Need a little insight. I use a pin type moisture meter which has been consistently accurate. Recently, (starting with my own attic sheathing) I have been getting readings of 15-18% in several houses. In each case, I have seen water staining somewhere in the attic but not necessarily in the areas I am testing. The one thing they all have in common is they have been snow covered. I have not had this issue in other winter seasons but since there have been a cluster of inspections with this kind of result, I am wondering if something else could be effecting the readings. Since anything above 15% is considered above average moisture content according to my meter, I always comment on it when it comes up. But if something else could be causing this, I would like to know what it might be. Any thoughts on this?
Sue, with 15% to 18%, I wouldn’t even comment unless there were other contributing factors, like staining, or older water damage, humidity etc. 
“In normal use the moisture content of wood varies between 8% and 25% by weight, depending on the relative humidity of the air.”
From: https://www.woodproducts.fi/content/moisture-properties-wood
P.S. Hope to see you on our forum more than every 2 years. 
Thank you, Larry. I appreciate your input. Just seemed like I was having a cluster of these reads and never had the issue before so I thought maybe something might be rotten in Denmark! lol!. Just wanted another set of eyes and opinion on it.
Accurate moisture testers are to be calibrated yearly. Sometimes they do go out of adjustment, sometimes a lot. Otherwise things like metals affect the readings.
Sue, Pin moisture meters measure electrical continuity which is more consistent than ultrasound which measures density.
You mentioned one variable, snow covered roof.
You must consider other variables other than water intrusion, in this case condensation.
The roof deck temperature with snow on it is much more consistent than a clear roof which has huge temp swings due to solar exposure, roofing materials, color of roof.
Next, what is the source of the moisture your measuring?
Moisture can come from three sources; building envelope leakage, Plumbing leaks and condensation. In your case as nothing else seems to line up (water staining), condensation does to me.
What is needed to produce condensation?
- Temps below dew-point temperature. You have a consistent 32F roof deck (or lower) with snow on the roof.
- A source of moisture. Attic ventilation is primarily there to control moisture from inside the house. So the moisture vapor is most likely always present in the winter because of higher temps and several sources of elevated moisture from inside the house which is always leaking upwards to the attic due to stack effect.
What moisture values your measuring are insignificant in this case.
Also I would never rely on meter values as to the % of moisture registered. These scales are “Apparent” moisture levels and they are calibrated for only one specific material. They should be only used to determine Delta-M.
There is only one accurate way to measure moisture, that is to weigh the material wet, dry it out and measure again. And we can’t do that. In reporting moisture levels I recommend you write it as being greater than 20% (or your base level) not the scale measurement as it is not duplicatable should I come behind you with my meter which will unlikely give the exact measurement. You should also note “with reference to wood” if the material is Sheetrock etc. Painted or unpainted etc.
I will not discuss nuclear moisture equipment.
Hope this is helpful.
Excellent reply David, should be material for a reference article on moisture meters.
It’s good to have more than one meter, the low cost ones you can buy at the big box stores fill the bill.
Thank you, Simon. This meter isn’t more than a year old but calibration is certainly a good consideration.
Thank you, David. Without observable signs of leaking, condensation was my most likely suspect. You give a great explanation and reference to stack effect. This is very helpful for how to explain it to my customers. Thanks!
Welcome.
Your a brave sole inspecting in MA! 
I’m from Acton.