Gotchas (or not) of Metal Roofing

I've spent the last few days in deep-bunker research mode for a new roof for our home.

Why I'm going with a metal roof:

  • Tired of trying to figure out where my asphalt shingle roof is leaking.
  • Better reflectivity of sunlight = cooler house in summer.
  • Lasts longer.
  • Better wind resistance

Right now I'm truly torn between a standing seam (concealed fasteners) roof and an exposed fastener metal roof. The cost of standing seam is about 2.5x higher, so I will go with exposed fasteners if I can manage to resolve the following issues:

Allowing for Temperature-Induced Movement

The metal panels will run from ridge to eave so that the water crosses no joints on it's way off. Using the formula provided here, I figure the expansion/contraction amount for my longest pieces will be possibly as much as a quarter inch. (Remember, on the hottest summer day, the metal will be hot!)

Just stop and think about that. My roof panels will be 1/4″ longer on the hottest day vs. the coldest night. That's a lot of movement!

So how do we deal with that? Is it really a problem?

Well, it isn't a deal breaker (according to link above) until a galvalume (steel core) panel gets up to about 40 feet in length. Short of that, you still have to plan for the movement but it's a winnable war.

So, let's think about this: The Sheet wants to grow so, on a hot day, the end screws start getting pressure pushing them sideways. The unstoppable force meeting the immovable object, as it were.

Assuming that the screw is firmly anchored into a strong material (like solid wood or plywood) that lies just beneath the metal sheet, the screw will not move. Screws are very, very stubborn. So in this case, the thing that gives is the metal sheet material that is pushing against the screw shaft. And so the roofing begins to get “wallowed out” around the screw shaft. So the critical factor here is: Can the flexible washer on the screw continue to hold a waterproof seal in the face of a widening hole and possibly sharp/irregular burrs around its edge?

But wait! We assumed that “the screw is firmly anchored into a strong material (like solid wood or plywood) that lies just beneath the metal sheet”. It turns out that this isn't how my two recommended local bidders want to do things. Let me explain.

As stated, my house currently has shingles. Both bidders want to cover the existing shingles with 30 pound tar paper and put the metal sheets down over that. So now you can see that there will be a .25-.375″ gap between my plywood decking and the metal panels. This creates a situation where, in my opinion, the screw will be flexing and probably bending and possibly, ultimately, breaking. Additional problems are that the washer is pushed to a non-flat angle and, worst of all, the downward pressure required to compress the flexible bushing will have the effect of creating little depressions wherever there are screws. Hmmm… low spots that encourage water to flow toward our screws…

So, to get the screw connection clean and strong, we need to either remove the shingles or run horizontal purlins (1x4s or 2x4s). These both allow the metal to be laid right on wood (perhaps with only the felt paper in between). I'm leaning toward the purlins, and here's why:

Being a “wood guy”, I could easily make the bottom 3-4 purlins be slightly mobile. First, I run an oversized (maybe 1/4″) hole all the way through the purlin for the shaft of the big purlin screw. Then I create a 1/2-3/4″ diameter hole for the pan head to sit down in. When the purlin screws are run, they aren't fastened tightly. This allows about the same amount of movement that the steel panels will produce. But the purlin screw will easily resist any upward lift. Doing these holes all at the same time wouldn't require too much time, and it seems to be an elagant solution to the problem.

Two problems I see with this:

First, in order for the purlins to move with the weather, it seems like I'll need to remove the shingles beneath them. So I end up with a big heap of work.

Second, this jacks the roof up by nearly an inch and a half. Wouldn't that mess up my gutter system? The water will be shooting off that roof a lot higher than before. Move gutters up?

Washers that Last

In my research, I keep running into the allegation that the standard neoprene flexible bushing in the screws will dry out and crack up after 10-15 years in the weather. One of my bidders offered an upgrade to a Zac screw. In researching, I find that this screw features more of a cupped washer reinforcement (complete weather protection of the sealant material?) and that the bushing in it is made of EPDM, which appears to be more weather-resistant. Am I getting this right?