-If only putting a layer of ice & water shield is considered insufficient underneath the closed valley, then why would the ice & water be sufficient for the first three feet of roof above the troughs?
Here's what I mean: since ice and snow sits on both of these areas, why should I sell you on the impermeability of ice & water shield on one area but not on the other? If you want to make argument that the joint is a vulnerable spot in the valley then I will tell you that a joint is a joint is a joint and there are many along the first three feet of roof unless your roof is 8' long (or less) on any side.
-Secondly, metal may facilitate the downward flow of water better than shingles do but when we talk about ice damming (which does occur over metal valley), we're not talking about the downward flow of water. We're talking about cohesive and adhesive forces that act to "pull" water from melted snow and ice up under the shingles. Metal valley does not extend more than 8" outward from the valley, whereas shingles can go as far out as you want them to. (In a closed valley, shingles "wrap" from one side of the valley to another). I've seen heater cables installed over metal valleys. I'm sure they are there for a good reason.
- As I mention on our product page, metal valley will be transported several times to get to the top of your roof. We've had calls about leaking metal valleys and the culprit would sometimes be a pin-sized hole, damage which is likely to have occurred during transport. Even the smallest holes do a whole lot of damage.
If you have any questions or thoughts you'd like to share regarding this topic, please comment.