I have an existing 8" thick concrete wall 10' above finish grade, and I would like to add a 6" metal stud plaster wall above the concrete wall.
With a 1" plaster coating, there will be a 1" recess from the outer face of the concrete wall. Is there any way to have a flush transition from the concrete wall to metal stud wall?
Additionally, do I have to have a horizontal control joint in the corner of an L shaped wall built as above, or is there anyway to hide that transition?
Best Answer
In order to join a plastered masonry wall to a new plastered stud wall, you must do surface prep and moisture barrier, offset the wire lath from the studs using some combination of sheathing and or furring, plaster, then do a skim coat to blend seamlessly.
You’ll account for any “missing” thickness such as the missing inch in your 6” new / 8” old joint problem by using furring or shims etc to separate the base of the coating away from the wall structure.
It is possible to flare the surface out so that the old and new wall sections can be different thickness along the bulk of their spans.
BUT BEFORE YOU START...
Are you sure it’s plaster and not stucco?
To be successful, you must be aware of what material you are seeking to blend into. Repair or add to like with like to minimize cracking and separation over time.
There are 3 major categories of spread surface covering.
To identify what you have:
If this is an outdoor wall anywhere that gets regular moisture you should hope that it’s not finished in gypsum plaster. Otherwise be extra diligent about maintaining a water barrier of paint or the like.
Lime plaster is hydrophobic and although it’s not perfect, it will do well as a moisture barrier. If you told me I’m going to be plastering on a long weekend, I would wish I was working in lime plaster. There is a bit of a learning curve to delivering an even texture though, and the drying process is... for some it’s burdensome and for others it’s a labor of love.
If you’ve used grout or mortar before, Portland cement will likely feel the most familiar.
Whatever coating you’re using, mix according to manufacturer’s directions.
Surface Prep:
Add furring strips to the studs so you can hang metal lath at thickness.
To calculate furring thickness:
For example, if this is an outdoor wall in the US, there’s a good chance the old work is probably Portland cement stucco, not plaster.
If you’re adding stucco over the new section, then the per-side application thickness for this calculation is 5/8 inch or 7/8 inch, but not quite 1 inch.
Apply paper or wire over the sheathing
If you’re doing the finish work yourself, do yourself a favor bear in mind which materials you’ll be working with in what climate. Proper match to the application is essential.
Apply your coating according to best practices for the materials, location relative to water sources / concentrators, and climate.
For example, Hunker provides an excellent training on stucco application.
No matter the coating, do a top to bottom skim coat at the end to bring up to final thickness and smooth out the joints.
Going into detail on best practices on all combinations of conditions is out of scope for this answer, but it’s fascinating to see how much of weatherproofing is surface-agnostic. It’s just another layer, you’ll do great.