below 40°F), shingles are brittle. Warm the sealant line with a heat gun before attempting to break it, or wait for a warmer day.

Step 2: Remove nails

Each three-tab shingle is held by 4 nails (6 in high-wind zones). They are located approximately 1" above each tab cutout and 1" from each end. Slide the pry bar under the damaged shingle, locate each nail head by feel, and lever it out. Then lift the shingle above and remove the nails that pass through the top edge of the damaged shingle.

Step 3: Slide out the damaged shingle and slide in the replacement

Slide the new shingle into position, aligning it with the adjacent shingles so the tabs are consistent and the exposure (the visible portion) matches the surrounding courses — typically 5" or 5-5/8" depending on the shingle type.

Step 4: Nail the replacement

Nail the new shingle following the manufacturer's nailing pattern. The nails should penetrate through the shingle, through the underlying shingle's top edge, and into the sheathing. Each nail should be driven flush — not countersunk (the nail head should not break through the shingle surface) and not proud (sticking up, which creates a bump under the shingle above).

Seal the lifted shingles back down. Apply a quarter-sized dab of roofing cement under each tab that was lifted, and press firmly. The sun will eventually reactivate the adhesive strip, but the cement provides immediate hold.

Chimney Flashing

Chimney leaks are the most common call for roof repair. The flashing system around a chimney consists of step flashing (L-shaped pieces woven into the shingle courses along the sides), base flashing at the front, counter flashing (embedded in the mortar joints and folded down over the step flashing), and a cricket or saddle on the uphill side to divert water around the chimney.

If the counter flashing has pulled away from the mortar joint, clean out the joint to a depth of 1" with an angle grinder and a diamond blade or a cold chisel. Press new counter flashing into the joint and seal with high-temperature urethane caulk. Tuck-point the joint with mortar for a permanent repair.

If the step flashing itself has failed, the shingles along the chimney must be removed to access and replace the step flashing pieces. Each piece should extend at least 4" up the chimney wall and 4" under the shingles, with each piece overlapping the one below it by at least 2".

Common Mistakes

• Walking on a wet or frost-covered roof. Asphalt shingles have zero traction when wet. Moss-covered shingles are worse. If the roof is not completely dry, do not get on it.
• Applying roofing cement as a permanent fix over flashing or shingles. Cement is a sealant, not a structural repair. It cracks and shrinks over 2–5 years. Use it as a supplement to proper mechanical flashing, not a replacement for it.
• Not weaving shingles into existing courses. A patch that simply overlaps existing shingles without being integrated into the nail and overlap pattern will leak at the seams.
• Nailing in the wrong location. Nails placed too high don't hold the shingle below. Nails placed too low are exposed and create a leak point. Follow the manufacturer's nailing line — it is indicated on the shingle.