The most simple thing is the flashing is leaking. Does it do it with just plain rain or wind-driven rain. Inspect the soft boot in the flashing. Do you see any light coming through when the attic is dark. is the boot pliable or hardened? The outdoor picture makes it look like there are some exposed screws in the flashing. That does not seem right to me.

Put some water on the roof uphill from the flashing, pour from a bucket or use gentle irrigation from a hose, and see if you get wet on the pipe.

Yes, flashing, but it could be the storm collar as well, the cone around the chimney just above the flashing.

I live in a semi-arid place, so big rains are rare. I noticed water dripping down my b-vent right into the mechanical room, and found the sealant around the storm collar...well, hadn't ever been put on. It displaces the water away from the flashing, since the seam faces upwards and can otherwise leak.

Provided there's no rust perforation of the flashing etc, you just need to re-seal it. I've used high-temperature silicone sealant, the red stuff. I don't know if it's what everyone uses, and time will tell how it deals with UV, but it worked for me, and I've seen HVAC people use it.

There is no light coming through when the attic is dark. So, the problem most likely comes from the base and up piece of the flashing and I should reseal them. Is it correct? See the picture {{gwi:1542292}}.

That's what I'd do. Nice to have pictures. What is currently around the lower part? Is it a seam, or sealant? It's not going to hurt to re-do them, but try to remove any old sealant that's there. If it's silicone, you can get silicone remover at the hardware store. As for the red, high-temp sealant, as I said, you can get it from automotive stores.

I do not know what kind of that white sealant on the low part. I am waiting for the good weather to go to the roof and see it. But, I found a blowoff shingle on the ground, most likely from that flashing where the screws exposed now. I think it should be covered by the shingle. I went to the HD. They recommended the Henry roof leak repair sealant (black). Is it good or I should choose the different sealant? See the sealant

That stuff looks like what I've used, you need to re-affix the tile of course, and apply that stuff. It's not suitable for the chimney itself, they get too hot - you need the high temp stuff I mentioned.

I would use roof flashing sealant. Below is a link of what Home Depot offers.

I highly doubt pipe would be hot at the roof line that you would need a special heat resistant sealant. I would think the pipe would be hotter sitting in the sun on a 90 degree day.

Here is a link that might be useful: HD roof and flashing sealant

Thanks for your comments. Yesterday I tried to climb the roof that is at 45 degrees to seal the flashing. But I finally gave it up because it is too difficult to climb it. The shingles have sand. It is very slippery. I will call a roofer to handle it though it is probably not difficulty to fix the problem, sealing it because by further carefully check the top of the furnace pipe in the attic; I did see light through on one side. The water must drop from there. I can fix the problem but I am unable to climb the roof at 45 degrees. Though I will call the roofer, I would like to hear what good ideas you have to climb the roof. If one uses a Harness, he still needs to climb to the top of the roof and nail on it before he can use it, right?

You need to find a shoe which will not slip on the shingles. There are shoes made specifically for roofers. The cougar paws boot is one example. Some claim basketball sneakers will also work.

One you get towards the stop you may want to sit on the deck while you do the repair. It helps to sit on a drop cloth to get more traction. You want a material which is not slippery.

coody,

You asked, "Though I will call the roofer, I would like to hear what good ideas you have to climb the roof."

Congers up the image of tying a rope to your spouse's car bumper while parked in the driveway. Then throwing the rope over the peak of your roof and going up from the rear of the house with the rope tied around your waist.

Of course, while up there you realize you need more caulk, so you ask her to run out and get some from the hardware store.

I'd suggest you let a pro do it... ;-)

Agree, let a roofing pro do it-----not an ordinary handyman. Have him seal the exposed screws or nails, nails under the shingles that are on top of the metal flashing, and the weather collar. A pro will know what to do in just a few minutes of looking at it. While he is up on the roof have him check the other vents and any other flashing. Make sure he has insurance. Use roofing caulk/sealant in a tube not tar or silicon based products.

I've done roofs like this. It's no fun and hard on your ankles. In our case (another 12/12 pitch roof), we build a 'ladder' from 2x4s and narrow 1x4 scraps. At the top, we put on 2x4s at a 90* angle. These would go over the crown of the roof and the ladder would hang down the slope. It was easy to stand on it and move it sideways as required. If you're close enough to the ground, you can use long ladders and 'ladder jacks' (similar to what painters used to go up the side of a tall house). These can be adjusted to the pitch of the ladder to give you a flat support for a platform between two ladders ( some 2x12s laid on top of each other).

Update. The roofer has come and sealed the flashing by charging $75. It�s pretty easy. I asked him how to safely climb the roof. He said we use the cushion. I wish I could know it but has learned it.

I'm glad it's fixed. Paying $75 to someone who knows how to do it safely? Priceless. A fall from the roof would not be fun.