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Author Topic: Cathedral ceiling  (Read 632 times)

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Offline Michael34

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Cathedral ceiling
« on: November 27, 2017, 03:33:26 PM »
Where do I find spanning requirements for the ridge beam in a cathedral ceiling? In IRC section 802.3.1 it only says "where ceiling joists or rafter ties are not provided, the ridge formed by these rafters shall be supported by a wall or girder designed in accordance with accepted engineering practice"

Offline Greenerpastures

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Re: Cathedral ceiling
« Reply #1 on: November 27, 2017, 04:48:10 PM »
Hi, try this link, http://www.awc.org/codes-standards/calculators-software/spancalc
there is a calculator so you will need to know
the dimension of the wood you are using.
But as far as am aware there is no support required under a ridge beam.
A ridge beam to my knowledge goes right up past the tops of the
rafters, this is so it will finish levell with the roofing material, and can
be used to screw a ridge tile or folded piece of material like aluminium
or galvanised steel to in order to close the gap where the two sides
of a roof meet.
The ridge is in fact is a very strong part of the construction as it is under
compression from the rafters on either side.

Now the rafters themselves, they will need supported at their centre,
depending of course on how long they are, short ones will require no
support, but the more support you give them the better the roof will
be at supporting any loads that may fall on it.

Offline Don P

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Re: Cathedral ceiling
« Reply #2 on: November 27, 2017, 05:50:16 PM »
You've described, very well, a ridgeboard which is non structural, it is simply a framing aid. A ridgeboard must be accompanied by rafter ties that connect across the lower third of each rafter pair. This is what puts those rafter tops into compression on the ridgeboard.
A ridgebeam is a different animal. Typically used in a cathedral ceilinged room where you don't want rafter ties. The rafters hang from the ridgebeam, each rafter being born half on the ridgebeam and half on the wall at their bottom ends. The ridgebeam then needs to be sized to carry half the roof load.  This goes outside of the awc spancalc and is part of the reason I made the calcs in the red toolbox on the lower left of the sidebar on this page. Holler if you need help.

Offline Greenerpastures

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Re: Cathedral ceiling
« Reply #3 on: November 27, 2017, 06:54:46 PM »
You've described, very well, a ridgeboard which is non structural, it is simply a framing aid. A ridgeboard must be accompanied by rafter ties that connect across the lower third of each rafter pair. This is what puts those rafter tops into compression on the ridgeboard.
A ridgebeam is a different animal. Typically used in a cathedral ceilinged room where you don't want rafter ties. The rafters hang from the ridgebeam, each rafter being born half on the ridgebeam and half on the wall at their bottom ends. The ridgebeam then needs to be sized to carry half the roof load.  This goes outside of the awc spancalc and is part of the reason I made the calcs in the red toolbox on the lower left of the sidebar on this page. Holler if you need help.
Hi, am interested to understand this, I think the terminology is putting me off.
I understand the ridge board, I understand what we over here call a purling,
this is carried at both ends, and depending on the length of the roof, may be
supported along its length in the center or at thirds or quarters or what ever
is required to keep it from sagging, this what I call purling is usually mid way
up the rafters and is cut in so it sits vertical, though others put it at right angles
pushed up and nailed to the back of each rafter, I like it vertical as it supports
its self better, each rafter has a notch out of it where the purlin fits in neatly,
then what we call collar ties are cut to fit under this purling and connect the
rafters on one side of the roof to its opposite mate on the other side, this keeps
the rafters from pusing out as weight is added to the roof, otherwise the walls
would get pushed apart by the bottoms of the rafters sitting on them.

Wish I was good at drawing, for I would like to know what method the OP is
reffereing to.

Offline Don P

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Re: Cathedral ceiling
« Reply #4 on: November 27, 2017, 11:06:53 PM »
We can't even agree on terminology across state lines  :D
This is a sketch that has been evolving. Factory attic trusses with a field framed small dormer section including a ridgebeam supported between doubled up trusses (a truss girder). Coming this way at the end of the trusses is a triple girder truss supporting the end of a 16' long ridgebeam. It is not drwn but there would be an adequate post under the near end of the ridgebeam (or a "load path" of posts and headers). Without ties down low on the rafters in order to prevent spreading the rafters are hanging from the ridgebeam. If the ridge is sized such that it does not sag, then there is no thrust on the walls, all loads are vertical.

The building here is 24' wide I think. Rafter span for our loading purposes is measured horizontally so each rafter spans 12'. The ridgebeam is supporting half of the rafter's vertical load the wall is supporting the lower half of the rafter. so on this roof the tributary area loading the ridgebeam would be 12' wide x 16' long= 192 square ft. Multiply that by the local design snow load+dead load and you have the load and span for sizing the ridge as a beam.


Offline Michael34

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Re: Cathedral ceiling
« Reply #5 on: November 28, 2017, 09:00:32 AM »
In the section R502.5, table 502.5 there are allowable girder spans. Is it what I should use? Here the minimum building width is 20' and for 2*2x4, 30 psf, i get 3-6,  the distance between posts supporting the ridge beam. But my building is only 10' wide; could the spans be increased?

Offline ChugiakTinkerer

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Re: Cathedral ceiling
« Reply #6 on: November 28, 2017, 03:47:18 PM »
It would probably be helpful at this point for you to describe what exactly you want to build, and whether there is a jurisdictional authority such as the county building inspector who will be inspecting your compliance with the locally adopted code.

If you want a cathedral ceiling and you don't want the roof to sag due to the walls bowing out, you need a structural ridge that runs down the center of the roof and is strong enough to support half the roof weight.  To start, you should know what the roof weight is.  For a small building with asphalt or metal roof, you can use a dead load value of 10 psf.  Combine that with the 30 psf snow load, you get 40 psf.  Multiply that by 10' and you get 400 lbs per lineal foot of roof.  A ridge beam will be supporting half that weight, while each wall supports a quarter of it.  So you need a ridge beam capable of spanning your desired room length and supporting 200 lbs per lineal foot.

If your room is 12' long, the total beam load is 2400 lbs.  Of that, 600 lbs is dead load.  You can use the Toolbox that Don P has created to help you find a beam that will meet your needs.  The URL is http://www.forestryforum.com/members/donp/beamclc06b.htm

Using that calculator and a beam of #2 Spruce-Pine-Fir (SPF), you'll need something like a 6x10 to support the roof.   A building inspector might let you build with that, or they might want to see something from the prescriptive tables.

Alternatively, you could use a couple of LVLs.  Looking at the Weyerhauser span table on page 14, a single LV 1-3/4" by 9-1/4" can support the load over a span of 12'.  If you have a longer span, say 20', then it would take a pair of 11-1/4" LVLs to carry that load.  The nice thing about LVLs or any other engineered product is that you can get a printed report from the retailer that should satisfy the building inspector.
Woodland Mills HM130

Offline Don P

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Re: Cathedral ceiling
« Reply #7 on: November 28, 2017, 08:49:26 PM »
Michael, see footnote c under the girder table. You can interpolate between dimensions on most code tables but you can't work off the edges. Yes the spans can be increased, you have entered the engineering required section of the code, different inspectors treat it differently. The calc Chugiak pointed to is one way. The wood frame construction manual WFCM at awc.org has beam load tables that are code referenced, you need to do a little engineering to get the beam loads which gets into the inspectors comfort level with you doing it. And then engineered products like the LVL that can come with an engineers letter for your job.


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