Replacing windows on a stucco house:

Should I leave a gap around the window ?

Dear RT,

I'm replacing windows on my stucco house, which involves breaking the stucco around each window to expose the nailing flange and some retrimming of the rough opening. After reading your website, I concluded that caulked expansion joints aren't necessary and I can stucco right up to the window frame (clad aluminum).

Question: I've already repaired the stucco brown coat around the first window and left a quarter inch gap for caulking. Can I go back and fill in that gap? Do I need to modify the brown coat mix to ensure adhesion in the gap, or will I be OK as is?

Great question. I'm glad you didn't leave a gap and fill it with caulk !

Probably the worst thing people do without realizing the consequences is to leave a gap between the stucco and the window. This is usually done by surrounding the window with casing beads, and filling the gap with caulk. We find severe rot when we tear off an installations done like this with woood trim or wood framing. This method is often specified on the plans. This needs to stop. The idea is to create a gap between dissimilar surfaces. Since mortar won't bond to wood, metal or whatever, a small crack (usually) appears between the stucco and wood, or whatever. We did this on commercial work, such as hospitals, schools and government buildings and it looked neat and usually there was no problem. But in commercial work, the dissimilar materials were usuallay concrete, metal door jambs, concrete, or whatever and not wood. Also, the substrate for the stucco , or interior plaster, was metal framing, exterior gypsum sheathing, block or other materials that don't rot like wood. If whoever draws these plans could see what we see, they would stop this practice. Speaking from experience in tearing off failed stucco installations, the casing bead and caulking traps water against the wood causing rot. The casing beads, also known as plaster stop, deteriorate and we find galvanized beads so badly rotted in less than 5 years that you can pull the off with your fingers. The worst we have seen with this method was a house in Alexandria, Virginia where the owner had his neighbor stucco the house 30 years before. TThe neighbor was a plastering contractor. The beads had rotted so bad, and the wood framing had rotted so bad, that we could pull ot the windows with our bare hands. The wood framing and sheathing bellow the windows had rotted so bad that the house sagged, and was infested with insects. The house had to be jacked up and supported with new lumber. I have seen this condition several times. I have a lot of experience in tearing off failed stucco.
Caulking around windows sometimes does more harm than good. There is a myth circulating that on an EIFS or one coat stucco house, the caulking needs to be scraped off and replaced every so many years, usually five years. The caulking, or lack of caulking causes rot. The reason I know this, is people call me looking for somebody to replace their caulking. I know someone here in northern Virginia who largely makes a living caulking EIFS and one coat stucco.
For example, I got a call recently from a woman who wanted caulking replaced. I get calls for EIFS repair and one coat stucco repair from web searches for a stucco contractor. She said she had a "hard coat" stucco house and had a damaged area under the window. This house is in a neighborhood near me. The houses are one coat stucco and many have foam EIFS details over the windows. She explained she needed the caulking replaced on the sides and bottom of the window because the caulking was bad. The bad caulking is what caused the damage. I know this is absurd, but I wanted to get off the phone so I recommended her to an EIFS contractor who does small caulk jobs. Usually someone tells me that so and so told them the caulking needs to be replaced every 5 years. The next time I get a call like this I'll explain that water runs downhill, and not horizontally through a solid object. The water is coming from above the window and not from the sides. For $ 200 I'll come over and show them what they need to correct their problem.
Yes, you may want to add an acrylic admixture to your dry mix mortar for patching the brown coat. We use flex-con or stong bond. If you can't find acrylic in small quantities, acrylic admixtures for ceramic tile should work fine.
Tar paper under window trim.

Carpenter puts on strips of tar paper before putting on window trim. I marked the tar paper with arrows because it is hard to see the black paper over the black celotex. Celotex is another obsolete material.

Back of wood trim sealed with protecto wrap.
On this historic job where we replaced the lath and stucco, the back side of the window trim is sealed by folding over 6 inch wide protecto wrap tape.
Window flashing
The top flange of the window flashing is sealed by overlapping with protecto tape and then tar paper. No way this is ever going to leak. Not taking the precations of the tar paper under the trim, or the folded protecto wrap, water that penetrates the stucco can run behind the wood trim. Caulking around the window will do little to stop the water infiltration, and more than likely, make the water infiltration worse b sealing water into the wall. Any sealing should be done before the stucco is done, instead of after. Tearing off stucco or EIFS on houses, we find rot behind the caulking where a gob of caulk is put around the window.