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Steel stud extension to block building...siding considerations

Steel stud extension to block building...siding considerations

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  #1  
Old 04-20-19, 10:32 AM
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Steel stud extension to block building...siding considerations

I'm about to build a steel stud bedroom on the side of an all block building. Currently the block does not have any siding inside or out. I live in the mountains of Costa Rica so there are no insulation issues. There will be no a/c or heat.

From a newbie perspective, what I would like to do is clean the block, build a steel stud bedroom with a cement or MGO based sheathing that I will install flush with the block, then use an acrylic stucco as the final "coat". I'm trying to be cheap, but I don't want junk.

So, the first question is am I doing something stupid by choosing this combination of methods and materials? If it seems like a sane plan, my second question is more technical. The steel stud wall will join the block at a corner so that the block wall will have a seam with the stud wall. I want that seam to be undetectable to the eye. That is, there will be one long wall, half which has the existing block backing, and half that has steel stud sheathing as a backing. I don't want to see the seam where the two meet. Will an acrylic stucco appear as a monolithic wall, or will you be able to see the two types of backing through the stucco? Or will the seam likely crack with time?

TIA!
 
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  #2  
Old 04-22-19, 03:12 PM
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Is the existing wall stucco now? Do you intend to stucco the whole wall at the same time? Do you mean acrylic stucco as in a Exterior Insulating Finish System or conventional three coat Portland cement plaster with an acrylic finish? Where dissimilar substrates are stuccoed they usually crack at the joint. I have been somewhat successful bridging dissimilar substrates with felt bond breakers and expanded metal lath with absolutely no fasteners over the bridge for about two feet either way. Actually I don't know if I prevented a crack or simply moved two cracks four feet apart and made them less visible because they did not seem to continue to move. I know stucco. I don't know much about steel studs in an application like yours. I like the idea for termite resistance, ease of working with steel studs vs lumber and fire resistance.
 
  #3  
Old 04-23-19, 01:34 PM
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I reread your post. What you propose is sure to crack. It will be only a straight crack. A texture in your finish will camouflage it. You should put some kind of sealant between your block and sheathing before you put on the finish. If you are worried about different substrates showing a difference through the finish there are a couple things you can do. First parge the blocks so the mortar joints are flush with the blocks. Second prime both surfaces with a primer suitable for your finish to even the suction before you finish.
Learn to like the crack. Oh one thing you could do is put a foam piece over the crack and mesh it and finish it like in an EIFS system but then there will be an offset like you are trying to avoid.
What is MGO based sheathing.
 
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Old 05-01-19, 08:19 AM
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Thanks so much for taking the time to engage this newbie. I've added an attachment to show what I'm planning just in case it isn't clear. The block in the attachment doesn't show its stucco, but it is stuccoed. So, to answer your questions:

Is the existing wall stucco now?
Yes. Looks good even after 10 years.

Do you intend to stucco the whole wall at the same time?
Yes, unless there is a reason not to. In the image attached, I plan to stucco all at once three coats over the entire wall...both the block and the board. But see my question below. It could save some money!

Do you mean acrylic stucco as in a Exterior Insulating Finish System or conventional three coat Portland cement plaster with an acrylic finish?
Just the finish and just because it seems to crack less. I'm open to any suggestions.

Where dissimilar substrates are stuccoed they usually crack at the joint.
Yeah, and this is what I'm trying to avoid. But, I get your main message that it is very difficult to keep this from cracking.

What is MGO based sheathing?
Magnesium Oxide board is used here in Costa Rica because it is more mold resistant than other cement boards.

The final question is, knowing the block already has stucco on it, do I just add another 3 caps on top of the existing block wall or do I offset the new wall so that the scratch and brown coats bring it flush with the block and then put on a finish coat (acrylic or otherwise)? It seems like this would be cheaper as I won't have to re-stucco the concrete with the scratch and brown coats, but then it seems more likely to show that I've got two walls joined together as opposed to one big wall. I'm not sure which option would be more likely to crack, but if it is going to crack one way or the other, I'll choose the cheaper route.

Thanks again for your time!

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  #5  
Old 05-01-19, 05:15 PM
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I think your ideas are good ideas
You say 'The final question is, knowing the block already has stucco on it, do I just add another 3 caps on top of the existing block wall or do I offset the new wall so that the scratch and brown coats bring it flush with the block and then put on a finish coat (acrylic or otherwise)? It seems like this would be cheaper as I won't have to re-stucco the concrete with the scratch and brown coats, but then it seems more likely to show that I've got two walls joined together as opposed to one big wall. I'm not sure which option would be more likely to crack, but if it is going to crack one way or the other, I'll choose the cheaper route."

Other than the crack issue I see no compelling reason to put three coats of stucco over the cement board, well let me amend that, if the cement board is intended for exterior applications and if the material will bond with it. You might have to, probably should do some sort of treatment at the joints in the MgO board. If you can get a texture match between the two different substrates a finish is all you should need especially over the blocks. If the texture is too coarse or too heavy then maybe a skim coat before the acrylic finish. Now about that joint. It will crack There might be a way to deal with it. You can cut a strip 4" or 6" or so of your MGO board and center over the joint. Attach it only on the new wall side. The joint will move behind it and the crack will be camouflaged in the slight angle between the block and new strip of board. Now if that really bothers you, think about adding a similar strip or two or three space appropriately on the wall for symmetry and balance. If this idea really does not suit you then learn to like the crack. Maybe leave space between the board and the block to put in a caulk joint before you finish. It will still crack, just stay sealed and maybe, though I can't quite say why, be less noticeable.
To be clear on the offset between the block and board, if any build it so it works out flush when you do whatever you do. If only a finish then install flush.
 
  #6  
Old 05-06-19, 08:36 AM
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Thanks so much. I have probably 5 hours searching the web to find what you just explained!
 
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Old 05-07-19, 01:51 PM
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What I meant to say was 5 hours searching the web and couldn't find!
 
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