Details, Details, Details: Stucco or EIFS on Flat Surfaces

In my last entry I talked about how decks installed into Stucco or EIFS require proper flashings.  Today I am going to discuss why stucco or EIFS should never be installed on a flat or horizontal surface.

There are some misconceptions out there that it is ok to install Stucco or EIFS on a flat surface such as a deck railing wall or parapet wall.  The truth is that Stucco or EIFS should never be installed on flat surfaces.  As with most cladding systems, Stucco and EIFS are not waterproof, rather they are slightly porous, breathable systems that allow vapor transmission to occur.

To put this into perspective, you have to think about the various forms of water.  There are water molecules (liquid version of water) & water vapor (gas version of water).  The size of a water molecule is dramatically larger than the size of water vapor (you can think of a water molecule as equivalent to the size of a car versus water vapor being the size of an orange).  Stucco & EIFS are designed to allow water vapor (gas or orange) to pass back & forth through the system which allows the system to be “breathable”.

When Stucco or EIFS is installed on a flat surface, in just a short amount of time the properties in the Stucco or EIFS will begin to break down (freeze/thaw cycles, impact damage, etc…) and the passage of water molecules (water or car) will begin to occur.  Once that happens the water will accumulate and do what water does when it contacts wood: damage it.

When Stucco or EIFS is installed on a flat surface, metal cap flashing needs to be installed on the top of the surface to protect the Stucco or EIFS from moisture intrusion.  If cap flashing is not desired for aesthetic reasons, an alternative is to slope the Stucco or EIFS a minimum 22 degrees and waterproof the surface underneath the final finish coat of the Stucco or EIFS.  With this method, the top surface is sloped to promote moisture drain off the are quickly and also the layer underneath the finish coat is waterproof to prevent moisture from entering the wall cavity.

So remember, Stucco or EIFS should never be installed on flat surface.  If it is, the area below should be tested to see if moisture intrusion has occured and the flat surface should be modified by one of the methods listed above.