A friend of mine has a skylight that leaks. It’s a flush mount unit so the standard flashing along the side isn’t applicable. I dug out the sealant that was bad and used a rubberized sealant that can be used to repair leaks on wet surfaces. It worked fine for 2 weeks and then it pulled apart. After reading all the qualities of the product I see that it states that it meets or exceeds all requirements of ASTM Specification D-4586, Type I **except for high-temperature sag. **. It appears I may have chosen the wrong product entirely.
I’m now back to square one and will have to scrape this stuff off the shingles and skylight and apply something that works. This stuff is still pliable and it doesn’t appear to stick well to the plastic skylight so hopefully I can get it all off. What product can I apply that will not sag/crack and will adhere to the skylight plastic?
You may want to try NP-1, generally available only in the construction industry. My dad was just here visiting last week and helped me close up a garage wall in my house (I never used the garage, and we were framing in an exterior wall to finish the project). We used a similar product to seal where the sill and concrete pad met (it gets a lot of water in the winter/spring). Dad just retired after many years in the trades, and most recently was an inspector for the state of NY. Swears by the stuff, though you generally won’t find it in your local Home Depot or Lowes. It’s sorta touchy stuff - works great, but you’ll need to follow the safety instructions to the letter. Gloves, etc. The MSDS is on the link, if you want to read it. NP-1 Sealant Home Page
(Scroll down to the NP-1 info)
Note: This is just one site of many that advertise NP-1. You might find it locally in a building supply store that caters to contractors. Ask one of the more knowledgeable associates regarding your particular situation. Often, the local stores employ people who used to work in the trades that can point you in the right direction - project wise - without blowing too much smoke your way.
This may or may not apply, but I had some vent stacks that didn’t want to stay sealed with just mastic. I found some fibreglass mesh intended for the purpose, and embedded that in the mastic. This has held up for a couple of years now.
A contractor friend of mine swears by Vulkem-116 for anything and everything that leaks. In my experience, Henry 208 is a temporary patch until you can have the roof properly repaired or replaced - after a year or two, it hardens and cracks. But, its biggest claim to fame is that it sticks and seals under water and in the rain.
Tell us a bit more about the skylight… Sounds like it’s on a pitched roof, and is a low-profile mount.
There has to be some sort of flashing - you can’t just depend on the good graces of caulk to last for a couple of decades. Properly flashed, the skylight should be leak-proof without needing caulk.
You’ll probably need to pull up the shingles immediately surrounding the skylight to see what’s really going on - the skylight may have a built-in flange on the edges, and this may have cracked, for example, or one piece of “step flashing” may have come loose, or the cap/counterflashing that covers the upper ends of the step flashing may have come loose, etc.