Ceiling Support Box: question about box height

[MaybeErnie]

After weeks (months?) of reading threads here, studying catalogs and doing research, I'm finally about ready to start ordering parts for my new installation. I'm planning on using DuraVent Duratech for the chimney. The first item I need is the ceiling support box and already I'm stumped. The square Duratech boxes come in 3 heights (11", 24" and 36"), and the round box comes in only 9.52". The roof is flat with an attic space above. My joists are 2x10, giving a total ceiling depth of ~11-1/2" including the ceiling boards (1/4" oak paneling with 3/4" nailers) and the attic floor boards. I also understand that the box should extend below the ceiling down into the stove room a minimum of 3-1/2".

Would it be best to have the support box match the total 11-1/2" height (joists + ceiling/floor material) or is that not essential? It seems like having the metal walls of the SB completely enclose the joist space would be desirable for safety, but maybe it's fine to just have a tall enough box to enable nailing to the framing and call it good. After accounting for the 3-1/2" room extension + 1" ceiling material, an 11" SB would give me only 6-1/2" to nail into the joists, but maybe that's plenty. Alternatively, I was considering ordering the 24" SB and cutting it (doable?) to match my full 11-1/2".

Your sage advice is appreciated!

 

[stovelark]

You normally use the round support box if the ceiling is flat, the square ceiling support box if you have a cathedral ceiling, but the choice is yours. You will have an attic shield too in the attic to keep insulation from coming into contact with the class A pipe. The choice is yours. Keep warm.

 

[MaybeErnie]

Oh -- I've seen lots of pics of square boxes on flat ceilings, but I do think that the round boxes look somewhat better. If I went with the round DuraTech box (9.5" tall) then that leaves only about 5" for nailing into the joists. That's OK?

 

[gthomas785]

Oh -- I've seen lots of pics of square boxes on flat ceilings, but I do think that the round boxes look somewhat better. If I went with the round DuraTech box (9.5" tall) then that leaves only about 5" for nailing into the joists. That's OK?

As long as everything above it is more than 2" from the pipe (or whatever your chimney pipe's spec is for clearance), then yes it's ok. If you have insulation above the ceiling you need a physical barrier to keep that away from the pipe, which should extend above the height of the insulation.

 

[begreen]

DuraTech flat ceiling support boxes are available in round or square. They are shallow and anchored to a box frame off the joists. Normally they will need an attic insulation shield on top unless chased when the chimney pipe continues up through a room. Cathedral ceiling support boxes (those 12-24" tall) are square and typically for sloped ceilings where the is no attic. There can be exceptions, like where the support box goes up in a shallow kneewall space close to the eave and the cathedral support box continues up to or through the roof. In this way it acts as both the support box and an attic insulation shield.

 

[MaybeErnie]

Ah, got it! In looking at the DuraTech catalog, I homed in on the "Square Ceiling Support Box" section (p. 10) where it says "Suitable for both flat and cathedral (A-Frame) ceiling applications." Those are the 11", 24" and 36" tall ones. I didn't catch that there was another product labelled "Unpainted Flat Ceiling Support Box" (p 12) that is 11.5" tall. It sounds like I can choose either the round box (9.5" tall) or the flat ceiling box and not worry about whether or not it extends all the way up to the top of the joists. As long as it extends up far enough to get some screws in, I'm peachy.

The appeal of just paying someone else to handle all this is becoming more apparent. I have a tendency to overthink these things.. But then again, we're talking about intense fires in our house, so maybe not.

 

[30WCF]

Mines not the same brand, but I had to frame it in between the joists, since the spacing was wider than the box, and added cross braces between the joists. I also added the insulation shield to keep any debris or anything a mouse might move around off the pipe.



It’s been this way for almost 10 years, but I’m gonna add another brace where the roof penetration is sometime this summer.
When I sweep I have to pull the top piece of class A off and sometimes the pieces below it spin from the attic when I’m trying to unlock the top pice on the roof. This should keep everything below the roof from spinning, and keep some of the weight off the ceiling box.

 

[MaybeErnie]

Mines not the same brand, but I had to frame it in between the joists, since the spacing was wider than the box, and added cross braces between the joists. I also added the insulation shield to keep any debris or anything a mouse might move around off the pipe.

View attachment 337899
View attachment 337900
View attachment 337901

It’s been this way for almost 10 years, but I’m gonna add another brace where the roof penetration is sometime this summer.
When I sweep I have to pull the top piece of class A off and sometimes the pieces below it spin from the attic when I’m trying to unlock the top pice on the roof. This should keep everything below the roof from spinning, and keep some of the weight off the ceiling box.
View attachment 337902

Thanks -- those pics are super helpful!

Adding the insulation shield seems like a great idea and I will add that to my materials list. I'm also still thinking about what all I need at the roof penetration and, although not required, including a roof bracket for weight relief has to be a good thing. That is especially true in my case because I will have a pair of 15-degree elbows between the roof penetration and the lower floor/ceiling where the support box will live.

 

[30WCF]

Depending on the height you may need to add some studs for a frame and add wall brackets to the studs in the attic area.
Needs a brace every 5’ I think. Anything above the roof gets a roof brace if over 5’.

 

[begreen]

Mines not the same brand, but I had to frame it in between the joists, since the spacing was wider than the box, and added cross braces between the joists. I also added the insulation shield to keep any debris or anything a mouse might move around off the pipe.

View attachment 337899
View attachment 337900
View attachment 337901

It’s been this way for almost 10 years, but I’m gonna add another brace where the roof penetration is sometime this summer.
When I sweep I have to pull the top piece of class A off and sometimes the pieces below it spin from the attic when I’m trying to unlock the top pice on the roof. This should keep everything below the roof from spinning, and keep some of the weight off the ceiling box.

Good illustration. That's proper. I would also seal up the 1/4" gaps with silicone caulk or wood strips so that no dust can get into the clean space around the chimney pipe. This would be important if blown in insulation was used around the shield.

 

[MaybeErnie]

Yes -- I will need roof bracing because the height above the roof will be close to 5-feet. I'm also going to need a snow diverter immediately above the pipe, but I may be able to fabricate that piece (big maybe, we'll see).

Thanks again for the input!