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I have a former window I want to close and stucco over. I did the framing already. Was wondering if I should use a cement board or 1/2" OSB or plywood with wire mesh as substrate to stucco over (to match the surrounding). If cement board is the way to go, should I still use wire mesh nonetheless?

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You can use any of those materials, or none of those. Waterproof paper (properly installed such that water is prevented from penetrating the repair seams) and wire mesh alone, with no substrate, may be just fine.

An important part of the job that you do not mention (and is not apparent in your picture) is the waterproof paper/membrane.

  • I'm guessing you're talking about roofing felt paper. If so, should that go before the substrate on the framing or over the substrate before wire mesh? – amphibient Nov 11 '15 at 22:09
  • Over the substrate (which should be recessed enough to allow proper stucco scratch coat, brown coat, and finish coat application). – Jimmy Fix-it Nov 12 '15 at 0:03
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The important part is breaking out the old stucco to create a rough edge, and expose the wire to tie into. Sheath what you have with wood. Slip new paper (two layers of grade D in the USA), under the old, and wire.

Now an alternative is to cut cement board in the shape of the old window and caulk the edges. But here you'd want to recess the cement board to make an architectural statement, as it would not be possible to hide the repair.

Ask yourself what happens if (when) water gets into that space. Consider a weep screed for the bottom.

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The simplest way to stucco the opening is to use cement board only because you'd be saving several steps by not having to apply each successive layer. Looking at your photo it appears that you have at least 3/4 inch of depth from the stud edge to the existing stucco surface. If you install 1/2 inch cement board over 30 # felt you just need to float a 1/4 inch finish coat. And Bryce is correct about creating a rough edge to finish the new stucco too. It make the repair less obvious and less chance of any hairline cracks appearing.

  • but do you think i will still need wire mesh or i can just stucco directly over the cement board ? – amphibient Nov 12 '15 at 18:00
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    Yes, it would ensure that the stucco stays in place. You will need to confirm that you are below the surrounding/ existing stucco by at least 1/8 inch so the new stucco can be troweled flush. – ojait Nov 12 '15 at 22:31

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