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Community and Q&A

Framed Chimney Chase and Flashing Details

dragonfly63 | Posted in General Questions on


I am a homeowner from Wa. County, Oregon so climate Zone 4c looking for advice on flashing detail for a framed chimney chase. We have just had a gas fireplace installed and inspection hopefully on Monday then we can start to button it up.  Chase is on the gable end of a single story home.  The chase will be clad with cultured stone (lumpy stucco) so we want to make sure we get all the flashing details right.  While I have a lot of questions I’ll start with the most pressing first.

The plywood Not OSB) has 1/8” gap between seams in the field and on the corners (inside and outside corners). I could use a peel and stick flashing membrane like Vycor as its readily available on all the corners and sheathing tape on the field seams but I was thinking it might be easier and a better result if I use Prosoco fast flash on the corners and and seams.  It won’t peel away or come undone and it looks like something a homeowner can accomplish.  I can also get their joint and seam filler.

Question #1 – Should I just use the fast flash or treat all joints with the joint and seam filler, then the fast flash or would a peel and stick be better for a DIY’er?  Yes I would have to buy a sausage gun but probably only 2 tubes of the fast flash and 1 tube of the joint and seam filler so I’d be into it for maybe $150.

 This chase will have sloped shoulders and since anything that isn’t vertical is a roof I will need to treat these should as such.  They will be capped with hearthsones.  Trying to figure how to incorporate the wrb and what to do on the shoulders.

Picture  shows yellow area where I would use fast flash, blue is wrb and red is ice and water shield on shoulders and over wrb with succeeding web layers shingled properly.

Question #2 – Even though Vycor is a non-roof flashing would it be acceptable to use on these shoulders or must I use an actual roof underlayment like ice and water shield? The shoulders measure 23” x 17” so not more than 8 sq feet.  I am going to need some for the roof to wall transition anyway. I can get a 108 sq foot roll of ice and water shield for $100 or a 150 sq ft roll of GAF Weathermate for $60.

Any comments or thoughts would be greatly appreciated.
Thanks for your time! KJ

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  1. user-2310254 | | #1


    I'll give your post a bump. It shouldn't be hard to get answers to these questions.

  2. dragonfly63 | | #2

    Thanks Steve, much appreciated!

  3. dragonfly63 | | #3

    Well I'm giving a bump to my post in hopes of getting a response. I'd like to accomplish the flashing while its warm and dry. Specifically if Prosoco joint and seam filler followed by the their fast flash would be appropriate for the inside and outside corners of our framed chimney chase.

    I'm still struggling with the sloped shoulder of the chase and what product to use on that.

    The chase will be clad with cultured stone.

    Many Thanks! KJ

  4. user-2310254 | | #4

    Hi Karen,

    My by-no-means-expert 2 cents on this. If it were my house, I would use the Vycor to seal all the joints. You could use liquid flashings as well, but you already have the Vycor if that's more readily available.

    If it were my house, I would simply wrap the chimney in cement siding (with a rain screen to promote drying). But it's your house, and I understand wanting to use a stone-like product. For that type of installation, I would follow the advice outlined here:

    1. dragonfly63 | | #5

      Thanks Steve for weighing in, I appreciate it.

      I really hate Hardie wrapped chases, they look like elevators.

      I've read just about every article I can get my hands on including the one you linked. I have the MSV installation guide on my computer. I believe veneer stone can be used it just has to be detailed correctly. We interviewed 7 masons for the job and only 2 knew what a weep screed was and what a rain screen was. We heard everything from hardibacker is waterproof to wanting to attach the lath directly to Tyvek and this is in Portland, OR! Its been very frustrating. I'm just a homeowner that wants to educate themselves on the work they want done on their house and make sure it's done correctly so it's a little frustrating when I have to explain what the above items were to licensed and bonded contractors. The mason we chose knew both.

      I was hoping for a pro or two to weigh in but that doesn't look like it's going to happen but I'm determined to move forward. I'd rather have confirmation that I'm on the right track or tell me if I'm not rather than an after the fact but I'm just going to do the best we can. I'm pretty meticulous and I feel confident in our mason but He can't start until we tear off the old siding on the wall, wrap with the 1st layer of house wrap, then the mason will come in and put on the Keene drywall rain screen and go from there.

      Thanks for checking up on me Steve. I'll be documenting everything so I may post something later because maybe it will help another homeowner down the line.
      Have a good weekend! KJ

      1. Expert Member
        MALCOLM TAYLOR | | #6


        The reticent to chime in on this one may come from it being a minefield of potential problems. So much depends on perfect detailing, especially of the sloped shoulders, and I'm not sure how you do that and still have them look presentable. That traditional shape of chimney spoke quite directly to the masonry materials that were always used. Water infiltration wasn't as big of an issue - although even with all masonry construction, you see a lot of retrofits on the shoulders of older ho0uses of metal caps to try and solve ongoing problems.

        You are dealing with both bulk water infiltration and solar inward vapour drive. The walls are fairly straightforward, but the shoulders have to be integrated with both the walls and plywood substrate the hearth stones will sit on. Flashing them is the obvious solution, but making that flashing invisible is difficult.

        If you are set on a masonry veneer cladding, my own risk adverse approach would be to reshape the whole chimney, either by tapering it over the whole height, or framing it out to the size of the fireplace enclosure.

        1. dragonfly63 | | #7

          Thank you Malcolm, I appreciate you taking the time to weigh in. I certainly understand the reticence to comment as I know adhered veneers are full of potential problems as a reservoir cladding, I guess I was just hoping if I was meticulous about flashing details, 2 layers of WRB and a 3/8 inch rain screen we'd be okay.

          The fireplace is already installed and the interior of the chase is insulated and sheet rocked so changing the shape is probably a no go but I'd have to talk to the inspector about adding to the framing and going straight up. Essentially we'd be adding 16" to each side of the upper portion. Not impossible I wouldn't be able to sheet rock the additional wall but I could insulate it or maybe it doesn't matter. I do have healthy overhangs on my house. I've attached a picture of the whole chase (temporarily house wrapped). Sorry if it's sideways, it won't let me change it.

          The shoulders are definitely the problem child. My plan was to ice and water shield the shoulders and shingle that over the house wrap. WRB's and Keene driwall on the shoulders as well. The hearthstones (or a single piece of bluestone) would overhang the stone on the walls by 2 inches.

          Obviously it would be easier to flash Hardie siding and put a mini roof on the shoulders (another water reservoir cladding) and in the end thats what I may have to do. I have a lot to to consider this weekend. I'm still holding out hope someone can show me an awesome way to flash those shoulders.

          Have a great weekend Malcolm,

          1. Expert Member
            MALCOLM TAYLOR | | #8


            Hopefully someone with more experience with this sort of situation will come along. Cheers, and good luck.

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