July, 2018
Inspection News and Views from the American Society of Home Inspectors

Capillary Action Sucks (Water Into A Home)


Here’s your mind-spinning science lesson for the day:

By definition, capillary action is a manifestation of surface tension by which the portion of the surface of a liquid coming in contact with a solid is elevated or depressed, depending on the adhesive or cohesive properties of the liquid. 

Or, try this explanation: Capillary action is defined as the movement of water within the spaces of a porous material due to the forces of adhesion, cohesion and surface tension. 

OK—got it? 
In practical terms, capillary action moves water through the thin spaces of a porous material. For instance, capillary action moves water from the roots to the leaves of a tree. Capillary action draws water into a paper towel. Water has very strong cohesive and adhesive properties; it can climb to the top of a 300-foot tree and travel through a masonry surface.

What this means for home inspections
Capillary action sucks water into any small opening in a home. The smaller the opening, the higher water will climb. When the gap is larger than about 3/8 inch, capillary action stops. The size of the gap is important to home inspectors and builders.

Roof-to-siding spacing 
To prevent capillary action, builders usually allow a space of about 1 inch between asphalt shingles and siding material or trim. If siding is tight to the roof, water will rot the siding.

Problems with gaps and cuts in trim
Trim should be cut to shed water. The side trim in this photo is cut improperly and butted against the horizontal trim, which traps water. This synthetic trim, just one year old, already is swelling from moisture. If this trim were natural wood, capillary action would carry the water even farther through the natural pores of the wood.

Trim with no clearance to flashing causes problems
Trim and siding should always be spaced at least 3/8 inch above horizontal flashing. Often, 1/2 inch of spacing is specified. If trim or siding is tight to the metal flashing, water will be drawn up into the material and water damage will occur. The flashing is there to direct water outside of the wall assembly, so it is designed to be wet.

Understanding the capillary action of water will improve your inspection skills. Review installation instructions for siding, trim and modern synthetic or composite materials; all of them will call for gaps and clearances to stop capillary action. If you see these issues during an inspection, make sure to note the condition in your report. And if you see rot, you’ll know why it is occurring.

Tom Feiza has been a professional home inspector since 1992 and has a degree in engineering. Through HowToOperateYourHome.com, he provides high-quality marketing materials that help professional home inspectors educate their customers. Copyright © 2018 by Tom Feiza, Mr. Fix-It, Inc. Reproduced with permission.

To learn more, attend Tom’s technical presentations at educational sessions for ASHI chapters.

Tom will be presenting “Describe That Defect” and “The Practical Science Behind Great Home Inspection” at InspectionWorld® 2019 in San Diego.

Tom can also provide his knowledge for your educational event; contact him at Tom@HTOYH.com.