Home inspectors will often look for “circumstantial evidence” of issues around the home. Things like water stains will be identified as evidence of at least past moisture conditions or possibly ongoing moisture issues.
For example if it has not rained for 2 months, a roof that leaks and stains a ceiling below might test negative for moisture at the time of inspection, but the client should be cautioned as to concluding the roof does not leak.
Other stains might only become active under specific conditions that are not duplicated at the time of inspection. For example, it is raining hard outside but the only time the stain would test positive for moisture is when the wind is blowing form the east instead of the more usual West. This is why most home inspectors are not going to crawl out on a limb and categorically state the roof does not leak. This is especially true if there is no actual physical explanation for leaking can be found at the exterior.
Stains should be monitored for signs of ongoing leaking, or if stains reappear after proper repairs the area should be further evaluated for.
Inspectors will often note stains at the corners of roof overhangs, the soffit area, and typically test them for moisture content with a moisture meter--if they can get to them. Gutter corner-seams often leak, and unless it is raining the leaking might not be noticed, especially if there is no mark on the ground consistent with leaking above.
I tell clients the best time to check your gutters for leaks and overflowing is when it rains---it likely will be difficult to tell definitively otherwise. We want “solid proof” not “circumstantial evidence” as to what is going on.
When it is really cold outside, solid proof gets a little easier to identify because perhaps the evidence actually remains after it has stopped raining.
Charles Buell, Real Estate Inspections in Seattle
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WA State, Home Inspector Advisory Licensing Board