Which Side Of Metal Roofing Overlaps – For A Watertight, Professional
Always overlap the “lap” edge of the top sheet over the “under” edge of the preceding sheet, moving away from the direction of prevailing winds.
This simple technique ensures that wind-driven rain is directed over the seam rather than being forced underneath it, preventing leaks and structural damage.
You have spent days framing your shed or workshop, and now it is finally time to button up the roof. Metal roofing is a fantastic choice for longevity and durability, but it can be intimidating if you have never handled large, sharp steel panels before.
The biggest question most DIYers face when they unwrap their first bundle is which side of metal roofing overlaps correctly to keep the interior bone-dry. Getting this wrong can lead to moisture infiltration that rots your rafters and ruins your hard work.
I have seen plenty of DIY projects suffer from “leaky roof syndrome” simply because of a backward overlap. Let’s walk through the mechanics of panel orientation so you can finish your roof with the confidence of a seasoned pro.
Understanding why knowing which side of metal roofing overlaps matters
Metal roof panels are designed with a specific profile. One edge is usually the “under-lap” (the flat or slightly recessed side), and the other is the “over-lap” (the side that sits on top to cover the seam).
If you overlap them in the wrong direction, you create a tiny, upward-facing ledge. When it rains, water will naturally flow into this gap, get trapped, and eventually wick its way into your building envelope.
When you place the over-lap on top of the under-lap, you create a “shed” effect. Water runs off the top panel, onto the surface of the next, and continues down to the gutter. It is a simple concept, but it is the most critical step in metal roofing installation.
The golden rule: Direction of prevailing winds
Beyond just the profile of the panel, you need to consider the environment. Always start your installation on the side of the roof opposite to the direction of the prevailing wind.
If the wind consistently blows from the west, start installing your panels on the east side of the roof and work your way toward the west. This ensures that the wind blows over your overlaps rather than trying to pry them open.
Working against the wind is like trying to comb hair the wrong way; it just doesn’t work. Take a moment to observe the typical weather patterns in your area before you set your first panel.
Step-by-step: How to identify and align your panels
Before you climb the ladder, lay two panels on the ground and overlap them. You will quickly see which side is designed to nest into the other. One side will have a small, raised rib or a subtle “V” notch designed to create a water-tight seal.
- Check the profile: Identify the under-lap rib and the over-lap flange. The over-lap should be clearly designed to fit over the under-lap.
- Snap a chalk line: Ensure your first panel is perfectly square to the gable end. If your first panel is crooked, every subsequent overlap will be problematic.
- Maintain alignment: As you place the second panel, ensure the fastener holes or ribs align perfectly with the first. Do not force the metal; if it feels tight, check for debris caught in the seam.
Common mistakes DIYers make with metal panel overlaps
The most common error is “stretching” the panel to compensate for a roof that isn’t square. If your building is slightly out of square, don’t try to force the overlap to make up the distance.
Another frequent mistake is failing to use the proper sealant. While the overlap itself is the first line of defense, most manufacturers recommend using a strip of butyl tape or an approved sealant between the overlaps, especially on low-slope roofs.
Finally, avoid walking on the overlapping ribs. If you crush the overlap while installing it, you compromise the seal and create a permanent path for water to enter. Always walk in the flat areas of the panel, preferably near the purlins or trusses.
Tools and safety gear for the job
Metal roofing is sharp. Before you even think about which side of metal roofing overlaps, you need to equip yourself for the task. Safety should never be an afterthought when working at heights.
- Cut-resistant gloves: Metal edges are like razors. Invest in high-quality gloves to protect your hands during handling.
- Magnetic hex driver: Use a driver with a built-in magnet to hold your roofing screws securely while you drive them.
- Snips and nibblers: Keep a pair of aviation snips handy for trimming panels at the ridge or hip.
- Fall protection: If you are working on a steep pitch, a harness and lifeline are non-negotiable.
Frequently Asked Questions About Metal Roofing Overlaps
Do I need to put screws through the overlap?
Yes, in most exposed-fastener systems, you will drive your fasteners through the overlap. Always follow the manufacturer’s specific fastener pattern, as they will dictate whether you screw through the flat or the rib.
What if my roof isn’t square?
If your roof is out of square, trim the first panel so it aligns with the edge of the roof, or start from the center and work outward. Do not try to “fudge” the overlap to hide a squareness issue, or you will end up with gaps.
How much should the panels overlap?
The amount of overlap is determined by the design of the panel’s rib. You should overlap until the ribs nest securely together. Most standard panels have a built-in “stop” that tells you exactly how far to slide them.
Can I use silicone instead of butyl tape?
Avoid standard silicone, as it can degrade over time and fail to adhere properly to certain metal coatings. Always use a high-grade, UV-resistant butyl tape or a sealant specifically recommended by your roofing manufacturer.
Final thoughts on your metal roofing project
Installing a metal roof is a rewarding project that significantly increases the value and protection of your workshop. By paying attention to which side of metal roofing overlaps, you are already ahead of the game. Remember, take your time, keep your lines square, and always prioritize your safety.
If you find yourself stuck or the panels aren’t nesting, stop and re-read the manufacturer’s installation guide. Every profile is slightly different. Keep your workspace clean, keep your fasteners tight, and you will have a roof that lasts for decades. Happy building!
