Best product for repairing a leaky skylight?

I have 2 skylights (Velux brand if that matters) in a bathroom. I'm guessing they're roughly 20 years old, and one is showing some signs of leaking. It's nothing major yet but I'd like to take care of it soon.

Right now, the plan is to get up on the roof, remove the cladding, and add (INSERT PRODUCT HERE) between the glass and the framework and then put it back together.

But what should I use to seal it? Butyl tape? Silicone caulk? Something more specialized? Both skylights are the same age, so does it make sense to preemptively do the same to both skylights, or is that opening pandora's box for no reason?

I use OSI Quad for almost everything outside. 

You need to find the source of the leak.  Bad flashing or a leak in the skylight structure itself.

best products for leaking skylights are plywood decking, tar paper, and shingles.

thanks, i'll let myself out.

srsly, i sat on my perfectly leak-free roof for about an hour before i sank the sawblade in to install skylights in my family room.  They were Velux brand.  I moved about 6 years later so i have no idea how they've survived since then.

AngryCorvair (Forum Supporter) said:

best products for leaking skylights are plywood decking, tar paper, and shingles.

Came here to say this.

Noddaz said:

You need to find the source of the leak.  Bad flashing or a leak in the skylight structure itself.

Of course. The signs of water intrusion are most evident on the underside of the glass, (about 4 inches above the roof deck) and inside the perimeter of the skylight frame rather than outside on the ceiling drywall. So it doesn't seem like a roofing or flashing leak, but a sealing issue with the glass/frame of the skylight. Affected area circled in red:

I picked up some solvent based clear Lexel the other day that seems pretty comparable to the OSI Quad that Toyman suggested, and well...it's already here. It's supposed to be super elastic, good indoors and out, on all kinds of surfaces and temps. It gets really good reviews, and it's attributes seem promising. I think I'll give that a try unless somebody here is staunchly opposed or has a better suggestion.

Streetwiseguy said:

AngryCorvair (Forum Supporter) said:

best products for leaking skylights are plywood decking, tar paper, and shingles.

Came here to say this.

My exact thought as well.  There are 2 types of skylights, the ones which leak, and the ones which will leak

I will fourth the plywood and shingles. Skylights always seem to be a problem. 

We have one skylight in our master bathroom.  I just keep expecting it to begin leaking any day now.  Just because it can.  If I ever replace the roof on this house, it will be removed without a doubt in my mind. 

JThw8 said:

Streetwiseguy said:

AngryCorvair (Forum Supporter) said:

best products for leaking skylights are plywood decking, tar paper, and shingles.

Came here to say this.

My exact thought as well.  There are 2 types of skylights, the ones which leak, and the ones which will leak

I mean, you could say the same about any roof. A well installed skylight will last decades without leaking. My current house has one that has outlasted one shingle roof so far without a single leak. From the skylight that is - the shingle roof was leaking like a sieve.