Biggest hint would be to talk him out of it.
Regarding wind slits we first have to discern the reason for them. We all assume that the slits will give stress relief by allowing the wind to pass through the banner putting less stress on the material and it's anchoring method.
So how much wind does it take to compromise the material or anchors? Well, how big is the banner? Certainly we wouldn't put wind slits in a 12" x 24" banner because we are sure that the material and anchors would easily withstand any stress the wind could put on it. So when does the banner become large enough to warrant wind slits?
Is 36"h x 144"w reaching that size? How strong is the material? Is it 10 oz? 13oz? 18oz? Is it hemmed? Is it installed by tying ropes into grommets at the corners or is there a rope running through the width of the banner sewn into the top and bottom?
Logic says to combat stress first look to use a strong material and a strong anchoring method. That may be all that is needed. This can depend on the size. I've seen a 48"h banner stretch all the way across a wide street in Columbus, Ohio that had what looked like 3/4" holes all over the material. It looked like window perf there were so many holes. Of course, the message was in such big letters that it could still be easily read. Like with window perf. Probably could have hung that with binder twine. Overall the area of the banner probably equaled one half it's size.
I've also seen banners stretched all the way across a street both using and not using wind slits. I never knew if one blew down and another didn't.
Imagine stretching two ropes across the street. How much pressure would it take to make either the rope or the anchor fail? That depends on the rope and the anchor I'd think. Now put a banner between the ropes. In a high wind, does that create enough pressure on the ropes to cause them or their anchors to fail? It would take an engineer to figure that out. I remember one who did.
In SignCraft magazine some many years ago there were a serial of articles by an engineer who explained such complexities. He talked about wind load on a sign, a billboard, banners, etc and showed examples using drawings of how to combat these stresses in confidence.
Regarding banners and wind slits he gave an example. He said, if you cut 6" semi-circles it would take 4 of them to equal 1 sq. ft. of relief against the wind. He also said you can accomplish the exact same relief by making the banner exactly 1 sq. ft. smaller. I know. My reasoning and logic says it would make a bigger difference if the relief were in the center compared to making a banner 2'h x 11'-6"w instead of 2'h x 12'w. Not so says he. It's the same thing. It's all about the size of the area standing up to the wind.
I never forgot that. And I never put wind slits in a banner. Instead, I've used strong material and a solid anchoring method to combat the wind.
36" x 144"? I'd use 18oz material with a 1/2" rope top and bottom sewed into the hem and anchored by tying it off onto something rock solid on either end.
According to the engineer, if you planned on making a wind slit, as you described, in two rows every 2' along it's length then you would have 12 slits. Needing 4 ea to make 1 sq. ft. of relief that would give you 3 sq. ft. of relief from wind stress. According to the engineer you can accomplish the same thing by cutting a foot off the end of your banner.
Think about this as well. When you've cut a wind slit into a banner you've already created a tear in the material. A well-planned, accurate, precise tear, but a tear none the less. The only difference between that and a rip is that you planned the cut you made. It turns into a rip as soon as it tears further than you planned.
You can help yourself somewhat by punching an 1/8" die hole at each end of your proposed wind slit cut. Then hand-cut just into the hole but not beyond it. That would end the tear with a punched hole and distribute some of the stress around that hole.
It's up to you and I guess ultimately up to your customer. It's also up to the material and anchoring method you choose.
I wish I could remember the issues of these articles by this engineer. It was many years ago; probably late 80's-early 90's. The wind slit issue is the only one I remember in detail partly because it was the only one I was able to comprehend in detail and partly because it was the only issue he'd covered that I'd dealt with up to that point. Plus it blew apart my logic on the wind slit subject. I no longer have those old issues. There was a wealth of information to solve a lot of common problems.
