Cinder block chimney flashing


I am finally getting to the part of the roof with the chimney and i’m curious what the best way to do the counter flashing is. Its about 20" x 20" made of 16" cinder blocks. The old flashing was just step flashing with loads of cement over the seams, and it leaked often. With the reroof i would like to properly flash it. Should i just do a long cut parallel to the roof pitch or should i cut steps into the blocks? The mortar lines are 8" apart and there would only be 2 giant steps if i used them. All i can find for examples are brick chimneys and i dont have the luxury of small steps… Any advise would be appreciated! thanks!


For block I would cut a slot diagonally following the plane of the roof.


Thanks! One more question, i plan to tile the chimney with a ledge stone veneer in the near future, are there any considerations i should take for that with regard to the flashing? I was planning on flashing the block, then when i add the veneer just bringing it down to the top of the counter flashing. I figured it would be best to have the flashing against the block rather than the veneer since the veneer and mortar are porous. My concern is that it will look odd though if the veneer sticks out more than the flashing. how would one properly flash a chimney with a facade or veneer?


You don’t cover the flashing with the veneer.


Right, because you cant replace the flashing if needed. Should i try to get the veneer on first and flash that or could i use a spacer to bump the flashing out so it matches the veneer’s thickness? Its getting to be the wet season here and i’d like to just get the roof on before fussing with the veneer. Thanks for your help, i just want to make sure i do things correctly so i dont have to touch it again for a few decades :slight_smile:


You could put a 1" cut into the chimney, then pack the bottom out with a 1x Before installing your step flashing. Bend your counter with a 2" top bend and a 3/8 back hem so it locks into your cut over the 1x. That will keep You watertight. Before installing your wire and base for your veneer install a z flashing over the counter and up the block 3" and the bottom of your veneer will rest on and overhang the z flashing.


Mpa described the best way to do it and exactly what I’d be doing.


That makes sense, thanks guys. I really appreciate you taking time to help homeowners. This is a fantastic forum.


Just to make sure i understand this correctly, does this look right?

I assume just attach the Z flashing with screws/nails? Thanks!


Looks good, nice drawing.


I’m looking at galvanized and aluminum for the chimney flashing, is there a preference? I’m leaning towards galvanized because that’s what i used everywhere else, but as i’m a novice i thought one of the pre-made kits might be a good idea, and they seem to be aluminum. Thoughts? Thanks!


Aluminum does not mix with masonry. Painted galvanized is better, copper is the best.


Aluminum is the most common chimney flashing in my area. I’ve never heard of or seen someone installing galvanized new in this area.


A galvanic reaction occurs when aluminum is exposed to Portland cement based masonry products. Typically paint deters the reaction but personally I don’t like relying on paint for the longevity of my work. If burying flashing behind stone veneer in would defiantly use copper.


I haven’t seen a lot of copper around here, and definitely not in my neighborhood. I’ll see what i can find for galvanized. Thanks!


OK, hopefully last question, i’m a little concerned about cutting a 1" reglet in the block. I dont know if they left it hollow or filled the voids with mortar, although i assume they are hollow based on the construction of other parts of the house. The side walls of the block that was used is only about 1" thick.
Am i going to cause a structural issue if i cut through the sides of the block and into the void? The front and back of the chimney have mortar joints i can use, so i’m not as worried about that. Would it maybe be better to step the counter flashing, even if it’s not on a mortar joint? Thanks!


The block is 8" wide, you can cut more than an inch into it before it becomes an issue.


They are 4" x 8" x 16" blocks, similar to this one:


It will be fine with a 1" cut.


I am having a heck of a time finding a galvanized or copper flashing kit. Moheco claims to have one of each by special order but no one seems to know about it and Moheco isnt answering phones or emails. I’m planning to use copper per MPA’s recommendation, but i’m not sure which gauge and width to go with. I used 4x4x12" steps for the walls, i assume a similar size is fine for the chimney steps. How tall does the apron and head flashing need to be? Should a vise and some angle iron be sufficient to bend everything for one chimney? Thanks!