How does stucco waterproofing work?

Question

(Note: Context is 1929 house in the Bay Area. I think this may be different than stucco in other places?)

If I understand correctly, stucco is water permeable and a water barrier is installed between wood framing and stucco (in other words, underneath the stucco).

This makes it extremely hard to manage the water barrier (confirm its condition, repairs etc). Why not apply waterproofing on top of the stucco and prevent stucco from getting wet in the first place?

Now consider this: Some holes (e.g. from previous conduits etc) exist through the exterior wall. This certainly affects the water barrier as well and the water barrier itself is broken and cannot be sealed (because it's underneath the stucco). We can only seal the hole from the outside. For example, push in mortar (not waterproof!) and seal with (Acrylic Based) Caulking. But then the water barrier is still broken! If stucco gets wet from rain it would seep into the hole in the barrier, even though the hole is closed and sealed. This doesn't make sense.

Finally, consider stucco cracks. The common advice is to seal all cracks to avoid moisture intrusion. But if the water barrier is underneath, why would we even care?

Can someone explain the full story how a stucco wall is water/moisture protected and why they are build how they are built?

Answer 1

I'm not a stucco expert. I've owned stucco from the 1940s (on concrete block), but never installed it in a modern context. I have seen it go on many new homes in the last 30 years as a builder, though. Here's what I've gleaned.

• Just about any siding should have a drainage plane installed behind it, over sheathing (and any exterior insulation). Stucco is no exception.

• Stucco cracks, as you said. These cracks cannot effectively be "sealed" from the outside. It's a game of whack-a-mole to try, and if that's your strategy you'll always be behind. A proper drain plane is a proactive solution.

• There are always joints. These are around windows and doors, at bricklines, etc. Caulk alone is not an ideal weatherproofing mechanism.

• There's not much to monitor with house wrap or other drainage membrane. It's well protected and presumably has a lifespan equivalent to the stucco itself. If there's damage to the stucco, or if modifications are made, appropriate repairs should be made to the drainage plane.

Answer 2

Many Florida homes have stucco exteriors. It is used over concrete, block and wood.

Stucco over wood is backed by Galvanized mesh with a waterproof backing. Stucco over concrete or concrete block is applied directly to the surface. Then when dry all stucco is treated with a primer/ sealer that bonds the tiny grains and seals the stucco. This preps for the paint and helps to further prevent water intrusion. Any openings for conduit, wires or such is sealed with a Polyurethane caulk.. Then the paint is applied and two coats should be done.

This is the basic method to apply and finish stucco over surfaces on the exterior of a home. If the process is followed the surface remains waterproof for years. Of course common sense is still expected, such as adjusting sprinklers to not blast the walls with water, repair blemishes, and caulk any cracks and touch up paint in a timely matter.