Whole roof IWS - what is correct way?

Roofing/Construction Questions
#1

If you’re covering the whole roof in IWS, what is the correct pattern to lay the IWS? Pretend this is a valley of a 6:12 roof.

A) Valley strip vertical, then overlap side to side starting from eaves (each ends 6" past valley center)

B) Valley strip vertical, then end lap onto valley by 6" minimum (3" sidelaps), starting at eaves

C) Eaves first, then valley strip, then endlaps 6" onto valley working way up

D) Eaves first, then side lap 3" up roof, then valley strip last with 6" endlaps

Or is it one of those modified? A combination of more than one? None of those?

How do you guys calculate how many extra squares you’ll need for all the overlaps? A percentage?

#2

You are over thinking this.

One way is that in general cut up roofs are usually 1/3 Ice & water shield, in these cases I usually just figure 40% of the exact area for Ice & water shield, you should have a little left over.

Another way is to count linear ft and use that since Ice & water shield varies in length per roll depending on brand.

No need to count the lap coverage in either case.

So long as all the laps are on down slopes and not bucking water it’s all good.

All that nice geometry you got there, the roofers will never do it or understand it.

#3

I was more wondering how you’d do a full roof, not just eaves, valleys, rakes, hips, and ridges. But any of those designs would work? Because the last 3 use less than the first one. The second would be the easiest.

It is more for my knowledge than for telling a roofer what to do.