Mold in a Metal Building: What Causes It and How to Stop It

People often toss around the word "gauge" when discussing metal buildings, but few stop to consider what it really means. For steel, its gauge is a system of determining how thick it is. The smaller the number, the thicker the material.

What Is Gauge?

Most metal buildings utilize 14-gauge steel for their framing. It’s light, sturdy, and can hold its own in most scenarios. However, some environments with higher wind speeds and snowfall demand thicker 12-gauge framing to carry the load and pass local building requirements.

And while you might not notice much of a difference in performance at first, that beefier framing makes it a lot harder for nature to bend or twist your structure out of shape.

Key Highlights

• 12-gauge = thicker and stronger
• 14-gauge = good for most small to mid-size builds
• Lower gauge = better wind and snow performance
• 12-gauge holds shape longer under stress
• Often comes with a better warranty
• Price bump is small compared to the upgrade in strength

Steel Gauge Chart

How Gauge Impacts Loads

Thicker framing doesn’t just sound tougher. It is tougher. Heavier steel stands up better to snow buildup, hard gusts, and the added stress from longer spans.

Engineers run the numbers for wind and snow loads based on ASCE 7 standards. Using this system, buildings can be rated for anywhere from 115 to 170 mph winds and 20–60 psf snow loads.

The local requirements for wind and snow loads depend on location, open exposure, and roof pitch. For example, a building sitting on a hill or near the coast deals with more pressure than one behind trees.

Upgrading to 12-Gauge Framing

At what point should you consider upgrading?

• High Wind Speeds

If your metal building will sit in an area with high wind speed calculations, you may need to upgrade to thicker framing to prevent swaying or failure.

• Heavy Snow Regions:

If you expect to see your fair share of deep snows this winter, upgrading your framing thickness is a smart move.

• Longer, Bigger Buildings:

Longer spans put more stress on the frame of your pre-engineered metal building.

• Certification Needs

Higher wind or snow ratings often call for thicker 12-gauge.

• Lower Maintenance:

Thicker steel can last longer and require fewer repairs over the course of its life.

Cost and Warranty Changes with Framing Upgrades

You can expect a small price increase when you upgrade from 14-gauge to 12-gauge framing. You pay for thicker steel, heavier anchors, and reinforced braces to match it. Usually, you’re looking at around 10–15% more on the framing side.

However, that extra spend often gets you a longer rust-through warranty, sometimes 25 years instead of 20. The thicker galvanizing takes longer to wear down, especially in damp or coastal areas.

So, while the up-front cost stings a little, you’re getting fewer repairs, less movement, and a frame that won’t quit early. In the long run, it’s usually worth every bit.

Frequently Asked Questions about Framing Gauges for Metal Buildings

Do you have questions about framing gauge, and which thickness is best for you? Below, we’ve answered some of the questions we see on this subject the most:

Is 12-GA stronger than 14-GA?

Answer: Yeah. It’s thicker, heavier, and stiffer under load.

What’s the real thickness difference?

Answer: 12-ga steel is about 0.1046 inches thick, while 14-ga runs around 0.0747 in.

Does 12-GA automatically mean better wind or snow ratings?

Answer: Not by itself. The full design, including anchors, trusses, and layout, plays a role. But thicker framing helps you hit higher numbers.

Does the 20-year rust warranty only cover 12-GA?

Answer: Usually, the heavier gauge gets longer coverage. Some 14-ga options still offer 20 years, but 12-ga often goes a few years beyond that.

You open the door on a cool morning and the underside of the roof is dripping. A week later there is a gray fuzz on the cardboard boxes against the back wall. That’s the most common way owners discover mold in a metal building, and the good news is the cause is almost always the same thing: condensation.

Steel does not grow mold. What grows mold is the dust, wood, cardboard, hay, and fabric sitting in a building that stays damp. Stop the moisture and you stop the mold.

This guide will provide you with handy information on what causes mold in a metal building, and how to put an end to it once and for all.

Key highlights

• Metal buildings "sweat" when warm, humid air touches cold steel. That water is what feeds mold.
• Insulation is the biggest fix, and it is cheapest to add when you order the building.
• Airflow matters almost as much. A sealed building with no ventilation traps moisture.
• Ground moisture comes up through dirt and bare gravel. A slab with a vapor barrier cuts it off.
• Unvented propane and kerosene heaters add water to the air every hour they run.
• Planning a new building? Call (208) 572-1441 and we will spec insulation for how you plan to use it.

Why metal buildings get condensation

It works like a cold glass of iced tea on a summer porch. Water beads up on the outside of the glass. The air around the glass had moisture in it, the glass was colder than the air, and the moisture turned back into water when it touched it.

Your building does the same thing. Steel changes temperature fast. On a clear night it gets cold, often colder than the air inside. When the morning warms up, or when you fire up a heater, that warmer air hits the cold roof panels and water forms on the underside. Then it drips.

A few conditions make it worse:

• Big day-to-night temperature swings. Spring and fall are the worst seasons for most owners.
• Humid climates. The Gulf Coast and the Southeast deal with this nearly year-round.
• Moisture sources inside. Wet vehicles, livestock, drying hay, a washing station, or people living in the space.
• No airflow. Fully closed buildings with nowhere for humid air to go.

Open carports rarely have this problem, since air moves right through them. It shows up once you close the walls in. If you are still deciding how enclosed to go, our comparison of open versus enclosed metal carports covers that trade-off.

Signs you have a moisture problem

Catch it early and the fix is simple. Watch for these:

• Water droplets on the underside of the roof panels, especially in the morning
• Drip marks or puddles on the floor with no leak above them
• A musty smell when you open the door
• Rust spots forming on tools and equipment
• Dark spots or fuzz on cardboard, wood shelving, hay, or fabric
• Damp spots along the base of the walls after rain

Water at the bottom of the wall after a storm usually means water is getting in from outside, not forming inside. That has its own fix, covered below.

How to stop condensation and mold

These are the fixes that make the most difference.

1. Insulate the building

Insulation puts a layer between the cold steel and the warm, moist air inside. The air never touches the cold surface, so water never forms. This is the single biggest fix, and it matters most on the roof, where condensation drips onto everything below.

The time to add it is when you order. Insulation goes on as the building goes up, so it installs cleanly behind the panels. Adding it later means working around everything already inside. Our options run from double bubble reflective insulation, which works well as a condensation barrier on shops and storage, up to heavier blanket insulation for spaces people spend real time in.

Insulating just the roof is a solid middle ground if your budget is tight. Fully insulating walls and roof is the right call for a heated shop or anything you plan to live in, like our barndominium shells.

Planning an enclosed garage or shop? See the 30x40x13 A-Frame Vertical Garage, or call (208) 572-1441 to talk through insulation options before you order.

2. Keep the air moving

Moist air that has nowhere to go ends up on your roof panels. Give it a way out. Vents high on the building let warm, humid air escape. A couple of walk-in doors or windows on opposite walls help cross-ventilation. In a closed shop, an exhaust fan does a lot of work for very little money.

Insulation and ventilation work as a pair. One without the other only solves half the problem. Ask about vent options when you quote your building.

3. Cut off moisture from the ground

Bare dirt breathes moisture up into the building all day long. So does gravel sitting directly on wet soil. You can’t see it happen, but it adds humidity to every cubic foot of air inside.

A concrete slab poured over a plastic vapor barrier stops most of it. If a slab is not in the plan yet, a thick plastic sheet under your gravel helps. Our guide to gravel versus concrete foundations explains when each one makes sense, and the site preparation page covers grading and drainage.

4. Keep outside water outside

If your damp spots show up after rain, the water is coming from outside. Check three things:

• Grading
  The ground should slope away from the building on every side, not toward it.
• Slab edge
  A slab poured with a notch lets wall panels drop over the edge so rain runs outside the concrete. A flush slab relies on base trim to close the gap.
• Roof runoff
  A vertical roof sends water straight down the panel ribs and off the eave. Our roof styles guide explains why that matters in wet climates.

5. Watch what you bring inside

Some habits add water to the air without you noticing:

• Unvented propane or kerosene heaters
  Burning fuel releases water vapor. A heater running all morning puts a surprising amount of moisture into a closed building.
• Wet vehicles and equipment
  A truck covered in snow melts into the air inside.
• Hay stored damp
  Let it dry before it goes in.
• Cardboard against the walls
  It soaks up moisture and gives mold a meal. Use plastic bins and keep storage a few inches off the floor and off the walls.

Want to know more about reducing mold in a metal building? Call (208) 572-1441 today.

Mold risk by building type

You can see configurations for each of our metal garages, metal barns, horse barns, and workshop buildings pages.

Already have mold? What to do

First, fix the moisture. Cleaning mold without stopping the water just means cleaning it again next month.

Then clean up:

• Wear gloves and an N95 mask while you work.
• Throw out moldy cardboard, fabric, and anything soft that stayed wet. Those rarely come clean.
• Scrub hard surfaces with detergent and water, then dry them fully.
• Open doors and run fans until everything is dry.

For bigger jobs, the EPA's general guidance is that mold covering more than about 10 square feet is worth professional intervention. The same goes if anyone in your family has asthma, allergies, or breathing problems. When in doubt, call a mold remediation company.

The steel itself is fine. Panels and framing do not absorb water, so mold on steel is sitting on dust or grime on the surface. It wipes off. Anything rust-colored is a separate issue and is worth checking on in your warranty and workmanship coverage.

Keep your building dry from day one

Mold in a metal building is a moisture problem, and moisture is easiest to beat before the building goes up. Insulate the roof, let the air move, keep ground water down, and keep rain outside. Do those four things and your building stays dry for the long haul.

Design your building in 3D at EngineeredMetalBuildings.com, or call (208) 572-1441 and tell us how you’ll use it. We’ll help you pick insulation that fits. Financing is available on the financing page, and you can check coverage on our service areas.

When you’re applying for a permit for a metal building, the drawings you submit can be just as important as the building itself. A set of generic plans may show the basic design and dimensions of a structure, but that doesn’t necessarily mean they contain everything your local permit office needs to approve the project. Depending on the location, building size, intended use, and local code requirements, the permitting authority may expect drawings that are specifically prepared for your property and project.

Understanding the distinction between the two before submitting your permit application can help you avoid delays, additional paperwork, or having to revise your plans after the application has already been submitted.

Key Highlights

• Generic drawings show the standard engineering for your building model. They are included with your order and they are free.
• Site-specific engineered drawings are calculated for your exact address and stamped by an engineer licensed in your state. They cost extra.
• Most permit offices that require an engineer's stamp will not accept generics, full stop.
• Anything occupied gets classified Risk Category II and always needs the stamped set.
• Drawings are ordered after your deposit, and the engineer typically takes 21 to 25 business days.
• Your permit review starts after the drawings arrive, not alongside them. Plan the calendar accordingly.
• Not sure which one your county wants? Call (208) 572-1441 with your address and we will check before you order.

What Each Drawing Set Actually is

Generic drawings are the standard engineering package for the building you bought. Dimensions, frame layout, panel details, and the loads the model is built to at standard ratings. They describe how the building goes together and what it’s rated for. None of it is tied to your parcel, your soil, your county's snow number, or your jurisdiction, and there is no engineer's seal on them for your state.

Plenty of counties are fine with that. If you are putting up an unoccupied barn outside city limits in a county with no meaningful snow load, generics are often all anybody asks to see. They come with your order at no cost.

Site-specific engineered drawings are a different document. An engineer licensed in your state takes your exact building, your exact address, and the wind speed, ground snow load, seismic category, and exposure for that spot, then runs the calculations and stamps the result. The output is a set of sheets showing how your building performs at your location, plus the foundation and anchoring details that go with it.

That stamp is the whole point. It is a licensed professional putting their name on the statement that this building, on this lot, meets the code your county enforces. That is what a plan reviewer is looking for when they say they need engineered drawings.

Which One Your Permit Office Will Accept

Four things decide it, and only one of them is up to you.

1. Whether anyone will occupy the building

   If the building is going to be lived in, slept in, or occupied under a permit, it gets classified as Risk Category II, and that means stamped site-specific drawings every time. A barndominium shell, a shop with a bathroom, a commercial space, an office. It does not matter how small it is. Our steel frame barndominium guide covers what that classification changes, and you can see the shells we build on the barndominium page.

2. Your county's snow and wind numbers

   Where ground snow load is high, generic ratings often do not reach far enough and the county wants calculations for your elevation and exposure. Same with high wind zones and coastal counties. We pull your address's numbers as part of quoting, so we usually know before you do whether the standard package covers you. The snow and wind load page shows how those ratings work, and our breakdown of how codes and snow loads move the price covers the cost side.

3. Your jurisdiction, which is not the same as your state

   Requirements are set county by county, and often differently inside city limits than outside them. Two neighbors on opposite sides of the same road can face different rules, and we have had that exact call more than once. Our state-by-state permit requirements guide is the starting point, and there is a deeper look at one market in our Idaho permits and plan costs post.

4. Size and what is holding the roof up

   Bigger buildings, wider clear spans, and anything commercial push toward stamped drawings even in relaxed jurisdictions. A 20-foot carport and a 60-foot-wide shop are not the same conversation with a plan reviewer.

"Give us your install address before you order and we will find out what your building department requires. It takes one phone call on our end and saves you a month on yours. Call (208) 572-1441."

Planning a wider shop or commercial building? Browse our commercial metal buildings or see the 50x50 clear-span metal building for a real-world clear-span configuration.

What Is Actually Inside A Site-Specific Set

A stamped set includes the foundation plan, which shows slab dimensions, footing sizes and depths, and reinforcement. It includes the anchor bolt layout, which tells your concrete contractor exactly where the anchors go before the pour rather than after. It includes base rail and connection details showing how the frame ties to the foundation. And it includes the edge condition, meaning whether your slab is poured flush or with a notch, sometimes called a sheeting ledge, for the panels to drop over.

The base trim that closes the gap at the bottom of your wall gets bent in the shop to match your slab edge. If your drawings show a lip and your contractor pours flush, or the other way around, the trim does not fit and you find out on install day. Read that sheet, hand it to your concrete guy, and tell us what it says. Our site preparation page covers the rest of what the crew needs from the site, and the 40x60 slab cost guide shows what a foundation on a larger footprint involves.

Generic drawings will not give you any of that at your specific dimensions. They will get a simple building past a relaxed county, and they will not tell your concrete contractor where to set anchors.

Generic Vs Site-Specific, Side By Side

Where Drawings Sit In Your Timeline

Drawings get ordered after your deposit, not before. Once the deposit is in, your order moves to our drawings department, they call you to confirm every detail of the building, and the package goes to the engineer. If you need the site-specific set, that’s when it gets quoted and paid for.

The engineer typically takes 21 to 25 business days. That is business days, so call it four to five weeks on a calendar. When the stamped set comes back, we send it to you, and then you take it to your building department and permit review starts. Review is its own timeline, set by your county, and it runs after the engineer, not at the same time.

Only once you have the permit and the site is ready does the install clock start. Add it up and a permitted building can run a few months from deposit to install, with most of that spent waiting on other people. Buyers who understand this at the start are relaxed about it. Buyers who find out in week six think somebody is stalling them.

The one thing that speeds this up is deciding early. Every change to the building after the drawings go to the engineer restarts part of that clock. Lock your dimensions, your leg height, your openings, and your lean-tos before the package leaves. Our installation timeline breakdown lays out the whole sequence, and the ordering process page covers what happens at each payment stage.

Lock your building configuration before engineering starts. Use the 3D Building Designer to configure your size, roof style, doors, windows, colors, and other options before the drawing package moves forward.

What Stamped Drawings Cost And Why It Varies

There is no flat price on site-specific engineering, and anyone quoting you one over the phone before they know your details is guessing.

The cost moves with the size and complexity of the building, your state, and the risk category. A simple garage in a mild county is at one end. A wide-span occupied building in high snow country with lean-tos attached is at the other, because every attached structure is more calculation. Lean-tos in particular are often engineered as their own elements, so a building with two of them is not the same job as a building with none.

Two practical notes. First, budget for it as a separate line rather than assuming it is folded into the building price, because it is not. Second, if the engineering cost is what is squeezing you, financing the building itself frees up the cash. Terms are on the financing page.

Still comparing certification requirements? Read our certified metal building permits guide for more detail on wind loads, snow loads, certification, and permit-ready engineering.

Four Mistakes That Cost People Time

1. Assuming your neighbor's answer applies to you.

   Requirements change across county lines and city limits. Ask your own building department, or give us the address and we will ask.

2. Waiting to ask about permits until after the deposit.

   Call your county before you order. A five-minute conversation tells you whether you need the stamped set, which tells you what your real timeline is.

3. Changing the building after drawings are ordered.

   Adding a door, moving a wall, or bolting on a lean-to after the package is with the engineer means revisions and more waiting. Get the whole build sheet right first. Configurations for metal garages, metal barns, and workshop buildings are worth reviewing before you commit.

4. Pouring concrete before the drawings arrive.

   The anchor bolt layout lives in that set. Pour first and you are drilling into cured concrete, or worse, your anchors are in the wrong places. Wait for the sheets.

Sorting This Out Before You Order

The right drawings can make the permitting process much easier, but the requirements ultimately depend on your local jurisdiction and the specifics of your building project. Generic drawings can be useful for showing the basic design of a structure, but they may not provide the site-specific information a permit office needs to verify that the building meets local codes.

Site-specific engineered drawings provide a more complete picture by accounting for the conditions and requirements associated with your particular property.

Design your building in 3D at EngineeredMetalBuildings.com, or call (208) 572-1441 with your address and what the building is for. We will check your county's requirements, pull your loads, and tell you what your real timeline looks like before you put a dollar down. Coverage is on our service area page.