Cathedral Ceiling Support Box

[bretzel.logic]

Does my support box need to reach all the way up to the underside of the sheathing? I think the one I bought will end up like the first image below

But we need it to be like the second image, right? And the top gets trimmed off?

Also thinking about what shape of hole I need to cut in the roof, is it better to cut a round hole with 2" of space all around the pipe, or a square one for the support box to fit snug? Or neither?

Roof is: 2x12 rafters, with 1" polyiso foam underneath and rockwool in between. 1 1/2 inch vent space under sheathing with a plywood baffle.

 

[begreen]

Like the second drawing. It gets a square hole and it needs to hang down into the room enough to meet the stovepipe clearance requirement.

 

[Eman85]

My cathedral support is inserted from the top and supported by the roof. I have a chimney in another building that has a flat ceiling and small attic and the box is supported by the ceiling rafters.

 

[bretzel.logic]

Is there a particular code or reason the support box would need to extend all the way up to the roof sheathing?

I would add blocking for the support box to screw into. I just talked to tech support at Selkirk and they said you don't need the metal to go all the way up. Just need to maintain the proper clearances to the chimney pipe.

I already have a support box, and would rather stick with this brand (Supervent) for the chimney as it is more readily available and quite a bit less money than the alternative brands that carry the 36" support box that I would need to reach through all the way.

I also thought I could get another support box (or just sheet metal?) and cover the framing if need be. Then trim off the top. Not sure if it would be necessary to add this 2nd box but let me know if you have any thoughts on that. This 2nd box would be flipped upside down and the side with the round hole cut off.

In the drawing below, the green is framing and orange is the second support box.

 

[begreen]

The support box will need to be firmly anchored to support the weight of the chimney. While Selkirk is technically correct. As long as the chimney pipe has at least 2" clearance all around it would work. However, it also should be impossible for debris to enter it once installed. That is the main issue I have is with it being open on the top. That provides a slim opportunity for crap to collect around the chimney pipe. For example, a roofing job later on can add a lot of trash into the support, though this is also possible with any sloppy worker. A simple extension or cap with a passage hole tight to the pipe could be pop-riveted on which help would prevent this.

 

[bretzel.logic]

The support box will need to be firmly anchored to support the weight of the chimney. While Selkirk is technically correct. As long as the chimney pipe has at least 2" clearance all around it would work. However, it also should be impossible for debris to enter it once installed. That is the main issue I have is with it being open on the top. That provides a slim opportunity for crap to collect around the chimney pipe. For example, a roofing job later on can add a lot of trash into the support, though this is also possible with any sloppy worker. A simple extension or cap with a passage hole tight to the pipe could be pop-riveted on which help would prevent this.

Have you seen these cathedral support boxes? They are, by design, wide open on the top.

 

[begreen]

Yes, I have installed them too. The box top either gets trimmed, folded over and nailed to the roof for as a supporting flange or it gets trimmed flush matching the roofline slope and anchored to box framing under the roof.

 

[Eman85]

My box was similar, cut the sides that stuck past the roofline and folded them over to make a flange. Attached at the sides and top flange.

 

[bretzel.logic]

I must be misunderstanding the concern over stuff falling into the box, then. Whether the box goes up to the sheathing or stops somewhere below that wouldn't change how much stuff can fall in during a roofing job. Still an open hole in the roof that could catch debris.

You said "A simple extension or cap with a passage hole tight to the pipe could be pop-riveted on which help would prevent this."

Are you suggesting to enclose the top around the pipe? At that point you are creating a closed in box of metal when the original design leaves it open at the top. I'm a total novice to this stuff but that seems like a terrible idea with no real benefit. Don't they make ventilated flashing so that air can escape from the roof/support box? Closing it in would prevent that from happening.

 

[begreen]

Methods to prevent anything from coming in contact with the chimney pipe are common. Look up attic insulation shields. They are capped.

Truth be told, this is the first time I have seen this question. Ultimately, this is up to you, and probably the insurance company and/or the inspecting authority. If Selkirk has said it's ok in writing, that may help.