Tall electrical masts like the one pictured here are common when the wires running to the mast need proper clearances above roofs, driveways etc.
When the mast gets to be more than 5 feet tall it typically will be required to have some sort of restraint brackets installed to resist the pull exerted on the top of the mast. The long run of wire to the mast can be quite heavy all by itself. Add to that weight, wind loads, ice loads and the occasional squirrel, and these brackets can be essential in preventing collapse of the mast.
The mast pictured above looks pretty normal and proper. My usual concern with these installations revolves more around how well sealed the attachments to the roof are and advising the buyer that these are connection points that are going to need to be monitored and maintained. There is actually a type of roof attachment that goes under the shingles and eliminates the maintenance issue, but these brackets are pretty rare. Leaking into the roof structure at these points is also common and a good thing to keep an eye on in the attic space.
You won't be able to tell from the first picture---but this installation is not going to leak---at least not from the usual reasons.
When this roof was replaced---somebody forgot to go back and bolt the brackets into the roof structure.
It is likely that the angle of the brackets and their digging into the roof a little bit has been sufficient to add enough support to the mast. That said, it would probably still be a good idea to properly bolt the brackets---who knows how many squirrels it would actually support.
Charles Buell, Seattle Home Inspector
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WA State, Home Inspector Advisory Licensing Board