What is Incidental Water Anyway?

By: Dominic L. Pusateri

Incidental water is a term used in the building codes and referenced in specifications but what constitutes incidental water?

The term is used in reference to the drainage plane of a wall assembly.  This is the portion of the wall behind the cladding and in front of the cladding substrate.  The strategy is not unlike a deck coating and wearing surface, in that the wearing surface takes the brunt of the weather and traffic but the waterproofing relies on the coating beneath. 

The International Building Code (IBC) quantifies incidental water by reference to American Society for Testing and Materials (ASTM) standards. For example, the IBC requires exterior insulation and finish systems (EIFS)  drainage systems to be tested in accordance with ASTM E 2273, Standard Test Method for Determining the Drainage Efficiency of Exterior Insulation and Finish Systems (EIFS) Clad Wall Assemblies. The test is of the ability of the systems’ drainage capacity; a specific volume of water is applied that represents the minimum performance.   The water is applied directly to the space between the cladding and the protected substrate, and the rate at which the water exits the substrate is measured.

ASTM C 1715, Standard Test Method for Evaluation of Water Leakage Performance of Masonry Wall Drainage Systems is a drainage test for installed brick cavity walls.  Similar to the EIFS test, water is applied directly to the space between the brick and protected substrate. This is a direct performance test where the building interior is inspected during and after the test for evidence of leaks.

The volume of water used for both the ASTM tests is similar, about 1.6 gal per 2 lf per hour.

Perhaps not coincidentally, the volume of water can be equated to the test used in ASTM E 1105, Standard Test Method for Field Determination of Water Penetration of Installed Exterior Windows, Skylights,Doors, and Curtain Walls, by Uniform or Cyclic Static Air Pressure Difference; the calibrated spray rack defined by this standard is often used to test claddings in the field of the wall away from openings, a  2" tall by 24" wide slot in this spray pattern equals the same volume, 1.6 gal per hour.

In my work as an inspector, and in my work as a consultant, I have often heard it said of the flashings, membranes, coatings, and other components that make up the drainage plane  that "This is just for incidental water" as if the word incidental is being taken as inconsequential.

Workers in the field undervalue the importance of this portion of a wall system. The attitude is that it might not be perfect but it is a second line of defense to the cladding, when in reality it is part of the cladding in the same way that a deck coating works with the wearing surface.

Terms such as bulk water as opposed to incidental water are too subjective.  So, in order to put this in a more clear light, we constructed a spray rack to deliver 1.6 gal/hr and directed it onto a piece of plexiglass.

Ladies and gentlemen, behold, incidental water.