Replies

1. 
   unclemike42 | Feb 07, 2021 04:07pm | #1

   If you need to cut one or more rafter pairs to have room for the chimney,

   hang the top end of each cut rafter from a joist hanger for such purpose, attached to a beam which runs parallel to the ridge. and size the beam for the load supported.

   then run the numbers to be sure your beams are properly supported. (you might have to double the rafters which support the beams, or make them taller) Carrying the load down also may be an option, if you have supporting walls handy that run down to the foundation, close to, but separate from the chimney.

   How big is the chimney at the ridge line, and what size roof are we discussing? How much snow load? How big are the rafter boards you plan to use? How far apart do you plan to install them?

1. 
   calvin | Feb 07, 2021 04:14pm | #2

   And remember the clearances of wood to masonry chimney.

2. 
   jofussino | Feb 07, 2021 08:59pm | #4

   Thank you for the response. I had envisioned that it would be to that effect--somewhat like framing second floor joists for stairs. My roof will be 12/12 pitch (I like the aesthetic--and head room on the second floor under the roof.), with planned 2x10s or 2x12s (for thicker insulation and strength) spaced 24 inches on center. This is Oklahoma, so snow is minimal for the most part--at least in central Oklahoma. I'm not exactly sure what the dimensions of the chimney will be at this point. (I'm thinking on the order of 3 x 3 ft; but I need to research that some more based on what would look most typical with late colonial era capes.) My thought before posting my question was that I would need to cut the ridge beam and one rafter set out (or, more accurately, frame so that the center four-ish feet of the ridge beam will be absent--creating a net two ridge beams). The more central ends of the ridge beams would have tripled rafter sets with cross bracing--like an A-frame--sandwiched between each these rafter sets to help support the more central end of the ridge beam. Thus, it would appear as rafter set, cross brace, rafter set, cross brace, rafter set --with the central ridge ends bearing on the cross braces in addition to having the rafters nailed to them. (See my attached diagram--Option A.) Then, as you noted, I would triple (my thought) parallel beams between the end rafters of that set to support load up to the chimney in the central portion of the roof (keeping appropriate clearances, as Calvin noted). Alternatively, I would triple up rafter sets and just put cross braces on each end of the tripled set for added support of the ridge beam (Option B in my diagram). What do you think? Too much? Too little? Too stupid?

   
1. 
   unclemike42 | Feb 08, 2021 06:55am | #8

   You did not include the roof dimensions, specifically the distance between outside walls.

   You also should specify if you intend to use an actual ridge beam, which will support the roof peak directly down to the foundation below (via wall members at each end), or if you will use a ridge board, which primarily transfers loads between the ridge ends of rafters on each side. (which then are passed to the outside walls)

   Follow the numbers. the span of your common rafters needs to be reviewed against a suitable span table for the dimension, species, grade and spacing.

   Since you are considering taller rafters to allow more room for insulation, the bonus is they will come with increased load carrying ability.

   The number of extra rafters should be determined by review of the capability. since you are only removing the end point of one rafter, the maximum loads on adjacent rafters would be increased by 50%. if the rafters were sized for the maximum span, you could double up each side and more than take care of the extra load. If you have beefier rafters than required for the load, you might be ok without any extra rafters to support the beams.

2. 
   2sheps | Feb 07, 2021 08:41pm | #3

   Here's a sketch of what it might look like in it's simplest form. The gray shade would be the actual chimney and the red would be the offset for fire prevention. Check you local codes.

   
1. 
   jofussino | Feb 07, 2021 09:09pm | #5

   Thank you for the reply. So was my proposed "solution" overkill?

1. 
   calvin | Feb 08, 2021 06:15am | #7

   The vaulted ceiling lacking ceiling joists will require a ridge beam and engineered connection/support at the masonry column (chimney) down to an approved footing. Or the proper columns down to a footing.

2. 
   unclemike42 | Feb 08, 2021 06:09am | #6

   The point here is that you need two inches of clearance between the masonry and stuff that burns.

1. 
   jofussino | Feb 08, 2021 09:49pm | #11

   Thank you UncleMike42 and Calvin for your comments. The house will be 44 feet long x 32 feet deep. The ridge member will function more as a ridge board than ridge beam (downstairs window on the side wall center line), I suppose. In retrospect, I think I'll be framing the roof 16" OC. I should have specified previously that there will be ceiling joists and 5' knee walls. (My diagram was more from the standpoint of how to frame the rafters than an accurate representation of how the entire upstairs would be framed.) My interest in the taller rafters is to enclose the entire (relatively small) attic space within the conditioned space--a function of watching too much youtube building science guys like Matt Risinger? I will actually be able to fit the mechanicals in a room within framed interior space upstairs; so maybe there's no significant value to my plan to insulate at the roof rather than the upstairs walls. Any comments on that?

3. 
   jofussino | Feb 10, 2021 05:08pm | #14

   Thank you. That's essentially what I thought might be the case (though as noted above I wondered if I might have needed more support on either side). Sorry for the late response.

3. 
   suburbanguy | Feb 08, 2021 09:08am | #9

   As a Canadian, it always amazes me to see common rafters used in the U.S. We've been using truss systems for decades.--literally from the '60s. They used to be made on-site with nailed plywood at each joint. Now, metal plates are used and the trusses are delivered on a flat-bed truck No sagging roof ridges here. (Basements and garages are for storage in Canada, LOL.)

4. 
   hridyard | Feb 08, 2021 06:48pm | #10

   My six foot wide center chimney is framed with double 2x8 rafters on either side of the chimney (30 foot wide house and 12/12 pitch roof.) Short rafters 16" OC, hung from double 2x8 beam parallel to the ridge, spanning the six feet (with several inches of clearance from masonry,)between the doubled full rafters. Assuming this met code 30 years ago in CT, with significant snow load....

5. 
   unclemike42 | Feb 09, 2021 08:51am | #12

   For discussion: (you really should consult an engineer or building codes person in your area)

   Your roof rafter span is 16 feet.

   http://www.southernpine.com/app/uploads/SPtable17_060113.pdf

   number 2 SYP you could get by with 2x8 spaced 16 inch centers. 2x12 would span up to 22'10".

   https://www.anthonyforest.com/assets/pdf/sfpa-header-and-beams.pdf
   Page 29.

   15 foot long rafter with design loads of 20 and 10 PSF would call for
   TL of 450 PLF
   LL of 300 PLF

   even a single 2x6 would handle the load, based on a 4 ft span.

   You would only need a single 2x12 for the beam.

   Not exactly clear if you could get by without increasing the rafters which carry the beams from a single 2x12. Might be ok if you went with no1 or select for the four rafters that reach to the peak next to the chimney,

   I would go with double rafters on each side of the chimney, and double 2x12 for the beams. (if number 2 SYP is available and in-budget.) will be overkill, but not as much as three, which would seem excessive.)

   The ridge board is supported by the rafters on each side.

   I think it a great idea to insulate the roof. Will let you use the spaces behind the knee wall for storage and not worry about sealing doorways. Ventilation will be easier if you make the ceiling below the ridge lower than the peak, (unless you really need a 16 foot tall ceiling)

1. 
   jofussino | Feb 09, 2021 06:49pm | #13

   Thanks again for the great information. As for the last sentence of your comment, I do plan to use ceiling joists with 8 foot ceilings (with some slope between there and the 5 foot knee wall). I'm hoping to do an unvented attic (again, for the aesthetic. I hate seeing attic vents on otherwise beautiful homes. The fewer roof penetrations, the better.) with medium-density SPF insulation.