It's not ideal, but could be worse. The old decking gave way. Whether the roofers saw that and covered it up or it gave through when the air nailer got it is another matter. Most roofers would probably keep going till the job was done and address isolated issues like this after.
If decking repairs or a warranty were in your contract then they should make it right. If you don't have a contract then it's at the discretion of the roofers if they want to do anything about it.
Over the life of the roof that spot may wear prematurely or give way if any weight hits it. It's under a vent so there is probably a metal flange over it. So it might hold up. Time will tell.
It's certainly fixable. To do it the right way you need to take it all the way down to the decking and patch in a new board. Certainly doable, but kind of a pita.
That's not right at all, but I can see that it's a tricky spot, and your roof deck is boards instead of plywood... I dunno. If that was the only spot, i'd make them go in the attic and stick some wood in that gap. They can bridge from the other two boards and it won't be great, but it won't be horrible.
If there are more spots like this, i'd have another legit roofer who specializes in repairs (but ideally not full roofs) come out to inspect. Especially if the spots aren't also obviously small, and difficult to actually add any real sopport. He'll give you an unbiased opinion and tell you exactly what needs to be done to make sure you end up with a good roof, and likely give you an estimate in case you want him to do the work.
I know a good bit about roofing, and unless the roof was obviously done impeccably except for that, i'd be rather concerned. Sometimes it's best to just leave something alone and not get into another several thousand dollars worth of repair, and I don't know what that area of your roof looked like when he got down to the decking. A good roofer will. It's time to get some outside advice. The municipality where you live also likely has roofing inspectors who have to close out any permits the roofer pulled. You can contact them and they'll be glad to take a.look for free, but you might not get someone with as much knowledge as if you hire someone.
Planks were used in older roofs (which you probably know) which makes me think the decking is pretty old and probably should've been replaced with plywood.
There's really no need to replace the decking unless it's damage, regardless of it's age. It's would be an unnecessary expense, and a large one a that.
That vent pipe is PVC which tells me it's a lot newer than this house so my guess would be that whenever that vent was added or replaced they destroyed that board in the process. Regardless, the roofers should have corrected it. It's possible they just forgot to circle back to it if this was the only issue.
Plank decking is way better than plywood. You replace solid wood with solid wood, unless a ton of it has been compromised. This should be specified in the contract for the roof. You even get insurance discounts for solid wood. If a roofer replaced my boards with plywood or OSB i'd be super angry.
If someone steps on that, they are probably going to go through. The membrane looks solid, I don't know if with heat or age if a hole could form. When we reroofed my garage, we didn't leave any holes like that. Any soft spots got resheeted in plywood. Small gaps are probably okay, but that looks like 1 foot+
I agree. The manufacturer of the membrane would not have designed their product to operate over a gap like that. This to me would be the cause of accelerated wear and tear.