| Author |
Message |
Chris Sawyer, CCS LEED AP
Junior Member
Username: csawyer
Post Number: 2
Registered: 02-2009
| | Posted on Monday, June 29, 2009 - 03:22 pm: |    |
I have several brick building rehab jobs undergoing exterior stucco work over brick and block infill.
The spec is a 3-part Portland cement stucco over metal lath. The plaster mix is ASTM C-926 type "C" for low-absorption substrates.
The first of these building is complete and the owner is complaining about hairline cracks. Control joints were not specified.
The contractor is going to "fix" the cracking appearance with a breathable acrylic finish.
What would be advisable to change in the spec for the next round of buildings to reduce the possibility of cracking?
Besides control joints, is it recommended to use a "paper-backed" metal lath and/or vapor retarder? Also, would a change in plaster mix from "C" to "CL" perhaps correct for dissimilar absorption rates of brick and concrete block (I'm told the existing brick is highly absorptive).
Chris Sawyer CCS LEED AP |
J. Peter Jordan
Senior Member
Username: jpjordan
Post Number: 336
Registered: 05-2004
| | Posted on Monday, June 29, 2009 - 06:10 pm: | |
Could be the plaster was not cured between coats. |
Richard L Matteo, AIA, CSI, CCS
Senior Member
Username: rlmat
Post Number: 332
Registered: 10-2003
| | Posted on Monday, June 29, 2009 - 06:18 pm: | |
When possible, I spec an acrylic finish coat, same as the one used for EFIS, providing you don't want a smooth finish.
Because it is elastomeric, it btidges cracks in the scratch & brown coats and is less prone to cracking. Also, the integral color is more uniform. |
Ron Beard CCS
Senior Member
Username: rm_beard_ccs
Post Number: 305
Registered: 10-2002
| | Posted on Monday, June 29, 2009 - 10:39 pm: | |
Chris:
It is not clear to me from your posting as to whether your stucco work is all new stucco over existing surfaces, patching and repairing existing stucco over metal lath, or patching and repairing existing stucco over old or new CMU [or maybe a combination of these].
You use the term 'infill.' Am I correct in assuming that you are dealing with an existing masonry wall and are filling in existing openings and are trying to match the existing wall? Since you selected a system with a metal lath, am I correct in assuming that the existing stucco system is a metal lath over a masonry wall or is the framing system something other than a solid surface? |
Colin Gilboy
Senior Member
Username: colin
Post Number: 192
Registered: 09-2005
| | Posted on Tuesday, June 30, 2009 - 09:59 am: | |
There is no framing. The new stucco system is metal lath over existing masonry in New York City. The entire existing stucco surface was removed from the brick facades of the building. Patching and repair of the brick and a few existing block surfaces was done as well as the filling in of some existing openings with block. I have a combination of existing brick with patching and old and new CMU with all new stucco.
[This was sent by email to me and not posted in the forum. I moved the email here.]
Colin Gilboy
Publisher, 4specs.com
435.654.5775 - Utah
800.369.8008 |
ken hercenberg
Senior Member
Username: khercenberg
Post Number: 27
Registered: 12-2006
| | Posted on Tuesday, June 30, 2009 - 10:40 am: | |
Chris, as you already noted, control joints may make sense.
In terms of your other questions, there is no panacea. The need for vapor barriers must be based on analysis. Take a look here - http://www.wbdg.org/tools/wufi.php for software and links to some good basic information that you can use for starters.
Any time you use brick and CMU in any combination, recognize that they don't play well together. Brick expands; CMU shrinks. Instead of an acrylic coating, please consider a 'one-coat' polymer modified stucco mix. See Sto Powerwall under http://www.4specs.com/s/09/09-2400.html; that's my preferred basis of design. Oddly, when done properly, you'll probably still end up with a 3-coat system, 3/4 inch thick, not the 3/8 inch thick 'out of the box 1-coat' system. This can usually be applied directly to the masonry substrate, no lath needed or wanted. You seem to have proper surface prep but may need to establish better separation between dissimlar materials. Contact your Sto (or whomever you decide to use) rep and have them come out to the project. They can probably assist you with the moisture vapor analysis and the system design.
Good luck. Please let us know how this turns out. |
Chris Sawyer, CCS LEED AP
Member
Username: csawyer
Post Number: 3
Registered: 02-2009
| | Posted on Tuesday, June 30, 2009 - 11:16 am: | |
Thanks Ken,
My firm is acting as the CM on this and I'm collating all of the recommendations. I'll be sure to post if any resolution is made.
I appreciate everyone's input.
Chris Sawyer CCS LEED AP |
Steven Bruneel, AIA, CSI-CDT, LEED-AP
Senior Member
Username: redseca2
Post Number: 184
Registered: 12-2006
| | Posted on Tuesday, June 30, 2009 - 01:17 pm: | |
Do the cracks follow any logical pattern, as in orthogonal cracks following the transitions from one substrate to another, or are they completely random and diagonal?
The former would seem to indicate a substrate problem as Ken notes above, while the later would seem to point to mix and curing issues. |
Chris Sawyer, CCS LEED AP
Intermediate Member
Username: csawyer
Post Number: 4
Registered: 02-2009
| | Posted on Tuesday, June 30, 2009 - 01:27 pm: | |
The cracks are "crazed" - random hairline. Nothing bigger than 1/32". I think that you are right to believe it was a problem with the mix specification and/or the application.
Chris Sawyer CCS LEED AP |
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