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Forums» International Building and Residential Codes -- Structural Issues» Cathedral Ceiling Ridge Beam and Reducing Rafter Spans

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Cathedral Ceiling Ridge Beam and Reducing Rafter Spans
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Re: Cathedral Ceiling Ridge Beam and Reducing Rafter Spans By  peach!!

Posts: 0

4/3/2008 4:21:00 PM

If it''s actually an engineered ridge beam, the rafters aren''t rafters any more.. they are simply supported beams...

The reason to use an engineered ridge beam is to open the room up for a cathedral ceiling (and thus remove the requirement for collar ties or ceiling joists.

Depending on your ground snow load, you are looking at (probably) less than the 40 psf floor loading for your roof.

Having said that, the not-rafter-simply-supported beams need to be properly supported..

does that make any sense?


   

Re: Cathedral Ceiling Ridge Beam and Reducing Rafter Spans By  George Roberts

Posts: 0

4/3/2008 4:42:00 PM

The rafters are still rafters. The ties allow a greater rafter span since they provide support.

Rafter ties and collar ties are terms used to attempt to simplify the prescriptions. All ties regardless of placement have some of the character of rafter ties d collar ties.


   

Re: Cathedral Ceiling Ridge Beam and Reducing Rafter Spans By  homebild

Posts: 0

4/3/2008 5:10:00 PM

Thanks to you both.

I [i]do[/i] understand that when a ridge is designed as an engineered beam or girder, the rafters become ''hung'' from the ridge rather than support it...but that fact does not answer my questions about rafter span de-rating.

George really succintly answers the heart of my concern by clarifying that ''ties'' indeed do increase the ability of a rafter to span a given distance...

And by eliminating ceiling joists, rafter ties, or collar ties (or whatever you want to call them)...or by raising them above the top plate of bearing walls for a cathedral ceiling...the spans of the rafters in the prescriptive tables must be decreased according to formulas noted in footnote b.

That said, this also appears to indirectly answer my 2nd question:

The prescriptive spans in Tables R802.5.1(1-8)[b]only apply[/b] when ceiling joists/rafter ties/collar ties are placed above the top plates no more than 1/3 the vertical distance toward the roof ridge.

When the placement of these ''ties'' is more than 1/3 above the top plate, the rafters themselves are required to be engineered.

Would you agree?


   

Re: Cathedral Ceiling Ridge Beam and Reducing Rafter Spans By  George Roberts

Posts: 0

4/4/2008 7:41:00 AM

You are being confused by the simplifications that provide the prescriptions.

In the prescriptive model the assumption is that the ridge is NOT structural and the lower portion of the rafter is a cantilever. The portion of the rafter below the tie fails in bending. Either by deflection or stress.

If you want to consider the ridge structural, you can argue that the rafter should be treated as an inclined beam with both ends simply supported (Maybe argue that the top end is fixed) and with the ties providing additional support. I guess that is what peach was saying.

With a structural ridge I would accept/use the tabulated allowable rafter spans between the lower support & tie and between the tie and the ridge. Without reductions. (Not the right answer but as good as most answers.)


   

Re: Cathedral Ceiling Ridge Beam and Reducing Rafter Spans By  homebild

Posts: 0

4/4/2008 12:27:00 PM

Aha!

Thanks so much again to you both for the help and clarification!


   

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