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Suppose I want to cover a roof with corrugated steel sheets. Imagine a sheet laying on the roof. It will have "tops" - zones that are further from the roof surface and "bottoms" - zones that are closer to the roof surface (the line is the sheet cross-section, the roof is below the line, the outdoors is above the line):

enter image description here

I need to decide where I run the screws that will hold the sheet.

In case it was roofing slate I'd definitely run them through the "tops" ("B" on the drawing) because when it rains water will run along the "bottom" and into any hole it finds there.

However seems like the typical approach with corrugated steel is to run the screws through the "bottom" ("A" on the drawing) which puzzles me a lot.

Which do I choose - A or B - in case of corrugated steel and why?

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  • This question is a matter of manufacturer requirements and opinion. It's essentially a poll. Voting to close.
    – isherwood
    Apr 20 at 12:42
  • If it's that form, the answer is different than when it's a series of rounded semi-circles (which are strong at "b" ) or a form like that with B much narrower (so, again, strong, not floppy/bendable) - In general, read and follow the manufacturer recommended method/placement/type for fasteners.
    – Ecnerwal
    Apr 20 at 18:41

9 Answers 9

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A

If you choose B you will dimple the steel, ruining the look while creating a penetration point for water because expansion and contraction of the metal due to heating and cooling will create an open access point for water to enter, and a big opening behind it to receive it.

A, however, will give you a tight seal against the wood, wood-steel-screw in a nice tight sandwich - resisting expansion/contraction gaps and thus protecting much more effectively against water penetration while preserving the look of the corrugated steel

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    +1 and also make sure to use only appropriate sheet metal screws with a water tight washer prevent water from getting in.
    – maple_shaft
    Jun 28, 2012 at 12:09
  • Why is the choice different compared to roofing slate?
    – sharptooth
    Jun 28, 2012 at 12:25
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    I have no idea why you would compare installing corrugated sheet metal to installing slate. They're different products and should be treated as such.
    – The Evil Greebo
    Jun 28, 2012 at 12:58
  • Well, both have "tops" and "bottoms", that's why.
    – sharptooth
    Jun 28, 2012 at 13:17
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    When you install slate, you are installing shingles, and you're overlaying them in a pattern where the top of the shingle is against the underlying surface (wood, tar paper) and the bottom of the shingle is over the top of the next course down. With slate as with asphalt, you're securing the top of the shingle tight to the wood underneath, and then covering those holes with the next layer. Corrugated sheeting works completely differently. So forget shingles - learn this product.
    – The Evil Greebo
    Jun 28, 2012 at 13:22
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In areas with extremely long and cold winters, many people prefer to put the screws in the bottom because roof rakes are less likely to to knock and displace screws. Same reason holds for going on the roof with a shovel.

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In Australia we use a lot of corrugated roofing .. and walls too. Probably half of all housing, and close to all housing in hot areas, use tin. Universally, we screw through the crest of the corrugation on the roof and through the trough on the walls. I have never seen any signs of water leakage when screwed this way. The main idea is to not screw down too tight. This leaves room for expansion in the heat, ( up 40 - 45 C. One day in 1960 it hit 51 C at Oodnadatta in SA. That's 123 in those funny F temps. ). Just tighten down until the rubber screw sealing grommet nips up. Manufacturers like Lysaght, https://professionals.lysaght.com/sites/default/files/LysaghtRoofingWallingInstallationManualJul2015.pdf, give instructions on this and it's easy to set the tension on your cordless to do it automatically. As an ex-insulation installer who used to take tin off routinely to get some ventilation and pass product through, I had a dedicated DeWalt tin screwer that was preset to the right tension. Of course, in Australia we have very little snow and I am unfamiliar with having to shovel snow off a tin roof, ( sounds deadly :) ), and perhaps trough screwing would be more normal then.

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    – Daniel Griscom
    Jun 20, 2019 at 2:58
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I am in NZ. We have built 4 houses so far all with corrugated roofing. And my father-in-law was a builder. Here they are always screwed on peaks (or crests). Always. Because water pools and runs down in the valleys and is more likely to get in through the screws if theyt work a little loose.

The one exception was that the TV guy screwed the aerial braces on the valleys. Because that is what he has always done. 2 years, later we had water pouring down our fireplace. I called the roofing guy to fix it under warranty. He found the aerial braces and spoke some choice French words to the aerial installer. It's all fixed now and has not leaked again in 3 years.

Heres wikihow

Pre-drill holes on the ridges. Some panels have pre-drilled holes. If yours do not, then use an electric drill to add them. Use a 3⁄16 in (4.8 mm) drill bit, and space the holes every 6 to 8 in (15 to 20 cm) horizontally and every 2 ft (0.61 m) vertically, drilling on the nearest ridge.

Heres an American site

Step 04: Screw on the top or bottom of the panel Corrugated metal roofing panels have two layers, high and low.

To screw, I always recommend screwing on the high position of the panel. Water or derby will not get stuck if screwed in a high position. If these are stuck, the metal roof will be damaged.

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  • A buddy of mine always used a hardened nail to make the holes. He said that he avoids a drill, as the chippings need to be cleaned or as they start to rust, and will corrode the coated roofing.
    – Martin
    Apr 20 at 13:02
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Manufacturers suggest it goes in the flat spots that’s why they sell the proper screws that are also coated to match its not sheet metal screws as one said. It gets screws in the flat spot just before each rib two next to a seam (where two panels meet) so from left to right it would be 

__/-\_.__._/-\___._/-\___.__/-\__.___._/-\____
__/-\___   <——-over lap fat lip on bottom

. = screw ^ every 2 feet

do one screw at a time from bottom to top to keep it from jogging and place the side with the longest lip on the edge so the longest or fat lip is on the bottom or the direction you are laying panels so if you’re going left to right the fat lip will be laid on right side So you’re securing the the last panels edge through the next panels first screw

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  • Check with manufactures screw pattern. And you may have local codes on the pattern to in high wind areas . Make sure you use the proper screw length and spacing ' Pretty wild in Florida when roof is not put on wright. See whole metal roofs come off using 1 inch screws into old roofing peel like a banana ..
    – user101687
    Jun 20, 2019 at 4:59
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Where is the water going? Off the highest point. Use manual nut driver and long enough screws. The valleys of the panels are the most likely source of leaks! Metal roofs with screws need far less fasteners than one would think. The roof will expand and contract better with less fasteners-No it will not blow off any quicker than a single screw tile roof (being we don't use 90# and mud anymore). Don't over or under tighten the fastener and use them about every 2 feet. At the eves fasten at every rib.

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As always, follow your manufacturer's directions. If they have to warranty it, they will expect their process to be employed.

The local mayor tinned his own shop roof, using basic labourers. It blew off the first wind, about a month later. It was as screwed through the strapping ok. They had used staples to hold the strapping down and only a few screws poked out beneath. Using three inch screws on the strapping works for me. I sink two into the meat of the studs at each intersection. If I have double strapping, that's four screws. if the wood is soft, I use four inch on the base layer. Because sheet goods are not broken up, like shingles, the vacuum pressure can be immense, and staples are not appropriate except for initial placement, for convenience. They won't hold the roof down. Although I might go 24 to o 36 inches on spacing, I would only do that with heavy tin, where the purloined are four feet apart. On wood, I prefer about 18". If one screw doesn't hold, the space in that location, is 36 inches. . . No one does that, but I never have to return to fix my work, and there is nowhere that it can flap or move at all. Consider your substrate, when making sweeping decisions about placement. Claiming by his labourers were contractors, he filed for insurance, so the whole community is paying to fix his workmanship. So he could play god and starve the real tradesmen. . . It's like a reward for the negligence, he really does have a knack for government, not construction! I miss the good old days, when adjusters called us first, then they knew what happened good, bad or indifferent. I'm pretty sure the lap is almost always screwed together. The manager, us the one who read the instructions, for that product, every time. Whether it's mixing chemicals, overlapping housewrap 12", or here following a recommended pattern. That's how I learn the right way, the first time, every time. I probably read the KD instructions 2000 times, until I knew the product and how I wanted to effect that result. If I hadn't mastered their technique first, I wouldn't have a baseline, to improve it yet.

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wiggle molding will allow you to screw on the high points of the metal without making a dimple in the sheet. It costs a little more but is worth it if you are unsure of your screw points and want it to last longer.

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The screw goes in the high rib. Be careful to avoid distorting the metal, or loosen the screw up a little. The flat is where the water goes. The water doesn't set on the high rib. I've done it both ways and I've been doing this for over 15 years, never had a leak yet.

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