Knowing exactly how to lap tin roofing is one of those skills that seems simple until you're standing on a ladder with a heavy sheet of metal and the wind starts picking up. If you don't get the overlap right, you're basically just building a very expensive sieve instead of a roof. Most people think you just throw the panels up and screw them down, but there's actually a bit of a science to how those sheets sit on top of each other to keep the rain out and the inside dry.
Whether you're tackling a backyard shed, a lean-to, or a full barn, getting the laps right is the most important part of the job. It's all about working with gravity and the prevailing weather rather than trying to fight it.
Why the Lap Matters More Than You Think
If you've ever seen a metal roof that leaks despite having brand-new panels, it's almost always because of a bad lap. Water is sneaky. It doesn't just fall straight down; it moves sideways with the wind and can even "wick" or climb upward through a process called capillary action.
When we talk about how to lap tin roofing, we're looking at two different types: the side lap (where the sheets meet side-by-side) and the end lap (where the top sheet overlaps the bottom sheet on a long run). If these aren't sealed or overlapped enough, a heavy storm will push water right under the metal, and you'll be chasing leaks for years.
Getting Your Tools and Materials Ready
Before you even think about dragging a ladder out, you need the right gear. You're going to need a good impact driver, specialized roofing screws with neoprene washers, a pair of tin snips (or a metal nibbler), and—this is the secret weapon—butyl tape.
Most DIYers skip the sealant tape because they think the overlap is enough. Trust me, it's not. In a high-wind situation, rain gets forced into those gaps. Butyl tape creates a gasket-like seal between the sheets. It's cheap insurance for a job you only want to do once.
Mastering the Side Lap
The side lap is the most common part of the process. This is where you lay your panels across the width of the roof.
Follow the Wind
Here is a pro tip that most beginners miss: check which way the wind usually blows at your place. You want the "lap" to face away from the prevailing wind. If the wind usually blows from the west, you want to start laying your panels on the east side and work your way west. This way, the "seam" of the lap is facing away from the wind, so rain isn't being blown directly into the joint.
One Rib or Two?
Most modern corrugated tin panels have a specific "underlap" and "overlap" edge. One side will have a slightly shorter rib. That's the one that goes down first. The next sheet has a slightly wider rib that sits perfectly on top of it.
For most standard roofs, a one-rib overlap is plenty. However, if your roof has a very low pitch (meaning it's relatively flat), you might want to go with a two-rib overlap. It uses a bit more material, but it gives you that extra layer of protection against standing water.
Using the Sealant
Apply your butyl tape right on top of the rib of the panel that's already been fastened. Then, set your next panel on top and press down. When you drive your screws through both layers, the tape will squeeze around the screw, creating a watertight seal that lasts for decades.
How to Handle the End Lap
If your roof is too long for a single sheet of tin, you're going to have an end lap. This is where the top sheet sits over the bottom sheet. This is the danger zone for leaks if you get the measurements wrong.
The Pitch Dictates the Lap
The flatter your roof, the more you need to overlap. If you've got a steep roof (like a 4/12 pitch or higher), a 6-inch overlap is usually fine. But if you're working on something flatter, like a 1/12 or 2/12 pitch, you really need to go with a 12-inch overlap.
Gravity is your friend on a steep roof, pulling the water down quickly. On a flat roof, water tends to hang out and pool, giving it more time to find a way under the top sheet. Don't be stingy with the metal here; that extra few inches of overlap can save you a massive headache later.
Bottom to Top
This might sound obvious, but you'd be surprised how many people get it backward. Always start at the bottom of the roof (the eave) and work your way up to the top (the ridge). This ensures that each higher sheet sits on top of the one below it, allowing water to run down the roof like shingles. If you do it the other way, you're essentially creating a series of buckets that catch water.
Driving the Screws the Right Way
You can have the perfect lap, but if you mess up the screws, it's all for nothing. When you're fastening your laps, you want to drive the screw through the top of the rib, not the flat valley.
I know, there's an age-old debate about this. Some people swear by the valley because it's more stable, but the rib is where the water isn't. Water collects in the valleys of the tin as it drains. If you put a hole there, you're asking for trouble. By putting the screw through the high point of the lap, you're keeping the fastener out of the main water channel.
Don't Overdo It
When you drive those screws, watch the rubber washer. You want it to compress slightly until it's flat against the metal. If you overtighten it, the washer will "mushroom" out and crack. If it's too loose, it won't seal. It takes a little practice to get the feel for it, but once you find the sweet spot, you'll move fast.
Common Mistakes to Avoid
Even if you think you know how to lap tin roofing, it's easy to make a silly mistake when you're tired or the sun is beating down on you.
- Cutting with a Circular Saw: A lot of people use a standard circular saw with a metal blade. It works, but it creates a ton of heat and sparks that can damage the protective coating on the tin. This leads to rust. If you can, use snips or a cold-cutting blade designed for metal.
- Forgetting the Drip Edge: Your lap should extend slightly past the edge of the roof (about an inch or two) to ensure water drops into the gutter or away from the fascia board.
- Walking on the Laps: Try not to step directly on the ribs where the sheets overlap. You can easily dent or crimp the metal, which ruins the tight fit of the lap. Always try to step in the valleys over the purlins (the wood framing underneath).
Wrapping Things Up
At the end of the day, learning how to lap tin roofing isn't about being a master carpenter; it's just about being meticulous. If you take the time to plan your direction based on the wind, use your sealant tape, and give yourself enough of an overlap at the ends, that roof is going to last thirty or forty years without a drip.
It's one of those jobs where the preparation takes longer than the actual installation. But once you get that first row down and see how the ribs lock together, you'll get into a rhythm. Just keep your lines straight, don't crush your washers, and always keep an eye on your overlap measurements. Your dry attic (and your wallet) will thank you when the next big storm rolls through.