Design of Cathedral Ceilings - InspectionNews

Terry Beck · #1 (**permalink**) 02-11-2008, 11:29 AM

Does anyone have any good online references to design of cathedral ceilings? I don't mean vaulted or raised ceilings (i.e. with scissor trusses), but true cathedral construction. I can see lots of stuff on ventilation issues, span tables etc for normal attic construction, but surprising very little on load and stress design for cathedral ceilings. I've checked the Internet sites of various manufacturers (APA, truss, etc), and would prefer something I can download for free (I'm cheap), but would also be interesed if anyone can recommend a good reference book.

One question for example, is in the absence of ceiling joists or collar beams to tie together the ends of the rafters, how do you account for the forces that would cause the sidewalls to spread? I saw reference to ridge beams as a structural replacment for ceiling joists, but don't understand how that would counteract the spreading forces to the sidewalls.

Jerry Peck · #2 (**permalink**) 02-11-2008, 11:41 AM

Quote:

Originally Posted by Terry Beck

I saw reference to ridge beams as a structural replacment for ceiling joists, but don't understand how that would counteract the spreading forces to the sidewalls.

Terry,

That would be a structural ridge.

In normal construction, the ridge is simply pinched between the two opposing rafters, with the loads trying to go vertically down (by gravity), thereby pushing against the rafters, which in turn push against the tops of the walls, pushing then outward when not supported by shear walls or pilasters or something else.

When a structural ridge is installed, the load of the ridge is carried by the two walls (or columns or something) are each end, then when the rafters are placed, the rafter load is simply carried vertically down from the ends of the structural ridge and vertically down at the walls, with no lateral outward push against the walls (stating it very simply, anyway).

Thus, with a structural ridge, nothing is needed to keep the walls from being pushed out by the roof load. However, something needs to keep the walls from being pushed in or sucked out by wind loads and other loads, so you typically end up with pilasters supporting the wall at specified intervals. The pilasters may be inside, outside, or both.

Nolan Kienitz · #3 (**permalink**) 02-11-2008, 11:54 AM

Terry,

Just curious ... are you in Missoula? I'm originally from NE Montana and just happened to be on the phone with a software developer in Bozeman this morning for an hour.

He was telling me there were 43 homes sold last month in Bozeman.

Hope things are okay for you further West in Big Sky Country.

Send me an e-mail when you can.

Nolan@NolansInspections.com

Terry Beck · #4 (**permalink**) 02-11-2008, 12:35 PM

Jerry,

After I wrote my question, I did find some other sources that refer to a couple of method of attaching the rafters to the sidewalls that counteract the stress that would push apart the side walls.

However, I'm still confused about what handles the roof load. If I get it right, in a traditional gable style house with a flat ceiling, the roof load is carried down the rafters to the side walls.

So are you saying that in a gable style house with a cathedral ceiling and a structual ridge beam, that roof load is now carried by whatever supports that ridge beam (the gable wall)? Or does the majority of the roof load still travel down the rafters to the side walls? I guess my question is, do the gable walls in a house with a cathedral ceiling and structural ridge beam(s) now become the main support walls?

Jerry Peck · #5 (**permalink**) 02-11-2008, 01:41 PM

Quote:

Originally Posted by Terry Beck

So are you saying that in a gable style house with a cathedral ceiling and a structual ridge beam, that roof load is now carried by whatever supports that ridge beam (the gable wall)? Or does the majority of the roof load still travel down the rafters to the side walls? I guess my question is, do the gable walls in a house with a cathedral ceiling and structural ridge beam(s) now become the main support walls?

Terry,

With a structural ridge (I would drop using the term "cathedral ceiling" as that has various meanings and could lead to confusion) the roof load is carried by BOTH the support for the structural ridge AND the bearing walls. The roof load is carried by the rafters and transferred to the structural ridge and walls on which the rafters are bearing. The structural ridge may be supported by the gable end walls or it could be supported by columns.

Think of it this way: You construct a post and beam 'shed'. You set 4 posts, then set 4 beams (2 sides and 2 ends), then install a center vertical column at the center at each end, then set a structural ridge (beam) on those two vertical supports - now lay rafters at each end from the ridge to the corner posts to keep that vertical column vertical.

Now lay the rafters on the structural ridge (beam) and on the side beams (between the corner posts).

No walls, nothing bows out, the roof load is carried to the 4 corner posts via the beams on the side.

No, install walls below the side beams. Same thing, no difference.

Now, let's change it a little, instead of side beams, construct two bearing wall setting on footings. Same thing, no difference (other than if the walls are long you will need to keep them from bowing in or out).

Bill Wieczorek · #6 (**permalink**) 02-11-2008, 01:58 PM

What Jerry said makes sense. My prior home had a 35x19 vaulted great room with 19 foot ceilings. There was a laminiated beam I'd guess 10 inches thick that ran the length of the room. I asked why the beam was so thick and was told me it was to transfer the weight to the gable ends as Jerry mentioned.

I should have kept that house, much better then what I bought.