What tio do! Leaking Big Agnes Copper Sour 2p

Be very confident in whatever fix you decide upon.  It’s looking like an unusually wet summer in Colorado this year!  Happy trails.

It appears it didn’t leak, but the water was from wind driven rain entering when the vestibule was open. If you are worried about it, you can coat the fly with mineral spirits and silicone about a 20-30 to one ratio. Just brush it in well.

Set it up again, button it up as you would in a storm and unleash merciless hell on it with a garden hose.  You will have your answer quickly thereafter.

If you find that it is indeed the fly leaking and not water ingress from other reasons, I would suggest you get hold of a can of Atsko silicone waterguard. That will add 2 oz or so to the fly weight. If you do it yourself you could end up with several extra ounces.
Make sure the fly is nice and clean first.
Atsko also has a liquid version. More expensive but it will last longer.

a can of Atsko silicone waterguard.

I do that every couple of years to my aging blue silnylon tent. Works well.

Cheers

It probably would be a very good idea to closely examine weather it is due to water leaking thru the fly fabric or if the water is simply being blown in from the sides and small openings, and the fly doesn’t have enough coverage.

In other words, if the fly were a little larger and came closer to the ground, would it still blow through? Would extra guy points help?

And geologically speaking, UV will ultimately be the demise of a good tent fly – even on a tent that was well tended to. Enough days out under the sun and no amt. of waterproofing spray will help any more.