Hi there, bit of background: live in a 300+ year old house, room with offending carpet is on the 2nd floor. We re-carpeted 2 years ago, a cream wool berber. This room has been a guest room for that time so has been used literally a handful of times. (Prior to this there was a carpet down which had no issues, i just didn't like it) So, small dark circular patches started appearing, particularly along the width of the carpet in a 1 foot strip... i put a rug over it to cover it for when guests came. This has spread over time and is now covers about 2/3rds of the carpet. I can't imagine for a moment that it is damp in that room - there is NO smell or sign of it...
The guy that fitted it said it was an end of roll and he didn't have any more - i'm wondering if it could have got wet and he fitted it anyway - any ideas Mumsnet?