Underlayment & Flashing: What Matters Most Under Your Shingles

The Layers Nobody Sees Are the Ones That Matter Most

When homeowners think about a new roof, they think about shingles — the color, the brand, the warranty. And that makes sense. Shingles are what you see every day.

But here's what most roofing contractors won't tell you: the layers underneath your shingles are far more important than the shingles themselves. Specifically, two components do the real heavy lifting when it comes to keeping water out of your home: underlayment and flashing.

A roof with premium shingles but poorly installed underlayment and flashing will leak. A roof with mid-range shingles but meticulous underlayment and flashing work will keep your home dry for decades. The difference comes down to the details that most homeowners never see — and that too many contractors cut corners on.

What Is Roof Underlayment?

Underlayment is a water-resistant (or waterproof) barrier installed directly on top of the roof deck (the plywood sheathing), underneath the shingles. Think of it as your roof's second line of defense.

Shingles are designed to shed water — they overlap like fish scales so rain runs down and off the roof. But shingles aren't waterproof on their own. Wind-driven rain, ice dams, and damaged shingles can all allow water to reach the layer beneath. When that happens, underlayment is what stands between that water and the wood structure of your home.

Types of Underlayment

For most residential roofs in the Lehigh Valley, the standard approach is synthetic underlayment across the full deck with ice and water shield at all vulnerable areas: eaves (minimum 24 inches past the interior wall line per PA code), valleys, and around penetrations like chimneys, skylights, and vent pipes.

Synthetic underlayment has largely replaced felt paper in professional roofing. It's lighter, stronger, lies flatter, won't wrinkle when exposed to moisture, and provides better traction for the crew during installation — which means a safer, faster job.

Why Cap Nails Matter More Than You Think

Here's where the first major quality gap appears between contractors: how the underlayment is fastened to the deck.

There are three ways to attach synthetic underlayment:

1. 01Staples — Fast and cheap. The staple punctures the underlayment but doesn't seal around the hole. Over time, water can wick through these punctures. Most manufacturer installation instructions explicitly prohibit or discourage staples for synthetic underlayment.

1. 01Standard roofing nails — Better than staples, but the small nail head can pull through synthetic underlayment in high winds. The nail hole is also not sealed.

1. 01Cap nails (plastic-cap or metal-cap nails) — The gold standard. Cap nails have a wide plastic or metal disc (about 1 inch in diameter) that distributes the holding force across a much larger area. This prevents pull-through in wind, and the cap creates a tighter seal around the penetration point.

At RoofOps, we use Stinger NailPac cap nails on every underlayment installation. The Stinger NailPac system is a coil of cap nails designed for pneumatic nail guns — each nail has an integrated 1-inch plastic cap that locks the underlayment firmly to the deck.

Why does this matter to you as a homeowner?

• ■Wind resistance: During a nor'easter or high-wind event, underlayment fastened with staples or standard nails can tear free and expose the bare deck. Cap nails hold.
• ■Warranty compliance: CertainTeed, GAF, and Owens Corning all require or strongly recommend cap nails for synthetic underlayment installation. Using staples can void your manufacturer warranty — and you'd never know until you file a claim.
• ■Water resistance: Every nail penetration is a potential leak point. Cap nails minimize this risk by sealing a larger area around each fastener.

This is one of those details that takes an extra 15-20 minutes on a typical roof but makes a meaningful difference in long-term performance. It's also one of the easiest ways to tell whether your contractor is doing the job right or cutting corners. Ask your contractor: are you using cap nails on the underlayment? If the answer is staples or standard nails, that's a red flag.

Learn more about every layer of a properly built roof system on our Anatomy of a Roof guide.

What Is Roof Flashing?

Flashing is thin metal (usually aluminum or galvanized steel) installed at every point where the roof surface meets a vertical surface, changes direction, or has a penetration. Its job is to redirect water away from these vulnerable joints and into the shingle system where it can drain normally.

Flashing is installed at:

• ■Chimneys — all four sides, with step flashing along the sides and counter-flashing at the top
• ■Walls — where a roof meets a sidewall or headwall (common on dormers and additions)
• ■Valleys — where two roof planes meet and water concentrates
• ■Vent pipes — plumbing vents, exhaust fans, and other roof penetrations
• ■Skylights — around the entire perimeter
• ■Drip edge — along the eaves and rakes (sides) of the roof

Flashing failures are the number one cause of roof leaks — not shingle failures. A shingle can be damaged and the roof won't leak if the flashing and underlayment are doing their job. But compromised flashing will leak regardless of how new or expensive the shingles are.

Chimney Flashing: Where Most Contractors Cut Corners

Chimney flashing is the most complex and most commonly botched flashing detail on a residential roof. A properly flashed chimney requires multiple layers of metal working together:

1. 01Base flashing (apron) — An L-shaped piece of metal at the front (downslope side) of the chimney that directs water around the sides
2. 02Step flashing — Individual L-shaped pieces woven into each course of shingles along the sides of the chimney, creating a stair-step pattern
3. 03Counter-flashing — A second layer of metal installed over the top of the step flashing, with its upper edge set into a reglet (groove) cut into the mortar joints of the chimney masonry
4. 04Back pan / cricket — A raised diverter on the upslope side of the chimney that prevents water and debris from pooling behind the chimney

The Counter-Flashing Difference

This is where the quality gap between contractors becomes most visible.

The shortcut (what most contractors do): Apply a bead of roofing sealant or caulk along the top edge of the step flashing where it meets the chimney. This is fast — takes about 10 minutes. It also fails within 3-5 years as the sealant dries out, cracks, and separates from the masonry. Once it fails, water runs behind the flashing and into your home.

The right way (what RoofOps does): Cut a groove (reglet) into the mortar joint of the chimney using a grinder, insert the top edge of the counter-flashing into the groove, and seal it with high-quality polyurethane sealant. The metal is physically locked into the masonry — it can't pull away, and the sealant is protected inside the joint rather than exposed to UV and weather on the surface.

This takes longer. It requires a grinder, skill, and patience. But it creates a flashing detail that will last 25-30 years instead of 3-5 years. Over the life of your roof, that's the difference between zero leaks and multiple service calls.

Crickets: The Detail Most Roofs Are Missing

A cricket (also called a saddle) is a small peaked structure built on the upslope side of a chimney. Its job is to divert water and debris around the chimney instead of letting it pool behind it.

Pennsylvania building code requires a cricket on any chimney wider than 30 inches. But even on smaller chimneys, a cricket is good practice — especially in the Lehigh Valley where heavy snowfall, ice, and leaf debris are common.

Without a cricket, water and debris accumulate behind the chimney. Standing water accelerates flashing deterioration. Wet leaves create a dam effect. Ice forms and expands against the flashing. Eventually, the flashing fails and water enters the home — often causing damage to the ceiling and walls of the room directly below.

At RoofOps, we install crickets wherever they're needed — not just where code requires them. It's one of the details that separates a roof built to last from a roof built to pass inspection.

See all six quality differentiators we bring to every project on our homepage.

Valley Flashing: Open vs. Closed

Valleys — where two roof planes meet at an angle — concentrate more water than any other part of your roof. How the valley is flashed determines how well it handles that water over time.

For the Lehigh Valley climate — with heavy rain, snow, ice, and freeze-thaw cycles — open metal valleys are the most reliable choice. The exposed metal allows maximum water and snowmelt flow, is easy to inspect during maintenance, and lasts the full life of the roof. We use this method on the majority of our installations.

Drip Edge: The Detail at the Very Edge

Drip edge is an L-shaped metal strip installed along the eaves and rakes of the roof. It serves three purposes:

1. 01Directs water into the gutter instead of allowing it to run down the fascia board and behind the gutter
2. 02Prevents wind-driven rain from getting under the shingles at the roof edge
3. 03Provides a clean, finished edge that protects the exposed wood of the roof deck

Pennsylvania building code requires drip edge on all new roof installations. Despite this, some contractors skip it to save time and material — especially on re-roofs where the old drip edge is removed with the shingles and not replaced.

Drip edge is inexpensive (typically $1-2 per linear foot installed) and takes minimal time to install. There is no good reason to skip it. If your contractor's estimate doesn't include drip edge, ask why.

How to Evaluate Your Contractor's Underlayment and Flashing Work

You can't inspect underlayment after the shingles are installed — it's hidden. And most flashing details are difficult to evaluate from the ground. That's why it's critical to ask the right questions before the job starts:

Questions to Ask

1. 01What type of underlayment do you use? (Synthetic is the standard; felt is outdated)
2. 02How do you fasten the underlayment? (Cap nails = right answer; staples = red flag)
3. 03Where do you install ice and water shield? (Eaves, valleys, penetrations at minimum)
4. 04How do you flash the chimney? (Counter-flashing cut into mortar joints = right answer; caulk only = red flag)
5. 05Do you install crickets behind chimneys? (Yes, where needed = right answer)
6. 06What type of valley flashing do you use? (Open metal is most durable for PA climate)
7. 07Do you install new drip edge? (Should always be yes on a full replacement)
8. 08Can I see photos of the underlayment and flashing before the shingles go on? (A quality contractor will document this — at RoofOps, we photograph every stage)

What to Look for During Installation

If you're home during the installation, here are visual cues that the underlayment and flashing are being done right:

• ■Underlayment covers the entire deck with proper overlap (typically 4-6 inches)
• ■Cap nails are visible — you can see the round plastic caps holding the underlayment down
• ■Ice and water shield is installed at the eaves — it's a different color (usually black or dark gray) than the synthetic underlayment (usually gray or light-colored)
• ■Step flashing is individual pieces, not one continuous bent strip
• ■A grinder is used at the chimney — if you hear a grinder, they're cutting reglets for counter-flashing (good sign)
• ■Metal is visible in the valleys before shingles are installed

The RoofOps Standard

At RoofOps, underlayment and flashing aren't afterthoughts — they're the foundation of every roof we build. Here's what we do on every project:

• ■Full-deck synthetic underlayment fastened with Stinger NailPac cap nails
• ■Ice and water shield at all eaves, valleys, and penetrations
• ■Precision chimney counter-flashing cut into mortar joints — never surface-sealed with caulk alone
• ■Crickets installed wherever water or debris could pool behind a chimney or wall intersection
• ■Open metal valleys for maximum water flow and longevity
• ■New drip edge on every full replacement
• ■Photo documentation of every layer before it's covered by the next

These details are part of what we do that others skip. They're why we're CertainTeed ShingleMaster certified — the top 1% of roofing contractors. And they're why our roofs are built to handle everything the Lehigh Valley climate throws at them: ice dams, nor'easters, summer thunderstorms, and decades of freeze-thaw cycles.

Schedule a Free Inspection

Want to know what's under your current shingles? RoofOps offers free, no-obligation roof inspections throughout the Lehigh Valley, Bucks County, and Montgomery County. We'll assess your roof's condition — including the underlayment and flashing — and give you an honest assessment of what needs attention.

Call (835) 248-0004 or schedule your free inspection today.

RoofOps is a veteran-owned, CertainTeed ShingleMaster certified roofing contractor serving Bethlehem, Nazareth, Allentown, Easton, and the greater Lehigh Valley. PA HIC #200014.