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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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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 '12 at 12:09
Why is the choice different compared to roofing slate? – sharptooth Jun 28 '12 at 12:25
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 '12 at 12:58
Well, both have "tops" and "bottoms", that's why. – sharptooth Jun 28 '12 at 13:17
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 '12 at 13:22

Were you considering using Horizontal corrugated closure strips?


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those are for sealing the ends, not for attaching the roofing to the sheathing. – DA01 Jun 28 '12 at 22:49
where do you get that idea? – Philip Ngai Jun 28 '12 at 23:03
am I wrong? I've never heard them used for installation, but for sealing the ends when used as siding. But maybe I'm completely wrong! – DA01 Jun 29 '12 at 0:06
For instance, this site doesn't mention them at all. But it's also not the most detailed instructions, either: corrugatedmetalroofing.net/installation.html – DA01 Jun 29 '12 at 0:09
My link says "Horizontal Wood Strips are used on every cross support". And page 2 of the following guide shows this in a picture. eplastics.com/pdf/… But this is fiberglass and perhaps metal is different. – Philip Ngai Jun 29 '12 at 7:24

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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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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