This is different from the water traps found in plumbing pipes or on a golf course. All the surfaces on the exterior wall of a home should slope away from the wall so that any rain that hits the wall will drain away. Areas that are flat, or—even worse—slope backwards, will allow water to puddle on them or funnel it back into the wall. Wood rot is the result, and possibly mold if the water gets inside the wall.

    Carpenters call them a “water trap,” which is definitely a bad thing, and they put a slight slant away from the wall in any near-level surfaces to eliminate them. Window sills and the top of trim over a window are classic water trap locations to check.

   Applying a strip of wood trim over the horizontal butt joint between two sheets of plywood siding to seal it is another example. Only caulk keeps the water from running behind the trim wood, which will fail over time and allow water to saturate the plywood behind the trim and cause rot. See our article What is Z flashing? for the solution. 

    When home inspectors search for wood rot on the exterior of a house, they are basically looking for water traps. Our article Where are the most common places to find wood rot on a house? talks about other common places to find water traps.

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Field Guide for Home Inspectors, a quick reference for finding the age of 154 brands of HVAC systems, water heaters, and electrical panels, plus 210 code standards for site-built and manufactured homes, and the life expectancy rating of 195 home components. Available at amazon.com for $19.95.
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Here’s links to some of our other articles about moisture intrusion problems:

• Why do new homes have more moisture and mold problems than older houses?

• How much ventilation is required for the under-floor crawl space of a home? 

 • Why is stucco that goes into the ground a problem at a wood frame house?

• What are the signs of stucco wall leaks? 

• How is water getting into the outside walls of my home? 

• Is mold contagious? Can mold spread to my home if there is a nearby house with mold?

• Why is there mold around the air conditioning vents? 

• What can I do to prevent mold problems in my home?