T1-11 siding was a great siding in the 80's and it can still look good today but it might not appeal to everyone and I get a lot of questions from people who ask if they can cover their T1-11 siding up with stucco.
Can You Apply Stucco Over Existing T-111 Siding?
Yes, you can go directly over T1-11 siding with any type of stucco system if the T1-11 siding is properly prepared. This involves removing any trim on the house on corners, seams, windows and doors. You want an even plane to start with to ensure that the stucco is properly installed and looks seamless after it is all finished.
Removing The Trim:
Like I mentioned earlier, you will need to remove all of the trim on the house/building before the stucco lath can be applied to the walls.
Now this is recommended for a clean stucco installation and in theory, you would not have to remove the trim if you wanted to keep it and stucco up to the trim but in most cases people want all of the trim removed for a number of reasons.
The pictures below illustrate some areas where the trim would have to be removed but does not point to all of the trim. These are to illustrate what I am talking about and remember that ALL of the trim will end up getting removed.
Standard T1-11: A typical T1-11 house will have trim on the corners, around the windows and around the doors and is considered the "standard" as far as trim goes. You might even find trim on some of the seams that covers up the Z flashing or along the bottom edges of the walls that run horizontally. These will all have to be removed prior to installing the stucco lath.
T1-11 With Batten Boards: Sometimes people use 1x lumber that is arranged in a vertical pattern that creates a "batten board" look. All of the trim, along with the batten boards will have to be removed in order to provide the best substrate for stucco.
What Shape Is Your Siding In?
Dry Rot Areas: Another important note worth mentioning is that all of your existing T1-11 siding should be in good shape and strong. Any areas that are rotted or soft will need to be patched in with a similar thickness plywood or OSB of some sort before covering the walls up with lath.
This doesn't have to be an involved process of replacing an entire sheet if the area is smaller in size, just cut out the affected area from stud center to stud center and patch the area with a cut piece of plywood/OSB/T1-11. Try to keep the paper underneath the siding in tact, if you can as it will make the wall more waterproof.
Bulges: If your siding is bulging out anywhere, you will need to try to get it to lay down flat on the wall as this bulge could show through in the finished product. Adding some extra nails or screws (usually works a little better) can help suck the siding flat to the studs.
Bottom Edges Of Siding: The bottom edges of the siding will also need to be inspected and corrected if any issues exist. These are usually in the form of rotting or the T1-11 siding is touching concrete, brick or some other dissimilar material. You will want a gap of 1" minimum between the siding and any other dissimilar material and can be achieved by snapping a line and cutting the siding with a circular saw, in most cases.
Don't Forget The Vents!
Usually there are a couple of different kinds of vents that are installed on houses that have wood siding like T1-11 but some will need to be replaced (or all of them) with proper stucco compatible vents. The foundation vents will usually have to be replaced if they are a flush (close to flush) mounted type because the stucco is typically 7/8" to 1 3/8" thick, depending on whether you are installing a three coat or one coat stucco system.
You will need foundation vents that have a flange around the perimeter that measures 7/8" or 1 3/8" and are specifically designed to have the exact standoff for stucco systems.
Gable vents and dryer vents are two other types of vents that will also need to be inspected and replaced if they do not have a standoff of 7/8" or 1 3/8".
Alternative To Replacing Vents: The only other alternative to installing new vents would be to install plaster stop around the vents to provide a ground (depth gauge) for the stucco, as illustrated below with a special made dryer vent. Installing plaster stop around every vent is WAY more time consuming and can cost just as much as buying new vents.
If you choose to go this route, all of your vents that are flush mounted will appear recessed, which some people like.
What About Windows?
How Windows Are Typically Installed On T1-11 Siding: On a typical T1-11 siding installation a water barrier is placed directly over the studs, the window area is cut out, flashing is installed, the window is installed and the T1-11 siding is installed over the window flanges.
This is a good method and works well so you could just run your stucco paper right up to the window and it would be fine, assuming that your windows have never leaked before.
An Alternative Method: The better alternative would be to remove the window, apply new flashing and reset the window so the flanges rest on top of the T1-11 siding and then your stucco paper can then be counter flashed with the window flashing, just like a new stucco installation would be done.
Window Reveal: You will need to measure the reveal of your windows to see if they are going to work for the stucco system you are planning on installing. Remember that a three coat stucco system has a thickness of around 7/8" and a one coat stucco system has a thickness of around 1 3/8".
With the trim around the window removed, measure the distance from the T1-11 siding to the face of the window to see if you have a measurement closer to 7/8" or 1 3/8". If the window has a reveal that is less than 7/8" then plaster stop will have to be installed around the window(s).
Doors are usually the easiest things to prepare for stucco on an existing T1-11 sided house because the door jamb is usually set flush to the siding itself so all you have to do is remove the trim and it is ready to go.
You can choose to remove the trim around doors and replace it with either plaster stop or another piece of wood trim that has a keyway for the stucco to fill up (special trim). If your existing trim is in good shape, then you could always keep it attached and go that route.