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How should I flash this tricky corner:

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This is at the junction of a 2 story section and 1 story lower flat roof, done in foam. The flashing was installed about 20 years ago, the wall below rotted and became a termite nest for various reasons, perhaps including the flashing.

This is the twin of the flashing at the opposite corner. There's no evidence of rot there, but then again, the first corner's problems were not in evidence prior to removal of stucco:

Stucco weep screed transition at corner over roof

Further back showing the junction of the 1 story addition. The major water originated in a window just above here, now fixed:

enter image description here

The water encouraged termites, who had probably decades to chew at the corner. This is what it looked just after removing the stucco. As is typical, none of this showed from the outside, the stucco shell showed no unusual cracks or problems:

termites in old growth doug fir

Flashing just prior to first removal of stucco. It was removed because of the crack you see:

enter image description here

And an overview of the good corner, twin of the bad corner:

Mediterranean House Water Problems at Parapet Joints

  • A photo from further back would be helpful, kind of hard to give advice with only a micro view. – Ed Beal Mar 7 '18 at 19:48
  • Done. The issue here is the design is inherently vulnerable to cracks. The wraparound corner has no stress relief except that flashing. – Bryce Mar 7 '18 at 21:14
  • The photo gives a better perspective, I would probably wrap the corner at least 6" up and then wrap the top horizontal corner. The problem I see is the stucco on the top any cracks there and the rot will start again, would it be possible to coat the upper area with tar? On the flat area? I would expect it to crack on that horizontal corner also. I will start a bounty to see if we can get some better answers I think the post has to be open for a couple of days I did not see the button to start a bounty. – Ed Beal Mar 8 '18 at 0:35
  • The foam roof itself is coated in a flexible elastomeric. This flashing was designed to take any water behind the upper wall and eject it onto the foam roof. The side wall (in theory) drains to the bottom inside the stucco. The transition point is the head scratcher - water behind the stucco has to go one way or the other, but at the transition some seems destined for the inside of the wall.. – Bryce Mar 8 '18 at 4:06
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+50

Cement is porous, not waterproof, it acts like a sponge and will hold water. It also allows water vapor to pass through so condensation does not build up in walls. Tarpaper also allows water vapor to pass as well but not water itself. Tyvek has the same properties too. The point I am making here is the tarpaper is your waterproofing layer and the stucco or any other masonry surface like brick, only provides protection for the tarpaper. If the tarpaper is not layered right or flashing is not installed properly you will get what you have now. The flashing prevents water from getting behind changes in plane, like a roof (sorry for the obvious). The mention about the paper going up to the top of the new extended flashing, is to treat the tarpaper install like there is no flashing there at all so the watershed properties of the tarpaper stay intact where the flashing and its "turnout" end. enter image description here

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  • Great answer. Any thoughts on the "good" corner on the other side? ("If it ain't broke, don't fix it" vs. if it was made the same way, there may be the same problem underneath or eventually will be, so fix it too.) – fixer1234 Mar 16 '18 at 21:26
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    @fixer1234,depending on the budget that Bryce has, I would take out a test corner to see what is going on. It would take about a 2X2 ft section to make a decent repair if everything was good and a small issue to fix while the other stucco was being worked on. But opening it up could literally open up a can of worms as well. That is where the budget comes in. Stucco can hide a lot until the failure is so bad, it has no choice but to buckle or crumble. – Jack Mar 17 '18 at 4:55
  • Stucco is amazing at hiding stuff. The open corner looked like nothing at all was wrong. – Bryce Mar 18 '18 at 6:30
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Just looking at the picture with the R13 in it. Just get a 6"x6" right angle flashing and lay it on the vertical corner above you other flashing. You will just slide it down over the other flashing but you will have to cut in a birds mouth right about at the R13 letters there. (you can actually just make a straight cut) It will give you that bow you need to prop over the other flashing. You can put a small piece of the 6x6 flashing under the bird mouth to cover wood that may be visible now. Also use tin snips to get that nice transition from the new to the old. Trial and error works well. If you get the new and old to line up nicely you can add a rivet right at the corner. Use flashing sealer as needed.

  • Note the existing flashing is detached and can be adjusted. What's a bird's mouth cut as you refer to above? What about an vertical expansion joint? – Bryce Mar 12 '18 at 7:11
  • Birds mouth on sheetmetal is kinda like making a V shaped cut so you are able to bend the opposing sides. The only way I see to flash that corner is with a vertical 90 cut and nipped so that it overlaps just right. You can use some butyl tape underneath but make sure to cover it as it should not be exposed to sunlight. Get a big piece of 6" 90 and just trial and error with it. You'll be an artist in no time. You may have to slightly trim back the orig flashing. – vin944 Mar 13 '18 at 20:06

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