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?

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  To learn more about roofs and attics, see these other blog posts:

 Why is my roof sheathing sagging between the trusses?

 Why is granule loss a problem for an asphalt shingle roof? 

 What are the mistakes to avoid when doing attic improvements?

 What causes roof shingles to curl up at corners?  

 What causes shingles to buckle along a line on the roof?

 What causes leaks at a fake roof dormer? 

 What causes a sagging roof ridge line?

 What causes bubble-like blisters in a built-up and gravel roof?  

 Why does it cost so much more to replace a steep roof than a low slope roof? 

 What is "ponding" on a flat roof?

 Is an attic required to have a light by the building code? 

 How can I inspect my roof for hurricane damage?

 Why is premature curl of roof shingles a problem?

 How can I tell if a roof has more than one layer of shingles? 

 What are the common problems with attic insulation? 

 What is the life expectancy of an asbestos cement shingle roof? 

 What's the average lifespan of a roof?

Is a house required to have an attic?

 Why is it a mistake to replace a roof and not replace its flashings? 

 Why is there no attic access hatch in the house?

 What is the building code requirement for an attic access hatch, scuttle, or door? 

 Does a roof with multiple layers of shingles last longer?

 What can I do to prevent roof leaks?

 Are roof trusses better than roof rafters (stick framing)? 

 Why is a popped nail in a shingle roof a problem? How do I fix it?

 What are the most common problems with wood roof trusses?

 What causes a lump or dip in the roof? 

 If my roof is not leaking, why does it need to be replaced?

 How can I be sure my roofing contractor got a permit?

 How many layers of roofing are allowed on a home? 

 What are the dark lines running parallel to shingles on my roof?

 How can I make my roof last longer?  

 What are the warning signs of a dangerous attic pull-down ladder?

 How can I find out the age of a roof? 

 Should I buy a house that needs a new roof?

 Should I buy a house with an old roof? 

 What are those metal boxes on the roof?

 What does "lack of tab adhesion" in an asphalt shingle roof mean?

 Why do roof edges start leaking?

 Why do my dormer windows leak? 

 Do home inspectors go on the roof? Do they get in the attic?

 Should I put gutters on the house? 

 How much of a roof truss can I cut out to make a storage platform in the attic? 

 What's the difference between an "architectural" and a regular shingle roof? 

 What does a home inspector look for when examining a roof? 

 Do stains on the ceiling mean the roof is leaking?

 How can I tell if the house needs a new roof?  

 Why does my homeowner's insurance want a roof inspection?

 What are the hazards to avoid when going into an attic? 

      Visit our ROOF AND ATTIC page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.

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