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Community and Q&A

Skylights that sit flush but don’t leak! Do these exist?

nathaliejordi | Posted in General Questions on

I am trying to daylight a currently unused attic under a shingle roof. Because we’re in a historic district, we’re not allowed to visibly alter the roof’s silhouette (so no dormers or protruding skylights). My architect has proposed switching to a standing seam metal roof with thin glass skylights slotting into the metal seams (as seen in the attached pic). I’m worried that the lack of flashing will lead to leaks. Does anyone have experience with skylights that sit flush but don’t leak? Or am I chasing a unicorn?

Your feedback much appreciated!

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  1. GBA Editor
    Martin Holladay | | #1

    The only way I can see that system working is with two layers of butyl glazing tape -- under and over the glass, around the entire perimeter -- with aluminum or stainless-steel bars to provide pressure. The metal bars would need to be fastened to the rafters.

    There are many more opportunities for bad detailing and failure than there are chances of success, but you could do it. I built a greenhouse this way 40 years ago, with cedar battens applying pressure to the butyl glazing tape, and it hasn't leaked yet.

  2. Expert Member

    It will leak, probably quite soon after it is installed. Skylights are problematic under the best circumstances. DIy-ing them is a very bad idea.

  3. nathaliejordi | | #3

    Don't worry, I'm not planning to DIY this job, a professional roofer would do it! But I am still concerned about leakage due to the lack of flashing--in effect, the only thing keeping water out would be the window's rubber gasket, subject over time to tears, shrinking, cracking etc.

    My contractor proposed a creative idea...installing the skylights in the existing shingle roof, but popped out with a few inches of flashing, and then laying a metal roof on battens above the shingle roof that would serve mainly as a rain screen. There would be openings in the metal through which the skylights would be visible. From the ground, the skylights would appear to be flush with the metal roof (thus satisfying the visual requirement), but the flashing would (theoretically) prevent the roof from leaking. The metal rain screen (standing seam or snap lock) would add longevity to the shingle roof and maybe even keep some water away from the skylights.

    What do you think of this idea?

  4. GBA Editor
    Martin Holladay | | #4

    I don't like the roofer's proposal. The roofer is apparently suggesting that water be allowed to enter the space between the old asphalt shingle roofing and the new steel roofing -- which is not a good recipe for success.

  5. Expert Member

    I didn't mean DIY as in you doing the work, I mean using a site-built skylight rather than a manufactured one. Your first idea simply won't work. Metal roofing expands and contracts with differing temperatures. That's why things like skylights are installed on a curb with flashing that allows for this movement.
    I don't know how to sugar coat this: Your contractors new idea is straight up nuts. I don't enjoy discouraging people, but I hope I am discouraging you. Unless you can find a manufactured low profile skylight, you are going down a road that will cause you untold future aggravation. Don't do it.

  6. jackofalltrades777 | | #6

    Skylights - the never ending nightmare. I've owned a home with them and swore that I never would own or install this device from hell. Unless they are installed 100% perfectly with redundant back-up systems, on a pitched roof, and one inspects them on a yearly basis, DON'T DO IT! You will curse the day you installed it.

    Besides the leaks, the big problem is also that it is basically an R-3 hole in your roof. You have a R-40 / R-50 roof and then this R-3 hole is there to rob you of your energy. Snow, ice and hail are other problems. Skylights are bad news and a flush skylight is a disaster.

  7. gusfhb | | #7

    What we do not know about the picture is how far the glass extends up the roof.If it extends up towards the peak significantly it may not leak.

    As a note I retrofitted 4 curb mounted skylights on my 3 1/2 pitch[shingle] and 2 1/2 pitch[rubber] roof that do not leak a drop [water, heat is another story] in 5 + years

    If I were to postulate a watertight solution to the ridiculous historic requirements.........

    If you were to mount a fixed skylight to the sheathing, with ice and water shield in such a way that it was water tight [say 3 feet to either side] prior to the roofing being installed.

    install sleepers such that the installed roofing was slightly higher than the top surface of the skylight.
    Install tempered glass panels in place of the metal roofing from roof peak to well below the skylight, above the surface of the skylight. Any break in this section of panel must be lapped shingle fashion.

    On the sides the standing seam metal will overlap the tempered glass by say at least 6 inches

    the edges of the tempered glass has a generous bead of sealant, continuous peak to eave. Inboard of that bead of sealant a series of diagonal beads of sealant angled downhill and not quite reaching the vertical bad of sealant or the edge of the overlapping metal roof. This guides water away from the edge of the glass.

    Basically think exposed metal valley and how those seal, mostly relying on gravity

    Is it ridiculous? no more than the historic districts rules, most likely a ridiculous overreach of their mandate, but I will bet it won't leak.

    A roof should be water tight without sealants due to gravity, and should have at least one double measure. If you envision my solution without sealants, it 'shouldn't' leak due to the pitch and 6 inch overlap of the metal over the glass. The sealants prevent wind blown water from moving sideways. When[not if] they fail to do their job the ice and water that is over a foot horizontally should guide the water to the roof edge. The actual skylight is just an air and thermal barrier

    God knows who you would get to install such a ridiculous system and how much they would charge, but hey, it would follow the rules............

  8. Expert Member

    Keith wrote:
    "If you envision my solution without sealants, it 'shouldn't' leak due to the pitch and 6 inch overlap of the metal over the glass."

    It would leak like a sieve. Capillary action would draw the water between the glass and metal. The main reason you don't see certain details in buildings is that they don't work. Relying on multiple layers of sealant, having no strategy for dealing with the expansion at the perimeter when the water freezes, nothing to deal with the differing coefficients of expansion of the two materials - it's a really risky solution.

  9. gusfhb | | #9

    Funny Malcom, open metal valleys do not leak.

    My point was that you should design a system that does not leak from gravity without sealants, that sealants should always be a redundancy. In this case the sealants are not so much sealants as dams to redirect wind driven rain back out onto the glass.

    All shingle roofs assume that there are 'leaks' that will then self drain

    My description was not so much a proposed solution as an explanation of how the picture might possibly be made to work, and show the bizarre extremes one might have to go to satisfy a ridiculous commission

  10. Expert Member
    MALCOLM TAYLOR | | #10

    Keith, I agree entirely with your post and the point you make about gravity and sealants. I was simply piling on agreeing with your conclusion that it isn't a good idea, no matter how well done, so that nobody ends up trying it.

  11. gusfhb | | #11

    I don't know why didn't think of it before, but corrugated roofing was invented in the 1820's so, some lovely corrugated metal roof with the oh so chic fiberglass inlay skylight panels.

    Wonder if the commission would let you put in a regular skylight after that submission..........

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