How Do You Install Flashing on Asphalt, Metal, Tile, and Flat Roofs?

What’s the most common mistake I see that leads to a leaky roof, even with brand-new materials? It’s always the flashing. A tiny gap or wrong piece here can funnel water right into your home’s structure, but doing it properly creates a bulletproof seal.

I’ll explain the critical techniques for asphalt shingle, metal, tile, and flat roof flashing to give you a watertight, long-lasting result.

Key Takeaways: What Every Homeowner Should Know First

Before you pick up a tool, get these core ideas straight.

• Flashing seals the joints where different parts of your roof meet. These joints are the most common leak points on any home.
• Installation is completely different for asphalt, metal, tile, and flat roofs. Using the wrong method guarantees a leak.
• Your personal safety is non-negotiable. A leak is annoying; a fall is life-changing.
• Proper flashing must integrate with the roof underlayment. It’s a system, not just a piece of metal.
• Flashing failure, not shingle wear, is the number one cause of roof leaks.

Simple repairs on a low-pitch roof might be a DIY project. For anything on a steep roof, around a chimney, or in a complex valley, you should call a professional roofer.

Flashing 101: What It Is and When You Can Repair or Replace It

Think of flashing as the rain jacket for your roof’s seams. It’s the metal (like aluminum, copper, or galvanized steel) or modern fabric strips that bridge gaps and direct water away from vulnerable spots.

Chimneys, valleys, and walls fail first because that’s where movement and water pressure are greatest. Your roof expands and contracts with temperature, and these junctions take the stress. It’s like the seam under the arm of a jacket. It gets the most strain and often fails first.

You probably have a few specific questions.

Can you replace flashing without replacing the roof? Yes, you often can if the surrounding shingles, tiles, or metal panels are still in good shape. I’ve done it many times to fix a leaky chimney without re-roofing the whole house.

Can you install step or kickout flashing on an existing roof? Yes, but it’s delicate work. It involves carefully lifting and disassembling the adjacent shingles or siding to tuck the new flashing in correctly. If done poorly, you create new leak points.

One of the best upgrades in the last 30 years is ice and water shield. This is a sticky, rubberized underlayment that acts as a critical backup layer. I install it under all flashing in valleys, against walls, and around chimneys. If the metal fails, this self-sealing membrane is your last line of defense.

The “Leak Point” Analysis: Valleys, Chimneys, Walls, and Pipes

Focus your inspection here. These four areas cause most of the leaks I get called to fix.

Roof Valleys: This is where two roof slopes meet, forming a V-shape. Water from two entire planes funnels into this one channel. The flashing here must be wide and sturdy enough to handle a concentrated river of water during a storm.

Chimneys and Skylights: These are massive objects sticking out of your roof plane. Water hits them and tries to go around. Flashing here is a multi-part system (step flashing on the sides, counter flashing on the chimney) that must allow the chimney to settle independently from the roof.

Roof-to-Wall Joints: This is where your roof meets a sidewall of your house. Water runs down the wall and can get sucked behind the siding by capillary action (like a paper towel soaking up water). Kickout flashing is essential here to eject water clear of the wall and into the gutter.

Pipe Vents: These small rubber or metal boots seal around plumbing pipes. The sun’s UV rays bake the rubber over 10-15 years, making it brittle and cracked. They’re a small, cheap part that causes big leaks.

In every one of these spots, the golden rule is to layer everything like shingles. Upper layers must always overlap lower layers so water sheds downward and out, never finding a path in.

The Pro’s Toolbox: Flashing Types and Material Choices

Think of flashing as your roof’s tailored raincoat. It’s custom-fitted to seal the spots where water wants to sneak in. Getting the type and material wrong is like wearing a wool coat in a monsoon. It just won’t work.

Common Flashing Types and Where They Go

Each flashing has a specific job. Using the wrong one is a common mistake I’ve had to fix on repair calls.

• Step Flashing: This is for where a roof meets a sidewall. It looks like little L-shaped metal stairs. You weave one piece under each shingle as you work up the slope. Step flashing creates a shingled, overlapping barrier that guides water down and away from the wall.
• Continuous (Apron) Flashing: Imagine a long, straight piece of metal bent at a 90-degree angle. You use this along the bottom of a wall or chimney where the roof meets it at the base. It’s a single, unbroken piece that kicks water onto the shingles below.
• Counterflashing: This is step flashing’s partner. You embed it into a wall’s mortar joint or siding, then bend it down to cover the top of the step flashing. Counterflashing acts as a cap, preventing water from getting behind the step flashing.
• Valley Flashing: Installed in the V-shaped trough where two roof slopes meet. It’s a wide, open channel that acts like a fast lane for rainwater to rush off the roof.
• Pipe Flashing: A collar, often made of rubber or metal with a rubber boot, that fits snugly around plumbing vent pipes. It seals the round hole in the shingles.
• Drip Edge: This is the metal trim installed along the roof’s edges. It isn’t just for looks. A proper drip edge controls water runoff, directs it into the gutters, and protects the underlying roof deck from wind-driven rain.

Choosing Your Flashing Material

The best material depends on your roof type, your budget, and how long you want it to last. I’ve worked with them all, including different types of shingles.

• Galvanized Steel: Steel coated with zinc. It’s strong and affordable. The zinc coating eventually wears off, leading to rust. I see it fail often on older homes. It’s a decent budget option for a short-term fix.
• Aluminum: Lightweight, easy to bend on-site, and it won’t rust. It’s my go-to for most asphalt shingle jobs. Aluminum is cost-effective and reliable, but it can corrode if it touches certain other metals or mortar without a barrier.
• Copper: The premium choice. It’s incredibly durable, develops a protective green patina, and can last as long as the house itself. The cost is high and it requires a skilled installer. Never mix it directly with aluminum or steel.
• PVC and Roofing Tapes: These are flexible membranes or peel-and-stick tapes. They’re fantastic for sealing complex shapes on flat roofs or as a backup sealant. They are not a standalone structural flashing on steep roofs.

Quick Guide: Best Flashing Material for Your Roof

Matching the material to the roof is half the battle. Here’s a simple guide from my years on the job.

• Asphalt Shingle Roofs: Aluminum is the standard. It’s compatible, easy to work with, and lasts. Copper is the upgrade for critical areas like valleys.
• Metal Roofs: Match the flashing to the roof metal. Use aluminum flashing for an aluminum roof, steel for a steel roof. This prevents galvanic corrosion.
• Tile Roofs: You need a malleable metal. Lead-coated copper is traditional and excellent. Heavy-gauge aluminum also works well under the weight of tiles.
• Flat Roofs (EPDM, TPO, Modified Bitumen): The system is key. Use prefabricated metal base flashings integrated with the membrane manufacturer’s peel-and-stick or liquid-applied flashing tapes.

Does Roofing Felt Go Under or Over Flashing?

This question causes more leaks than almost any other. The rule is simple, but you must follow the order.

Base flashing, the part that lays on the roof deck, always goes over the roofing underlayment (felt or synthetic). Think of it as a sandwich. The roof deck is the plate. The underlayment is the bread. The base flashing is the meat, placed on top.

Here is how you layer it correctly, step by step.

1. Install your roof deck sheathing (plywood or OSB).
2. Roll out and secure your roofing underlayment over the entire deck.
3. Now, install your base flashings. This includes step flashing, valley metal, and the bottom piece of chimney flashing. These pieces sit directly on the underlayment.
4. Install your roof covering (shingles, metal, tile) over the base flashings, weaving them in as you go.
5. Finally, install the counterflashing. This piece is embedded into the wall or mortar and bends down to cover the top of the base flashing and roof material.

The counterflashing is the final seal. It always goes on top of everything else. Getting this sequence wrong traps water under the flashing, guaranteeing a leak into your home’s structure.

Roof Work Safety: Gear and Risks You Can’t Ignore

Let me be clear from the start. Installing or repairing flashing is not a beginner’s project. If your roof is steep, high, or complex, hire a professional. I’ve seen too many close calls from homeowners who underestimated the pitch.

Non-Negotiable Safety Gear

If you are tackling a low-pitch, single-story roof, this gear is your first line of defense. Don’t skip any of it.

• Fall Arrest System: This is a roof-rated safety harness, properly attached to a certified roof anchor. A rope and a belt from the hardware store won’t save you.
• Sturdy Footwear: You need boots with a defined, rubber heel and deep tread. Sneakers on roof granules are like ice skates.
• Roof Jacks or Brackets: For any slope you can’t comfortably sit on, you need these to create a stable, level platform to work from.
• Eye Protection: Metal flashing can throw sharp slivers when cut. Roof cement and debris can also get kicked up.

The Hidden Risks on Every Roof

The roof itself is often the biggest hazard. Here’s what we look for before we even set a ladder.

• Pitch Steepness: A roof that looks manageable from the ground can be a sheer wall up close. If it’s steeper than a 6-in-12 pitch (imagine a 6-inch rise for every 12 inches of run), it’s pro territory.
• Rotten Decking: The plywood or boards under the shingles can look solid but be completely rotten from a slow leak. Your foot can punch right through, leading to a severe fall or leg injury.
• Power Lines: Always look up and around. Aluminum ladders and metal flashing are perfect conductors of electricity.
• Weather Conditions: Never work on a wet or even damp roof. A light morning dew is enough to make it slick. Wind can turn a sheet of metal flashing into a dangerous sail.

My rule is simple: if a single part of the safety checklist gives you pause, that’s your sign to call a licensed roofer. A fall from your roof can change your life. A professional repair bill is just an invoice.

Asphalt Shingle Flashing: Step-by-Step for Walls and Valleys

Most leaks on an asphalt roof start at the joints. The spot where your roof meets a sidewall is prime territory for water trouble. That’s why step flashing is the number one DIY repair I get asked about. Getting it right keeps your attic dry.

Here is my crew’s method for installing step flashing along a wall. Follow these steps in order.

1. Carefully lift the shingles along the wall using a flat bar. Go slow to avoid cracking them.
2. If you’re in an area with ice or heavy rain, tuck a 12-inch wide strip of ice and water shield under the shingles against the wall. This membrane is your backup plan.
3. Start at the bottom. Slide the first L-shaped flashing piece under the underlayment and over the shingle course below it. The vertical leg goes against the wall.
4. Secure the flashing by nailing only through its horizontal leg, into the roof deck. Use two roofing nails placed high so the next shingle will cover them.
5. Lay the shingle back down over that flashing piece. The shingle should completely cover the horizontal leg.
6. Move up to the next course. Install the next flashing piece, overlapping the one below by at least two inches. Repeat the process: under the underlayment, over the shingle, nail the horizontal leg, cover with the shingle.
7. Seal the top edge where the flashing meets the wall with a bead of roofing sealant. Do not rely on sealant alone to hold the flashing.

Can you use flashing tape on a roof? Yes, as a supplemental sealant on seams, but not as a primary flashing replacement. I use high-quality flashing tape to seal the overlaps between step flashing pieces or around nail heads. It is a great helper, but it will not replace properly bent and installed metal.

Roof valleys work like a funnel for rainwater. Installing valley flashing correctly is non negotiable. You have two choices: open metal or closed cut. An open valley uses a wide, exposed strip of metal down the center. Shingles are trimmed back about three inches from the center line. A closed valley has the shingles woven over the flashing. No matter which style you choose, keep all roofing nails at least six inches away from the valley centerline to avoid creating your own leak points. For most homes, I recommend an open metal valley for its superior water flow and longevity.

Installing Step Flashing on an Existing Asphalt Roof

Fixing bad flashing on an old roof is a common call. You will need a flat bar, roofing nails, a hammer, roofing sealant, and pre bent step flashing pieces. A pair of gloves saves your hands from sharp edges.

The sequence is what makes it waterproof. You must install one piece of flashing per course of shingles, and each piece must be overlapped by the shingle installed above it. Think of it like putting on a raincoat one button at a time. If you try to install multiple flashing pieces and then shingle over them, water will find a way in.

I have re paired dozens of roofs where the mistake was simple but costly. Never, ever nail through the vertical leg of the step flashing where it touches the wall. Those nail holes direct water straight into your sheathing and wall framing. I once spent a week replacing rotted wood on a garage because a previous handyman made this exact error. Only ever fasten the horizontal part that lies on the roof deck.

Metal Roof Flashing: Valleys, Seams, and Penetrations

The biggest shift when you move from asphalt to metal is the system itself. A metal roof is like a car with specific, proprietary parts. The flashing for standing seam or snap-lock panels is often designed by the panel manufacturer and must be used for the warranty to be valid. You cannot simply grab generic flashing from the supplier. Using the wrong profile is a guaranteed leak.

Installing a Proper Metal Roof Valley

An open metal valley is the standard and most reliable method. We use a wide piece of painted steel or aluminum, often 24 to 30 inches wide, as the valley liner. The key is how you attach it. You never, ever put a nail or screw through the center of the valley channel where water flows. That’s asking for trouble.

Instead, you secure the valley metal with cleats or clips. These are small metal tabs that get fastened along the edges, outside the main water channel. The metal roofing panels then overlap the valley liner, leaving a clear gap in the center for water to run off. Always seal the overlap between the panel and the valley metal with a high-quality, UV-resistant sealant made for metal, applying a consistent bead during residential metal roof installation. I’ve seen too many valleys fail from using cheap, non-compatible caulk that cracks and shrinks in a year.

Sealing Pipes and Vents on Metal Panels

This is where many DIY jobs go wrong. The standard rubber plumbing boot for asphalt shingles will not work correctly on the ribs of a metal panel. You need a boot designed for the task.

• Gasketed Metal Flashing Boots: These have a custom-fitted metal base that matches your panel’s profile. A rubber gasket seals around the pipe. You secure it with screws through the raised rib, never the flat panel.
• Butyl Tape is Your Friend: Before setting any flashing, apply a thick bead of butyl tape sealant under the entire base. This non-hardening sealant conforms to the panel’s shape and creates a primary water barrier. It’s far superior to trying to caulk after installation.

The Critical Importance of Precise Cutting

Cutting a metal panel is a final act. There’s no fixing a bad cut. If you notch a panel too wide for a pipe or cut a valley angle poorly, you’ve broken the weathertight system. Always measure twice, account for the panel’s overlap, and use proper metal snips or a nibbler. Avoid using a grinder if you can. The intense heat can burn off the protective factory coating (the galvanized layer and paint), leaving a raw edge that will rust quickly. A clean, precise cut preserves the panel’s integrity. On my crew, we treat every cut like it will be front and center on the house, because in a way, it is. Knowing the best cutting techniques for metal roofing panels helps keep edges tight and coatings intact. Familiarize yourself with the right tool, blade, and technique before you start.

Tile Roof Flashing: Chimneys, Hips, and Delicate Work

Tile roofs are beautiful and durable, but flashing them is a whole different ballgame. The material itself dictates the rules. I have a vivid memory of a client who tried to save money by having a general handyman flash their chimney. He cracked three Spanish tiles just walking up the roof. That mistake cost more to fix than proper flashing would have in the first place.

Handle with Care: The Brittle Nature of Tile

You cannot treat a tile roof like an asphalt shingle roof. Tiles are brittle ceramics or concrete. Stepping on them directly will crack them. You must distribute your weight by crawling along the roof battens (the wooden strips the tiles hang from) or by using thick planks laid across multiple rows of tiles. If you’re hiring someone, watch how they approach the roof. If they just walk up, send them away.

Flashing a Chimney on a Tile Roof

This is where true craftsmanship shows. Rushing leads to leaks and breakage. Follow these steps in order.

1. Remove tiles carefully. Work from the top down. Gently lift the overlapping tiles and slide out the ones around the chimney. Have a soft place to lay them so they don’t chip.
2. Install a cricket or saddle. This is a peaked structure built on the roof, uphill of the chimney. It is absolutely critical on tile roofs. A cricket splits water and snow around the chimney, preventing a dam from forming. Without one, you are asking for a leak.
3. Lay the base flashing. This is the first layer of metal, usually a continuous piece that goes up the cricket and sides of the chimney. It gets integrated under the roof underlayment or ice and water shield. Every seam must be sealed.
4. Embed the counterflashing. The final layer. A groove (reglet) is cut into the chimney mortar. The top edge of this metal flashing is bent and tucked into that groove, then sealed. The bottom edge laps over the base flashing. This two-part system lets the roof move independently of the chimney while keeping water out.

Integrating Flashing with Tile Profiles

This is the fine detail work. A flat piece of metal won’t work under a curved mission tile or the interlocking ridges of a barrel tile. Step flashing along a wall must be custom-bent or shaped to follow the contour of the tile above it. For a mission tile, the flashing often needs a pronounced “kick” or bend to tuck up under the tile’s rounded lower edge. The goal is to direct water onto the top of the tile below, not behind it.

Choosing the Right Metal

Aluminum or galvanized steel, common on shingle roofs, are often too stiff for tile work. We almost always use lead-coated copper or pure lead for critical tile flashings. Why? Malleability. You can gently form these softer metals to the exact shape of the tile profile without springing back. They also last for decades. It’s a case where the right, more expensive material saves money on future repairs.

Flat Roof Flashing: Pipes, Edges, and Preventing Leaks

A flat roof changes the game. On a sloped roof, water runs off. On a flat roof, water sits and finds a way in. Your flashing can’t just be good. It must be perfect.

Flat roof flashing is less about shedding water and more about creating a permanent, watertight seal that works with the roof’s drainage system. If the seal fails, water pools right on top of it, guaranteeing a leak.

Installing Flashing Around a Vent Pipe

Pipes are the most common leak source on a flat roof. Here’s how to flash one correctly.

1. Cut the Membrane: Using a sharp utility knife, cut an X in the roof membrane directly over the pipe’s center. Make the cuts just long enough to fit the pipe through.
2. Install the Boot or Flange: Slide a pre-fabricated pipe boot (for smaller pipes) or a metal flange (for larger stacks) over the pipe. Pull the membrane flaps up and around the pipe’s base.
3. Seal the Base: This is the critical step.
   • For PVC or TPO roofs, the base is heat-welded to the main membrane using a hot air welder. This fuses the materials into one piece.
   • For modified bitumen or built-up roofs, apply a heavy layer of approved roof mastic (asphalt-based sealant) under the boot’s flange and press it firmly into place.
4. Reinforce: Cover the boot’s entire flange with a layer of flashing tape or reinforcement fabric, embedding it in more mastic. This creates a secondary, flexible seal that moves with the roof.

Edge Flashing (Gravel Stop)

You don’t have gutters on most flat roofs. Instead, you have a metal edge called a gravel stop or fascia. Its job is simple but vital, especially since gutters can sometimes cause leaks and water damage on flat roofs.

The gravel stop is a lipped metal edge that contains the roofing materials and directs all water off the roof in a controlled sheet, right into the scuppers or off the edge. It’s installed first, with the roof membrane lapped up its inside face and sealed thoroughly. If this seam fails, water runs behind your entire roof system and into the walls.

Flashing at Drains and Scuppers

This is where water is supposed to go. The flashing must be flawless.

Roof drains and scuppers (wall drains) have special metal flashing rings. The roof membrane is carefully cut to fit and is then bonded to this ring with weld or mastic. Any gap or weak seam here means ponding water will back up and seep directly into the building at the drainage point, the exact opposite of its purpose. I always double-seal these areas and clear them of debris after every job.

Sealing a Pipe Boot on a Modified Bitumen or EPDM Roof

Let’s get specific. For a common torch-down or EPDM roof, patching a pipe boot is a frequent repair. You need the right materials.

• Pipe boot (EPDM rubber for EPDM roofs, modified bitumen for torch-down)
• Membrane primer (cleans and preps the surface)
• Lap sealant (a high-quality, flexible roofing cement)
• Reinforcement fabric (a mesh that strengthens the seal)

The process is about creating one solid piece.

1. Clean: Scrub the area around the old boot with a brush and roofing cleaner. All dirt, gravel, and old sealant must be gone. Let it dry completely.
2. Prime: Apply a coat of membrane primer to the cleaned area. This helps the new sealant form a chemical bond with the old roof.
3. Seal and Layer: Apply a thick bed of lap sealant where the boot’s flange will sit. Press the new boot into it. Now, cover the entire flange with more sealant. Immediately lay the reinforcement fabric onto the wet sealant, pressing it down. Finally, apply a top, smoothing coat of sealant over the fabric, feathering the edges to create a monolithic, waterproof bond that flexes with temperature changes.

Keeping It Dry: Inspection, Maintenance, and Mistake Avoidance

Flashing needs a regular checkup, just like your car. I tell every homeowner to inspect it twice a year, in spring and fall. Grab a pair of binoculars and scan your roof from the ground; it’s the safest way to check high spots without climbing a ladder. Look for specific signs that trouble is brewing.

What exactly are you looking for? Rust or corrosion on metal flashings means the protective layer is gone and water will find a way in. Search for cracks in the material or in the old sealant. Keep an eye out for lifted edges where the flashing isn’t lying flat against the roof or wall. On my last inspection job, I found a leak because a piece of step flashing had curled up from the freeze-thaw cycle.

After decades on roofs, I’ve fixed leaks caused by the same few errors. Avoid these common mistakes to keep your flashing watertight for years.

• Relying on caulk instead of proper overlaps. Caulk is a helper, not a solution. I’ve peeled back gallons of old silicone to find rotted wood underneath. Flashing must be layered shingle-style so water flows over it, never through it.
• Improper nail placement. Nails belong high on the flashing leg, away from the direct path of water. When I see nails driven low or without a dab of sealant, I know a leak is following close behind.
• Using mismatched metals. Mixing metals like aluminum and copper causes galvanic corrosion. It acts like a battery and quietly eats the metal away. Always use compatible materials to prevent this silent damage.
• Ignoring manufacturer instructions. Every roof type has specific requirements for flashing size, pitch, and attachment. Skipping these steps is a sure way to void warranties and invite failure.

Good maintenance is simple and mostly about keeping things clean. Clear leaves, pine needles, and debris from roof valleys and around flashings every few months. A clogged valley forces water sideways under your shingles. I keep a leaf blower handy for this quick job.

Reapply sealant only where you see a small, clean gap, and use a high-quality roofing sealant. Don’t smother everything in caulk; it can trap moisture and hide bigger problems. If a sealant fails repeatedly, the underlying installation is usually wrong.

Know when a repair is beyond a DIY fix. If multiple flashings are damaged, the roof deck feels soft, or you’re uneasy on a ladder, call a professional. I’ve been hired too many times to fix a well-intentioned repair that made the original leak worse.

Here’s the bottom line from my experience. Proactive care of your flashing is the simplest way to extend the life of your entire roof. A little attention now prevents big, wet, and expensive surprises later.

Quick Answers

How do I properly inspect and maintain my roof flashing?

Inspect flashing from the ground with binoculars each spring and fall, looking for rust, cracks, or lifted edges. Keep valleys and areas around flashings clear of debris, and reapply a bead of high-quality roofing sealant only where you see small, clean gaps.

What’s the safest way for a homeowner to check flashing on a steep roof?

Your safest move is to not get on a steep roof at all. Use binoculars for a visual inspection and call a pro if you see problems; a repair bill is far cheaper than a hospital visit. For tips on what to look for, check out our roof damage inspection guide.

What’s one hidden flashing mistake I might not see until it’s too late?

Mismatched metals, like aluminum against copper, cause galvanic corrosion that silently eats the metal from the inside out. Always ensure flashing material is compatible with your roof and adjacent metals to prevent this slow failure. This is especially crucial when installing flashing and fasteners for tin metal roofs.

Ensuring a Watertight Seal for Years to Come

No matter your roof type, always install flashing with careful overlap and a complete seal. I’ve seen this single habit prevent countless leaks and save homeowners from major repairs.

Make roof inspections a regular part of your home care routine, and never compromise on safety when checking for issues. Keep learning about durable materials and methods, as informed choices are the foundation of a trustworthy roof.