Today we found a roof that was pouring water into a building by the gallons. The water was draining underneath an APP flat roof, this flat roof was retaining a huge amount of water. The reason this roof had such a bad leak was because the installers used a continuous pan flashing running vertical down the wall and underneath the shingles. This continuous pan flashing is a very bad roofing technique and leaks in  almost every situation I have seen it used.

Below is there is an example of the failed flashing technique (continuous) used on some roofs. There is also a diagram with an example of proper step flashing. Continuous versus step is no contest as evidenced by the video and the testimony of an over whelming majority of competent roofing professionals.

 

Continuous flashing
Continuous flashing is a single piece of L shaped metal, installed before the actual shingles are applied. Normally roofing cement is applied to the metal that is on the roof, and the shingles are set in this cement. As the shingles are being applied roofing cement is used in between the shingles to ensure a continuous seal. Counter flashing or siding is then installed over the flashing that is on the wall.

 

Step flashing
Step flashing is installed along  walls and chimneys. Step flashing is an L shaped piece of very thin sheet metal (usually aluminum), normally 7 inches long by 3” by 4”. It is placed along a wall or chimney and then covered with a shingle and then another piece over laps this step just above the exposure of a shingle. .This process is repeated all along the wall and then either is covered by siding or counter flashing.

Below are some quotes from roofers around the nation on the subject of continuous flashing
I have replaced too much sheeting at chimneys and walls where 1 piece flashing was used. The water migrates along the top of the shingle and as soon as it hits the metal it wicks under it and travels to the nail and causes damage.

Keith, Anchorage

That type of flashing always leaks from what we see on repair work.

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WWW.jbennetteroofing.com

You step-flash rake edges along base of walls, chimneys, etc. L-flashing is a quick way of doing it, but it is wrong and often times leaks.

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Professional Roofing & Waterproofing Consultant

Lloyd Williams Chattanooga,Tn
Common step flashing in my area is 4x4x8 and is
laced in every coarse of shingles. It’s all I have
ever used and I started shingling in 73. The few jobs
I did see that used continious leaked. Continious
is used for headwall and fronts of chimneys.