Example of mastic instead of proper flashing

Here is a good example of why roof mastic is no substitute for proper flashing.


You’ve got that right, Randy!

Thanks for sharing.

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Thanks for sharing! One problem leads to another and another and another…

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Good Post, Randy. Here is another example. The chimney had a bunch of old sealant at the roof and no cricket either.
I cheated and used one of those gadgets (pure gimmick I use to look smart in front of clients) :rofl: :wink: to find that the wall in the living room was soaked (no visible stains) from water running down the exterior of the chimney through the roof and down into the living room.
(Notice my Safety-Buddy when I’m up on a steep roof, top right corner :wink:)

Cheater :rofl:


I refer to mastics by type. In this case, roofing plastic cement, asphalt bitumen mastic.

Sheet metal chimney flashing/s, roof deck step flashing and chimney counter flashing/s, are unique. The flashing/s allow for expansion/contraction component independence, movement, between the roof deck sheathing component and the chimney component, allowing required sheathing expansion/contraction clearance.

A: Step flashing/s are secured to the roof deck. B: Counter flashing secured to masonry. The sheathing can expand and contract.

By using roofing plastic cement, asphalt bitumen mastic on the roof deck covering and masonry surfaces, the 2 components are secured/sealed at the clearance intersection until expansive/contractive forces tear or buckle the seal. The intersection/s are no longer weather tight.

Thank for sharing.

In what manner would you secure step flashing to a roof deck?


Brian, First the rotted roof sheathing has to be replace at the chimney. Then standard flashing like in the graphic below: (Note my chimney did not warrant a cricket)

Nice graphic. I see they show the “side cap flashing” which I call counter flashing to be embedded or tucked into a mortar groove joint. Often this “embedding step” is skipped which defeats the purpose in my opinion.

I was asking Robert about his statement “step flashing attached to the roof decking”. It should be attached to the sidewall, not the decking in my opinion.

Fasteners. As you fasten shingles the same fasten applies to each step flashing. Installed shingle roofs for over a decade. Fashioned and Fastened flashing for decades. Bent sheet metal for decades.

Randy, yes that style of chimney flashing you have exhibited in the illustration allows for roof deck sheathing movement. As in your neck of the woods and mine Counter flashing is sealed in mortar joints. That way you can unlock the mitered Counter Flashing corners and manipulate new step flashing under each shingle when recovering the roof.

The point I was making is that only mastic bonds the sheathing to the chimney. Once the deck contracts or expands the sealant will fail. Same expansion contraction applies on 3 point caulking contacts on wide gaps.


From the article.

Each step flashing piece is nailed to the building wall only, and uses a single nail in the uppermost or top corner of each step.

This is how I was taught. But I am in the “down south”


You are correct, Brian…there is some mis-information going by on this forum. Make sure you vet it and the source. :grin:


I don’t think it is misinformation as opposed to other methods.
Iko How To Install Roof Step Flashing.
This is on a abut wall. Same as chimney. Note the high step flashing fastening. The shingle should be notched back 45 degrees at the top inside like valley.

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I went GAF and watched their video, they also attached the flashing to the roof deck. (video from 2016). Apparently, different methods are acceptable.

I personally do not like penetrating the roof at high water volume areas and will likely continue to advocate for sidewall fastening. Most important in either installation is the overlap in my opinion.


I apologize to Randy. I just noticed Counter flashing is installed only in strips. Mostly one piece full Counter flashing in my neck of the woods.
Thanks for sharing. Flashings threads are always great learning tools.

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