Cathedral Ceiling Roof Rafters Thrust
Cathedral Ceiling Roof Rafters Thrust
(OP)
I am designing a roof for a single family residence and the owners would like a cathedral ceiling WITHOUT tie rafters. I was thinking about designing the top plate rectangular frame (seen in plan) and its connections at all four corners of the wall as a rigid frame that will take the horizontal thrust of the roof rafters.The slope of my rafters is 10 in 12 and the span is 22 feet. Has anyone looked at this problem this way? Input is appreciated






RE: Cathedral Ceiling Roof Rafters Thrust
I don't think you can make the top frame rigid enough. Maybe steel beams on their side?? Using say a 40 psf total load - check the side thrust generated. Usually very high.
I usually use a ridge beam that can carry the tributary area. There are are often quite sizable. Use Microlams or equal.
Just a thought.
RE: Cathedral Ceiling Roof Rafters Thrust
woodengineer
RE: Cathedral Ceiling Roof Rafters Thrust
In this situation, if I can't hang it from a ridge beam or framing above it, I usually use a bent steel beam.
RE: Cathedral Ceiling Roof Rafters Thrust
RE: Cathedral Ceiling Roof Rafters Thrust
RE: Cathedral Ceiling Roof Rafters Thrust
I take it this is a new building? Consider designing the roof with one side pinned and the other on a roller support (scissor trusses or something similar should work). You still need a ring beam as there will still be friction forces, but the horizontal thrust is much reduced due to the sliding joint. You also need to ensure that the finishes allow for the sliding joint...
Coefficients of friction between various materials are available in text books and from web sites. We tend to use 0.3 for greased steel to steel and 0.15 for PTFE bearings. Greased steel to steel requires regular maintenance PTFE although much more expensive, can usually be left alone.
For your roof also consider polyethylene sheet as a cheaper alternative, but make sure it is sourced from a reputable supplier and is not simply damp proof membrane...