1. In step 2 they say to leave 2" between chimney and combustible. Then this means that I need to make the hole 2" bigger because of the OSB sheathing on the roof?
Correct, I would personally go about 2-1/2" to be safe. Unless you cut a perfect oval, it may not give the 2" in some spots as required if cut too tight.
2. I understand the water management concept for the slot. How far off from the cone should it be? I fill like it should be so it touches the cone for maximum protection.. right ?
You can have the slot anywhere you desire, I personally would keep it 2"-3" higher than the cone of the flashing and seal it very well between the roof and the part of the flashing base that slides into the slot.
Running it tight to the cone may crate water damming issues, and also creates a tight spot for any debris that may accumulate on the roof to get caught in.
It can be a pain in the ass, but I would cut the slot so that the flashing sliding up into is is tight as possible without buckling the flashing. Small gaps on the sides if not sealed really well can end up dripping, if not sealed well. Those two spot if done good will not be a problem, but if not sealed good, will be the Achilles Heal of that flashing set up.
Webby, where do you cut your slot in?
3. Online video shows a guy installing it (on a shingle roof but nonetheless) where he makes a square cut... I thought I need to make a round hole? The guy online then connect that interior metal box (not right term I think) to the inside of the rectangle hole and places the flashing on top...
It looks like he is using the cathedral ceiling support box to double as an attic insulation shield. This will keep any insulation in the attic from contacting the class A, and is a must/required. If you have an attic with insulation, you will need an attic insulation shield between the ceiling of the room below the roof and the roof deck underside, or higher than any insulation in the attic and then with a ring on top similar to the storm collar, to keep anything from falling into the attic insulation shield.
Is you roof open to the room below? Or is there an attic place, and if so, is it going to be insulated?
Typically an oval hole is cut through the roof to make up for the slope in the roof for clearances around the class A. Since he is running his ceiling support box all the way up, and it is square, that is why he cut a square hole.
Not relating to your metal roof install, but an FYI, I always run roof flashing base portion under the shingles top and both sides, Only the bottom is exposed when I install roof flashings. It looks better, and protects the nail heads that fasten the flashing down. I cut the shingles in about 1/2" away from the cone, and use sealant or roof cement under the shingle tabs and over the nail heads. To be even more thorough, I would put roof cement under the entire base flange of the roof flashing. Just sucks is you ever have to remove for a re-roof etc. Sometime with particular home owners, we woule even roof cement a tab over the bottom of the flashing for aesthetic purposes. Make sure you use a good sealant over all nail heads. I prefer urethane caulk as silicone if not worked in properly does not always adhere as one would like. Just dabbing it over the heads does not always work without working it in with a finger.
4. What is the best sealant out there?
For non-high heat purposes, I use urethane caulk(nail heads, flashing through slot area etc. Silicone for the storm collar, although I think urethane would be fine there also. Both work in real nice with a finger coated in saliva, and the caulk sticks less to your finger that way. If you don't work silicone in, many times it releases and peels away, or does not give a tight seal. That is my opinion and experience from my roofing, siding & metalwork days.
5. Also they mention that anything over 5ft needs a brace. My entire cabin is 12 ft so in a 10 ft radius my ridge/peak, on a 4/12 pitch roof, should be about 2 ft higher than the lowest part (overhang) and where my chimney will be quite close to.... Then from my estimate - 3 ft minimum chimney height will be about 1 ft higher to 0.5 ft than my ridge so I need another foot - 0.5 ft which will = 4-3.5 ft of chimney. I don't mind to go above this minimum. I like to over build than under build. Then how much more than the ridge should I go? Should I do a brace any way?
Your going to need 15' minimum. You might, I say "might", get away with 14', but most likely will have a lazy draft. You may want to consider figuring a placement of the stove to go through near the peak, and then add on the remaining piping needed to reach 15'. Depending on what is extending outside the roof, you will need a brace for every 5' of pipe. Your pipe is going to be most likely closer than 10' to the ridge(the highest part of the roof), so you will need to be 2' above the ridge with the pipe. And you will prolly need more to get your 15' minimum required for a good for draft.
And again, you guys are really helping me and I can't wait to do this and share photos.[/quote]
This is why we are all here. We all learn much here and many pass it on.