How To Look At A House
McGarry and Madsen's home inspection blog for buyers of
site-built, mobile/manufactured and modular homes
How do you flash skylight, chimney, and pipe vent roof penetrations on a metal roof?
Wednesday, February 6, 2019
The principle behind flashing on any sloped roof, and especially a metal one, is downlapping. The bottom edge of one piece of roofing overlays the upper edge of the piece below it, which overlays the upper edge of the piece below that, and so forth.
Roof penetrations like skylights can be challenging when trying to maintain downlapping. The diagram below, from Gulf Coast Supply and Manufacturing, shows the recommended way to flash a skylight. Using a combination of endwall and sidewall trims as shown, with chimney flashing across the upper side, start from the downhill side working up, so that the laps provide a continuous drainage path. Any laps should be sealed with butyl tape or sealant.
Penetrations wider than 30-inches will require a cricket at the uphill side to divert water around the penetration. See our blog post What is a roof cricket? for more on this.
And here’s an example of how not to do it. The arrow in the photo below is pointing to the top edge of a skylight with up-lapping over the PBR ribs and gobs of clear caulk to cover it all up. There was also puddles of water sitting at the caulk joint when we examined it, that you can see if you look closely.
Vent pipe penetrations require a rubber boot that is screwed through a malleable metal ring at the base into the roofing, as in the diagram and photo below.
And here’s another example of how not to do it, where the installer simply cut a hole in the panel and caulked around the pipe penetration.
Also, see our blog post Can metal roofing be used on a low slope/pitch roof?
• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •
To learn more about roofs and attics, see these other blog posts:
of Blog Posts
Top 5 results given instantly.
Click on magnifying glass
for all search results.
Buying a home in North/Central Florida? for a team inspection by two FL-licensed contractors and inspectors. Over 8,500 inspections completed in 20+ years. In a hurry? We will get it done for you.