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Roofing/Skylight on flat tar and gravel roof good or bad idea


Hello Brad,

I am interested in having a fixed velux skylight installed in my windowless bathroom. Although, I am concerned about having it installed on my 6-year-old flat tar and gravel roof. In your experience, would you advise against installing skylights on a flat tar and gravel roof? Also, what steps would you take to install a leak-proof skylight on tar and gravel? I have heard of contractors applying asphalt primer, emulsion, and wet patch to create a tight seal between the flashing and tar, but I am worried about cracks forming over time. I have also heard of using galvanized sheet metal flashing. Not sure how the metal flashing is perfectly sealed to the tar. Could you please explain to me the details involved in installing the flashing? Thank you in advance!


You and all those roofers are barking up the wrong tree.  Most roofers use a skylight made for shingles and try to fudge all kinds of things to try to keep it from leaking but every one of their ideas fail over time then they all call me and I laugh at all the goofy ways they try to do this. Home Depot and Lowes stock things for shingle roofs since that's most of the roofs in the country. In the cities you get flat roofs. So roofers don't know how to do it so they go to Lowes or Home Depot and buy a skylight (which is a shingle roof skylight) and they put it on the roof and try to put a ton of roofing cement or "flashing" around it. Flashing doesn't work on flat roofs.

So here's what you do. You get a Velux Fixed skylight that has a curb mount. It's called an FCM on Velux's system. So if you wanted a 22" x 30" Velux FCM you would order FCM2230. You also have to pick what kind of glass you want and then order it. Lowes and Home Depot will be able to order it for you but they won't stock it. YOU DO NOT NEED A FLASHING KIT. They will try to sell you one because they don't know anything about roofing.

You will then build the curb on the roof. You will use 2x6s to make a box on the roof. The curb has to be at least 4" so up it to 2x6 or larger. Once that is installed and your opening is made you then need the roof done around the box and including the box. It's the same material your roof is made out of. You probably will need a roofer for that part.

Once done you just place the skylight on the frame and screw it in through the side holes. No sealant is need. No cement. No flashing. No caulking. No stupid roofers. I have seen so many of these headaches due to roofers guessing how to do this it's a shame.



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Brad Zacharia


All aspects of residential Roofing. This includes shingles and flat (low slope) roofs. I have knowledge in the installation as well as the design of roofs from an engineering standpoint.


I have been doing roofing for 40 years. This was my father's business and I took it over in 1980.

I have written responses to artcles that I felt needed a response to and those responses have been published in roofing trade magazines.

BSEE Drexel University

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