Attaching corrugated sheet metal to a shed roof

You know, I was kind of hard on Mr. Sleepy Home Depot Guy in my last post, so I will offer this in his defense: he did give me one piece of useful information, and that was to use a special lag screw designed for attaching sheet metal to roofs. The lag screw comes with a rubber washer already on it, so that when you screw it down, it seals the hole you have just made. He also picked out a bolt head bit for my drill drive that would fit the lag screw, both of which are shown in the photo below (click the pic for a closer view; bolt bit is on the blue drill drive):

Very handy. Thank you, Mr. SG.

Other than that, my questions seemed mostly met with a puzzled, vacant, vaguely cranky, straining-to-be-polite-to-the-customer-because-I'm-basically-a-nice-guy look, as in, "Lady, why are you making this harder than it has to be?"

Or perhaps it was, "Lady, why don't you ask your husband?"

Or, to be fair, maybe it was merely, "Lady, I haven't had my coffee yet."



For example, when I asked about using a drip edge, he responded, "Well, I guess you could use one. Most people just slap the sheet metal on there and let it overhang. The overhang is all you need to keep the rain off."

Well, that may or may not be true, but I didn't want a huge overhang because I just don't like that look. I didn't want to get into a discussion about the aesthetics of "long, unwieldy, trashy, half-hearted overhangs that look like an afterthought versus short, neat, tidy, trim overhangs that look like you care about good things in the world like, for example, beauty" with him, so I let it pass...and loaded the drip edge on the cart anyway. Mr. SGuy didn't go so far as to shrug openly when I did so, but I could see that he was apathetic about my choice.

And he did answer other questions, such as whether I needed supports under the sheet metal ("Not really. Most people just screw the sheets to the studs and skip the supports underneath."), and which ridge cap to use.

By the time I had checked out, I was feeling vaguely nervous about both the project and about my own general can-do-ness; nevertheless, I loaded the material onto my trusty trailer and headed home.

I need not have worried. Easy. As. Pie.

Here is the process, explained below, with photos for reference:

First, I put down 30 pound roofing felt, using staples, and covered the edges with drip edging, using roofing nails (drip edging prevents rain from coming into contact with the wood of the roof), shown here in this photo with the ridge cap and the first corrugated sheet already in place:

I then measured the ridge cap to length and cut it. Because the ridge cap has "wrinkles" into which you slip the corrugated sheets, I wanted enough overhang to match the two components, and so when I cut it, I allowed for a one inch overhang on either side. Later, I would make a tiny snip in the center and fold the excess down and cover it and the sheets with a wider drip edge (strictly for looks).

As you can see in the photo, the wrinkles in the cap (I'm sure they have a name, but I like "wrinkles"), are actually a few inches down the roof from the ridge itself, which means that the edges of the sheets are not the same length as the length of the roof. So in order to measure the sheets to length, I placed the ridge cap on the ground, slipped one of the sheets into the wrinkles as far as it would go, then measured how long it needed to be, allowing for a 1/2 inch overhang.

I then cut all the sheets to length. See my earlier post here.

Then I cut some scrap wood and screwed this into the rafters on the underside of the roof. You see, Mr. Sleepy Guy had given me an idea when he said that most people just screw the sheet metal into the studs. By putting the scrap boards approximately where I wanted to place the lag screws, I could just measure down from the ridge (15") and up from the bottom (11") and screw them in along that line without worrying about where the "studs" (in this case, the rafters) were. I believe there is a construction name for boards that exist for the sole purpose of nailing or screwing things in them, but for the life of me, I can't remember it...This is called "strapping."

Anyway, once all the sheets were cut, I tacked the ridge cap to the roof and slipped the first sheet up under the corresponding wrinkles, and tacked that (see the second photo, above). Since I was doing all this by myself, I held each sheet in place with clamps at the bottom to position them while I attached the lag screws. To screw them in, I first used an electric drill (more power than a portable drill) to make a hole (shown with the bit in the first photo), and then followed that with a portable drill with the bolt bit and lag screws.

Once the first sheet was tacked on, I followed it with the other sheets, overlapping each by at least two ridges, and clamping each down until they were all positioned the way I wanted them:

After that, it was simply a matter of measuring to the board on the underside of the roof and screwing the lag bolts into the boards underneath. Once this was done, I cut wider strips of drip edge to length and attached them to the edges, using the same lag bolts, to give the roof a neater, more finished appearance:

I also used a rubber hammer to apply some "carpenterial persuasion" to the wrinkles to make them adhere more neatly to the corrugated metal below. The ridge cap is made from heavy gauge aluminum, and so this is very easy to do.

The only other counsel I would offer about roofing is to make sure that any edges that have to overlap (as in drip edges, or shingles, or whatever) are "shingled" so that water runs down off one onto the other below it. In other words, the lower piece of roofing material is always underneath the higher piece.

And there you have it, a weather-proof shed, just in time for the next snow: