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Stucco patching (no air gap possible)

severaltypesofnerd | Posted in Energy Efficiency and Durability on

When patching historic stucco, what’s the best WRB to use?
Here I’m patching a couple hundred sf of 1938 stucco, installed with over single layer of paper (quite thin tar paper apparently, where it can be found, which is not everywhere due to termites).

Do I go with two layers of Grade D paper, and hope the outside layer wrinkles to create a bit of an air gap? I’d like to go with a drainage mat, but there’s a need to match the existing wall thickness.

What’s the best practice for stucco patching?

I have read:

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  1. GBA Editor
    Martin Holladay | | #1

    A couple of hundred square feet of stucco is a lot.

    I don't think we can advise you over the Internet. This is a judgment call. If this is "historic stucco," a lot of questions arise. The most important question is, "Is it worth patching this stucco? Or is it time to strip off all of the existing stucco and do a proper repair job with all new materials?"

    To evaluate the risk of water entry, we would need to know your geographical location (so that we know the annual rainfall amount); we would need to know about the width of this building's roof overhangs; and we would need to know about existing window flashing, if any.

  2. Tyler_LeClear_Vachta | | #2

    Martin raises excellent points.

    Is there board sheathing in this scenario? Board sheathing disperses moisture a lot better than some modern sheathing, but it sounds like termites may be involved.

  3. severaltypesofnerd | | #3

    In this particular case:

    Single family home built 1938, with a single layer of thin tar paper that's mostly gone behind stucco that's now rock hard. West Coast San Francisco Bay Area. The replacement area was due to use of tar as caulking on a 1938 tile flashing (when the tar dried out 40+ years or so ago, it left a quarter size water entry that metastasized for a couple of decades -- someone then replaced the wood but did not fix the leak -- the new wood rotted in about 5 years).

    Thus we're going from 3/4" old growth diagonal sheathing to something modern.

    There are NO overhangs as it's mission revival.

    Photos here:

    The home is now MUCH more vulnerable to leaks, due to recent blow in cellulose insulation.


    If THIS case is suitable for a rainscreen, what about even smaller patches where there really is no option for a rainscreen? What's the thinnest rainscreen mat material made?

  4. GBA Editor
    Martin Holladay | | #4

    Obviously, if there is no choice, and you need to patch, you will be installing asphalt felt (two layers) and crossing your fingers.

    If you can afford to strip the walls and start from scratch, that's the better approach.

  5. severaltypesofnerd | | #5

    I'd strip to the parapet except: the roof is foam. Stripping to the parapet means disturbing the foam. The foam roof guys in this area charge a minimum $3500 truck roll fee.

    The blown in cellulose in the walls has already caused significant damage, due to previously inconsequential leaks. I can tell you that was a bad move....

  6. GBA Editor
    Martin Holladay | | #6

    OK, I see the parapet. Why can't you:

    1. Cut the stucco at the top of the wall (right under the parapet flashing) with a circular saw. (I'm assuming, of course, that your parapet wall has cap flashing. But it may not -- which may explain your moisture problems.)

    2. Repair your wall with new WRBs, some type of rainscreen gap, and new stucco.

    3. Install new cap flashing on your parapet -- wider, of course, than your existing parapet cap flashing (if there is any).

    If you do this, you won't disturb the roofing.


  7. severaltypesofnerd | | #7

    Martin, this home does not have a parapet cap. A signature architectural feature of this style of home is the rounded parapet tops. A square edged top cap would make it look more like a strip mall building.

    The parapet tops were originally hot mop tar capped with stucco. At this point they're a foam creation, as shown in the diagram. Thus the "foam truck".

  8. GBA Editor
    Martin Holladay | | #8

    I think like a roofer. The so-called "foam cap" may be as effective as metal flashing, but there are several reasons why it might not even be waterproof.

    Roofers see water entry problems all the time that can be traced back to defective cap flashing on parapet walls.

    If you like the look of a parapet wall with no cap flashing, the decision on how to proceed is yours to make. I wouldn't sleep well if I owned such a building. I would want a metal cap flashing on my parapet walls, and I would make sure the cap flashing has a lip with a drip edge far from the plane of the walls.

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