Reasons Why the Wrong Metal Roof Decking is Used & How to Fix It

Common Issues: Improper Metal Roof Decking

What Happens When the Wrong Roof Deck Type is Used For a Metal Roof and How to Fix It

When the wrong decking is used under a metal roof, the system can look fine for a while but slowly develop movement, noise, oil canning, structural problems and moisture damage. It can cause a ton of issues because decking is the foundation of the roof. Just like with a house, if that foundation isn’t right, what's above it will eventually be affected.

This guide from the metal roofing experts at LOA explains why the wrong metal decking might be installed, what can happen and how to fix the problem before it creates bigger issues.

Types of Decking and Why the Choice Matters For a Metal Roof

Decking, also called sheathing, is the layer the metal roof sits on. It's typically made of plywood, OSB or structural purlins over open building framing. The roof decking is there to:

Provide a flat, stable surface for panels and underlayment
Tie rafters and trusses together structurally
Help resist wind uplift and live loads
Support fasteners so they hold long‑term

Metal roofing is thin, reflective and moves with temperature fluctuations, so any weakness or irregularity in the deck quickly telegraphs through as visible waviness, noise or leaks.

When a metal roof is being installed, the roofer needs to select the appropriate type of decking for optimal performance. The most common types of decks are:

Type A Deck/Narrow Rib - This is the original metal roof decking that isn't commonly used today because it isn't as strong. However, it is used for construction projects on older homes and buildings to match the existing roof.

Type B Deck/Wide Rib - Type B is the most commonly used metal decking that is lightweight, easy to install and offers good strength. It's also used to support built-up roofing systems and roofs that have rigid board insulation.

Type F Deck/Intermediate Rib - The ribs for this deck type are 1.5" deep. It's not as strong as Type B decking, which is why it's often used for reroofing projects and roof repairs.

Type N Deck/Deep Rib - It's called deep rib because the ribs are 3" deep, making this the type of decking to use for long spans with supports that are far apart.

1" Roof Deck - This one has 1" deep ribs and is similar to N decking in the uses except 1" roof decking is used for short or medium spans.

Here’s a concise comparison chart you can drop straight into your blog. It focuses on practical differences a designer, contractor, or building owner would care about.

Deck Type	Typical Depth	Rib Style / Spacing	Typical Sheet Width	Relative Strength & Span	Common Uses Today	Notes
Type A	1.5 in	Narrow rib, approx. 1 in between ribs; weakest of the 1.5 in profile	36 in (most manufacturers)	Lowest strength and shortest spans among A/F/B; often limited to short spans and light loads	Legacy roof decks, patch/repair work where existing A deck must be matched	Considered an older, “narrow rib” profile; generally not recommended for new construction because B deck is stronger and more economical
Type B	1.5 in	Wide rib, approx. 2.5 in between ribs, 6 in on center	36 in	Highest strength‑to‑weight of the 1.5 in decks; longer spans than A or F (e.g., 22 ga B can span up to 6 ft in typical tables)	Most common structural roof deck for commercial roofs, supports rigid insulation and built‑up / single‑ply roofing	Modern “wide rib” standard; usually the go‑to choice for new 1.5 in roof decks under metal or membrane systems
Type F	1.5 in	Intermediate rib, approx. 1⅞ in between ribs, 6 in on center	36 in	Intermediate strength; spans shorter than B but slightly better than A (e.g., typical FM and SDI span limits sit between A and B)	Re‑roofing and patch work where existing F deck is present; small roofs, canopies, awnings	Legacy “intermediate rib” profile; still available but usually reserved for matching existing decks, not for new large projects
Type N	3 in deep “deep rib”	Deep rib, approx. 3 in between ribs, 8 in on center	24 in wide sheets	Much stronger long‑span deck; commonly used for spans of 8 ft and greater, ideal over widely spaced joists/joists or trusses	Long‑span roofs in gyms, auditoriums, warehouses and other large open spaces where fewer framing members are desired	Higher material cost but allows fewer structural supports; often paired with thicker insulation and various roof coverings, including metal and membranes
1″ Roof Deck	1 in deep ribs	Shallower rib profile than N; various trade names (1.0C, HD, HF, UF1X, etc.)	Typically 36 in (varies by manufacturer)	Strong for short to medium spans, but not a long‑span deck like N; sits between fluted form deck and 1.5 in structural roof deck in capacity	Roofs, canopies, awnings where spans are modest and a shallower profile is preferred	Often used where structural depth is limited but a true structural roof deck is still required; similar applications to N Deck but for shorter spans.

What the Wrong Decking Looks Like

With a number of options to choose from, it's easy for an inexperienced roofing company to choose the wrong type of decking. Using the wrong decking under metal means:

Using non‑structural board like old 1x skip sheathing where a solid deck is needed to support horizontal and vertical loads
Using undersized or low‑grade plywood/OSB that can sag or delaminate
Decking that’s too thin for the panel type, span or local wind/snow loads
Decking installed over rotten, spaced or uneven boards without proper prep
Decking that isn’t flat, properly fastened or adequately ventilated beneath

Each of these scenarios sets the stage for long‑term problems, even if the metal roofing materials looks okay right after installation.

What Happens When the Wrong Decking is Used

As you can imagine, installing the wrong decking is problematic. Eventually you'll run into the problems below.

Sagging, Bounce and Soft Roof Areas

Thin or low‑grade decking can:

Sag between rafters or trusses over time
Flex under foot traffic or snow loads
Create “soft spots” when you walk on the roof
Lad to panels bending to match the dips
Cause fasteners to work loose as the deck moves
Cause more pronounced oil canning in flat‑pan profiles

Oil Canning and Visible Waviness

Metal panels want a flat, even plane to sit on. When decking is warped, uneven or loosely fastened:

Panel flats show ripples and waves
Standing seam ribs may wander or lean
Reflections on the roof surface look distorted

Sometimes owners blame the metal for these issues when in fact the real culprit is the deck below.

Fastener Problems and Leaks

Wrong or weak decking affects how well fasteners hold. When that happens:

Screws can strip easily in thin or degraded sheathing
As the deck sags or moves, fasteners tilt and oval the holes
Loose fasteners let water seep in around washers and seals

In high‑wind or storm conditions, this can lead to:

Panels rattling or lifting
Water intrusion along fastener lines
Progressive loss of fastener clamping force

Moisture, Mold and Rot

Some decking will actually trap moisture in certain conditions. Other problems noted above can also lead to:

Old boards left under a new deck can hold moisture against the sheathing
Poor ventilation plus temperature swings cause condensation on the underside of the metal, which drips onto susceptible decking
Low‑quality OSB or improperly rated sheathing that swells, delaminates and eventually rots
Dark staining and mold growth on the underside of the deck
A musty smell in the attic
Structural weakening where wood fibers break down

Structural and Safety Concerns

If decking is not rated or sized correctly for spans and loads it can lead to big issues with the roofing system.

Live loads (snow, workers, equipment) can exceed its capacity
Panels and underlayment may be the only thing between occupants and the outdoors in a localized failure
Code violations and insurance issues can arise if the roof structure doesn’t meet local requirements

In extreme cases, significant deck failure can pose a safety hazard because it is impacting the structural integrity of the roofing system.

Why the Wrong Decking Gets Used in the First Place

How does this major issue happen? Common reasons why the wrong decking is installed include:

Cost cutting: Using thinner or lower‑grade material to save money up‑front
Reroofs without proper tear‑off: Laying new deck or metal over questionable old boards to avoid labor time
Assuming “any deck will do”: Treating metal roofing like shingles and not accounting for its sensitivity to substrate quality
Ignoring manufacturer and code guidance: Not matching deck thickness and spacing to the specific metal system and local load requirements

How to Tell if Decking Might Be the Problem

From inside and outside, there are ways to tell when the wrong decking was used. The signs include:

Roof planes that look wavy or uneven from the ground
Noticeable oil canning in flat areas despite correct panel install
Soft spots when walking on the roof
Thin, patchy or obviously sagging sheathing
Delamination, swelling or rot
Excessive moisture staining or mold in the attic

If roof leaks or panel movement keep returning despite fastener and sealant work, the deck is often the reason for the leaks and movement.

How to Fix Metal Roof Problems Caused by the Wrong Decking

Assess the Extent of the Problem

A qualified roofing contractor should:

Inspect the attic and underside of the deck for thickness, grade, fastener pattern and damage
Check panel condition for corrosion, oil canning and fastener issues.
Evaluate whether issues are localized or widespread

This determines whether spot repairs or more extensive work is needed.

Localized Decking Repairs

This approach works well for known weak areas near eaves, valleys or past leak zones. If only a section of decking is inadequate or damaged the localized repair should include:

Carefully remove metal panels in the affected area
Cut out and replace bad decking with proper rated plywood or OSB in the correct thickness and exposure rating
Refasten decking correctly to rafters/trusses with appropriate nails or screws
Reinstall underlayment and metal panels, ensuring good alignment and fastening

Larger‑Scale Deck Replacement

If the wrong decking covers most or all of the roof, or if widespread sagging and rot are present the repair process should involve:

A full or major tear‑off
Remove metal panels and existing underlayment
Replace decking with properly specified material:
- Correct thickness for span and load
- Proper grade and exposure rating
- Installed with correct edge spacing and fastening
Install appropriate underlayment and reinstall panels or upgrade the metal system

This is a significant project but often the only way to restore structural integrity and long‑term performance when the problem is widespread.

Address Moisture and Ventilation Issues

If moisture contributed to decking failure that needs to be addressed as well. Do so by:

Improving attic ventilation intake and exhaust
Sealing air leaks from the conditioned space into the attic or roof cavity
Considering condensation‑control strategies if metal is installed over unvented assemblies

Stopping moisture at the source helps ensure the new decking doesn’t fail prematurely.

How to Prevent Decking Mistakes on Metal Roofs

Preventing problems is always easier than fixing them. If you plan to have a metal roof installed there are some things you can do to prevent decking mistakes before they happen.

Use the Right Decking Type and Thickness

Follow both building code and metal manufacturer requirements for minimum thickness and span rating
Choose appropriate sheathing (plywood or OSB) rated for roof use and exposure
Avoid using old or non‑structural boards as the primary deck under modern metal systems unless the system is engineered for open framing

Ensure a Flat, Solid Substrate

Plane, shim or replace warped or twisted framing before sheathing
Stagger panel joints and maintain proper gaps between deck sheets
Fasten sheathing correctly to avoid squeaks, bounce or uplift issues

Coordinate System Design

Match deck design with:
- Metal panel profile (corrugated vs. standing seam)
- Fastening method (exposed fasteners vs. clip systems)
- Insulation type and location
Make sure everyone (designer, builder, roofer) agrees on the decking specification before work starts

Hire Contractors Experienced With Metal Roofing

Ask specifically about their decking practices for metal roofs
Request that decking type, thickness and fastening be listed in the contract
Have them explain how they’ll handle any existing deck irregularities on reroofs

A metal roof is only as good as the deck beneath it. When the wrong decking is used the roof may look fine for a while but slowly develops waves, leaks and structural problems. The right deck, installed correctly, is just as important to a long‑lasting metal roof as the panels you see from the street.

Do you need your metal roof evaluated by expert roofers? Planning to install a metal roof and want to make sure everything is done right? If you're in Central Texas, contact LOA to schedule a complimentary roof inspection. We have years of experience installing, replacing and repairing metal roofing systems that perform well and last for decades.

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At LOA, we believe success is defined by customer satisfaction. We're proud to say we have some of the happiest clients in the roofing industry! You can find hundreds of 5-star reviews like the ones below online.

"I got three inspections and decided to use LOA Construction for a new roof. LOA’s Dan Harvey worked with my insurance inspector to make sure all the hail/wind damage was accounted for, gave us a good choice of shingle colors, and was great at communicating. The workers did quality work, found and repaired rotted wood and cleaned up. Highly recommend!"

— Brian McFarlin, 5-Star Google Review

"Brandon at LOA construction made replacement of our roof very easy. He was the only roofing contractor that I could find that would work directly with our insurance company. They showed up on time and made the whole process very easy. And we have a great looking high quality roof! I Would definitely recommend them to anyone looking for a new roof. Give them a call if you need a roof or think you might have wind or hail damage. I had several other bids that were higher cost and didn't think our insurance would cover it. Our insurance did cover it, I think they were just unwilling to work with the insurance company. Not Brandon, not only was he will to work with our insurance he met with the adjuster and made the whole thing a breeze. Thanks again LOA and Brandon!!!!"

— Mike A., 5-Star Google Review

LOA Construction has exceeded my expectations again. This is the second roof installation with them and I couldn't be happier. Specifically, Tyler Griffiths is professional, attentive, and made the entire process of working with my insurance company effortless. My tenant sent a glowing account of their experience as well. Knowing it was a rental, they still diligently protected all property and plants. They were on time every time and went out of their way to not inconvenience the residents. LOA Construction has integrity and takes pride in their work. I couldn't recommend them more.

— Sheri Moran

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— Tony Smith, 5-Star Google Review

From the very start, I knew I was in good hands. The professionalism was top-notch, but most important, was the caring attitude of the project manager, Dan Harvey, the roofing supervisor, Ambrocio, and the roofing crew. They made sure every detail was covered. They are definitely an asset to LOA Construction. I highly recommend them. Dennis and Suzy S

— Dennis Sanger

Andrew was very great at getting myself and my wife all the information and getting us started from start to completion. His workers were friendly and professional. If ever I had to contact him, he was quick to respond back. We are very happy with the finished product. Yes, I would and have recommended him and his company to get their work by LOA Construction and Austin. Andrew made sure my wife would be satisfied with whatever roof she liked. Can't thank you enough Andrew and LOA Construction.

— Paul Coleman

As a first time home owner with no experience in how to make claims and start the process of getting a new roof I was scared to the point of avoiding what needed to get done after the bad hail storm. August Walker did an amazing job walking me through each step along the way. He answered all my questions and calmed my concerns. They inspected my roof and drove from Austin to Belton just to be there with the ins inspector, and followed up with me in person after the roof was installed. He also made a special trip back to drop off samples! I’m grateful to August and LOA construction for being highly professional and friendly!

— Angela Baker

From start to finish Lee and his team took great care of us. He made us feel comfortable during the process and answered any and all questions we had. I would definitely them recommend to anyone.

— Taneisha Jones

Arthur Harris and his team with LOA Construction did an excellent job on our roof. They finished it in one day. Arthur was very good in explaining all the information to us.

— Crystal Hill

Will, has been so awesome and easy to work with he gets everything we want done the way we want it and also keeps us updated as we are over the road truck drivers !!! Hands down to LOA Construction for their great service and great customer service !!!

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Dan Harvey is Very Knowledgeable and directed me straight to the products that will save us money in the next 3 years the savings on energy costs alone will pay for the installation itself. I highly recommend LOA Construction and Dan Harvey! Actual Customer Jefferson Fara

— Jefferson Fara

Mark from LOA roofing was very professional from the git, the initial estimate process, he quickly identified all the issues that needed to be addressed. The company was very prompt, to coordinate the crew, and for a very reasonable price, got my roof repaired without any hassle. They also took care of all the cleaning, so that it all looked like nothing happened. I would recommend Mark from LOA to anyone needing a roof repair, with no kind of hesitation.

— Ale León

Loa Construction provides quality services in installing roof and related repairs. I’ll recommend it to my friends and family.

— Peijie Li

Thanks to Landon Wells of LOA Construction for the beautiful new roof! It was all completed in one day including a meticulous clean up job afterwards. The workers were all polite and professional. Landon made sure everything went smoothly and that all of our questions were answered. I am thrilled with our new roof. I would definitely recommend Landon and LOA Construction to all of my friends and family. Thank you Landon!

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Improper Decking With Metal Roofs