What Goes Under Metal Roofing – Essential Layers For A Leak-Free
The primary material that goes under metal roofing is a high-quality synthetic underlayment, which acts as a secondary moisture barrier. In addition to underlayment, you must install ice and water shields in vulnerable areas like valleys and eaves, along with a sturdy drip edge and closure strips to prevent pests and debris from entering.
For some applications, a radiant barrier or furring strips (battens) may also be used to improve energy efficiency and provide an air gap between the roof deck and the metal panels.
Installing a new metal roof is one of the smartest investments you can make for your home or workshop, offering decades of durability. However, many DIYers focus so much on the color and profile of the panels that they overlook the critical components hidden beneath the steel.
I promise that if you take the time to understand the sub-roofing assembly, you will avoid the common pitfalls of condensation, rot, and premature failure. A metal roof is only as good as the layers supporting it, and getting the “sandwich” right is the key to a professional-grade finish.
In this guide, we will preview the essential materials required for a successful install, from modern synthetic barriers to the specialized flashing that keeps your structure bone-dry. Let’s look at exactly what goes under metal roofing to ensure your project stands the test of time and weather.
The Foundation: Understanding what goes under metal roofing
Before you ever lay down a single sheet of steel, you need to address the roof deck itself. Most residential metal roofs are installed over solid decking, typically made of 7/16-inch OSB or 5/8-inch plywood, which provides a flat, stable surface for the panels.
The layers that sit between the deck and the metal are designed to manage moisture and temperature. Metal is a fantastic conductor of heat, which means condensation can form on the underside of the panels when warm indoor air hits the cold metal surface.
Without the proper materials underneath, this moisture would sit directly on your wood decking, leading to mold and structural decay. Choosing the right components ensures that any water that gets past the metal—or forms beneath it—has a way to escape or evaporate safely.
Synthetic Underlayment vs. Traditional Asphalt Felt
For decades, 30-pound asphalt felt (tar paper) was the industry standard for roofing. However, in the world of modern metal roofing, synthetic underlayment has almost entirely taken over the top spot for professional and DIY installs alike.
Synthetic underlayment is typically made from woven or spun-bonded polyethylene or polypropylene. It is significantly lighter than felt, much stronger, and doesn’t tear when walked upon or during high-wind events before the metal is installed.
One of the biggest advantages of synthetics is their UV resistance. If you are working on a large DIY project and can’t get the metal panels down in one day, synthetic underlayment can remain exposed to the sun for weeks without degrading.
High-Temperature Ratings
Metal roofs get incredibly hot in the summer sun, often reaching temperatures well over 150 degrees Fahrenheit. Traditional felt paper can become brittle and “bake” onto the roof deck or the underside of the metal panels in these conditions.
High-temperature synthetic underlayment is specifically engineered to handle this thermal load. Always check the temperature rating on the product label to ensure it is compatible with a metal roofing system, especially if you are using dark-colored panels.
Slip Resistance for Safety
If you are a DIYer working on a pitched roof, safety is your primary concern. Synthetic underlayment often features a textured top surface that provides better traction for your boots than oily, smooth felt paper.
While you should always use a harness and proper fall protection, having a non-slip surface under your feet makes the installation process much more manageable. This added grip is a “pro” feature that beginners should never overlook when selecting materials.
Ice and Water Shields: Protection for Vulnerable Spots
Even the best underlayment needs help in high-stress areas of the roof. Ice and water shields are self-adhering membranes that provide a “self-healing” seal around fastener penetrations and in areas where water tends to pool or back up.
You should always install these peel-and-stick membranes along the eaves (the bottom edge) of the roof. In cold climates, this protects the deck from ice dams that can force water upward under the metal panels.
Valleys are another critical area where two roof planes meet. These sections handle a massive volume of water during heavy rain, so a double layer of protection is often recommended to prevent leaks in these high-flow zones.
Self-Healing Properties
What makes ice and water shields unique is their rubberized asphalt or butyl composition. When a roofing screw passes through the membrane, the material seals around the shank of the screw, creating a gasket-like fit.
This is vital because every screw you drive into your roof is a potential leak point. By using these shields in valleys and around chimneys or skylights, you add an extra layer of insurance against the elements.
Proper Application Tips
When applying these membranes, ensure the roof deck is clean and dry. Dust or sawdust will prevent the adhesive from sticking properly, which can lead to “fish-mouthing” or bubbles in the material.
Use a weighted roller or firm hand pressure to ensure the membrane is fully bonded to the wood. Remember that these are generally not breathable, so they should only be used in the specific areas that require maximum waterproofing.
The Essential Role of Drip Edges and Eave Flashing
A drip edge is a L-shaped or T-shaped metal strip that installs along the perimeter of your roof. Its job is to direct water away from the fascia board and into the gutters, preventing rot and water damage to your home’s trim.
In a metal roofing system, the drip edge is installed before the underlayment along the eaves (bottom). This allows the underlayment to overlap the drip edge, ensuring any moisture on the barrier flows right off the roof.
Along the rakes (the sloped sides), the drip edge is typically installed over the underlayment. This configuration prevents wind-driven rain from getting underneath the protective barrier at the edges of the roof plane.
Choosing the Right Material
Match your drip edge material to your roofing panels to avoid galvanic corrosion. This occurs when two dissimilar metals (like aluminum and copper) touch in the presence of moisture, causing one to corrode rapidly.
Most modern metal roofing kits offer matching trim and flashing. Stick with the manufacturer’s recommended components to ensure the colors match and the chemical properties are compatible for a long-lasting build.
Overhang and Water Shedding
When installing your drip edge, ensure it has a slight “kick-out” at the bottom. This small bend helps the water break its surface tension and fall cleanly into the gutter rather than hugging the fascia board.
Check your local building codes, as some areas require specific drip edge profiles. A properly installed edge is the difference between a dry basement and a rotted-out soffit system three years down the road.
Using Battens or Furring Strips: The Air Gap Advantage
While many residential installs go “direct-to-deck,” some pros prefer using battens or furring strips. These are horizontal (and sometimes vertical) wood strips that are screwed into the roof deck to create an air gap.
This air gap provides several benefits, most notably ventilation. By allowing air to circulate under the metal panels, you significantly reduce the risk of condensation and help the roof stay cooler in the summer.
If you are installing a metal roof over an existing layer of asphalt shingles (where allowed by code), battens are almost always required. They provide a flat, level surface for the new metal while separating it from the abrasive old shingles.
The “Above-Sheathing Ventilation” (ASV) Concept
ASV is a technique where the air gap acts as a thermal break. Instead of the heat from the metal panels transferring directly into your attic, the moving air in the gap carries much of that heat away before it reaches the insulation.
This can lead to lower cooling costs and a more comfortable home. However, using battens adds complexity and cost to the project, as you have to ensure all your flashing and trim are adjusted for the increased height of the roof.
Structural Considerations
When using battens, you must ensure they are properly fastened into the rafters, not just the thin roof sheathing. The battens must be able to withstand the uplift forces of a windstorm, as they are now the primary anchor for your roof panels.
Typically, 1×4 or 2×4 lumber is used for furring. If you choose this route, make sure the wood is kiln-dried to prevent warping, which could telegraph through the metal and create an uneven appearance.
Sealants and Closure Strips: Finishing the System
Even with the best underlayment, the “openings” in your metal panels need protection. Metal panels have ridges and ribs that leave small gaps at the eaves and the ridge cap. These are entry points for pests, snow, and rain. Closure strips are foam or rubber inserts shaped to match the specific profile of your metal panels. “Inside” closures go under the panels at the eaves, while “outside” closures go on top of the panels at the ridge cap.
These strips act as a physical barrier. Without them, a strong wind can blow rain right up under your ridge cap, bypassing your underlayment and causing a leak in the center of your building.
The Importance of High-Quality Sealant
You will inevitably need to use caulk or sealant around pipe boots, chimneys, and overlapping trim. Never use standard silicone from a big-box store; it often lacks the adhesion and UV stability required for metal roofing.
Look for a high-grade polyurethane or tri-polymer sealant specifically labeled for metal. These products remain flexible through thousands of freeze-thaw cycles, ensuring the seal doesn’t crack as the metal expands and contracts.
Vented vs. Non-Vented Closures
At the ridge of the roof, you have a choice between vented and non-vented closure strips. If your attic relies on a ridge vent for airflow, you must use vented closures. These are made of a mesh material that allows air to pass through while blocking bugs and snow.
If your roof is unvented or uses gable vents, solid foam closures are the way to go. They provide the tightest possible seal against the elements and are generally easier to install for a first-timer.
Frequently Asked Questions About what goes under metal roofing
Can I install metal roofing directly over old shingles?
Yes, in many jurisdictions, you can install metal over one layer of shingles. However, you should use a layer of underlayment or furring strips between the two to prevent the shingles from scratching the back of the metal panels.
Is a vapor barrier the same as underlayment?
Not necessarily. While all underlayments provide a water barrier, some are “breathable” (vapor-permeable), allowing moisture from the attic to escape. Most metal roofers prefer a non-permeable or low-perm synthetic underlayment to keep exterior moisture out.
Do I need a radiant barrier under my metal roof?
A radiant barrier can be a great addition in hot climates to reflect heat. Some underlayments come with a reflective foil side. If you use one, ensure there is an air gap (using battens) for the reflective surface to actually work effectively.
What happens if I skip the underlayment?
Skipping underlayment is a recipe for disaster. Condensation will form on the underside of the metal and drip directly onto your wood deck, leading to rot and structural failure within just a few years. It also voids most manufacturer warranties.
Final Thoughts for the DIY Roofer
Building a roof that lasts fifty years isn’t just about the steel you see from the curb; it is about the engineering of the layers hidden from view. By investing in high-quality synthetic underlayment, proper flashing, and the right sealants, you are protecting your home’s skeleton.
Always prioritize safety when working at heights. Use roof jacks, harnesses, and sturdy ladders. If a section of the roof feels too steep or complex, don’t hesitate to call in a pro for that specific part of the job. Your workshop or home deserves a roof that is as tough as the projects you build inside it.
Take your time with the prep work. Once those metal panels are screwed down, it is very difficult to go back and fix a mistake in the underlayment. Lay a solid foundation, follow the manufacturer’s specs, and you’ll have a roof that you can be proud of for decades to come.
