I have a general question regarding roof framing forces. My understanding is that rafter ties ( from top plate to top plate) resist the outward force of a non-trussed roof system. Do collar-ties necessarily perform the same function? And I’m aware of flitch plates allowing an open cathedral w/o either of the aforementioned elements; Does a ridge _beam_ also allow the same? (say in contrast to a ridge _board_)? I know this is pretty basic and yet I appreciate hearing different ways people explain it to give me a more comprehensive understanding of the options and trade-offs.
Thanks,
David
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Rafter ties (in the bottom third of the rafter span and typically doubling as ceiling joists), if properly fastened, are to resist outward thrust.
Collar ties (in the upper third of rafter span) are to resist uplift at the ridge (and used to be used also to stiffen undersized rafters against sag).
I don't know where you would use a flitch plate in roof framing, but a STRUCTURAL ridge is engineered to take half the roof load (load-bearing walls the other half) and eliminate out-thrust and the need for rafter ties.
Solar & Super-Insulated Healthy Homes
Thanks, that pretty much answers my questions. But if anyone else has any other thoughts feel free to chip in.btw- can you give me an idea of how much uplift is generally allowed for and what causes it (wind only?) I live on the West coast and we don't have much in the way of tornados/ hurricanes so I'm curious.
Edited 3/8/2008 6:41 pm ET by peedee
There was an article in FHB a while ago where steel plates were sandwiched between doubled up rafters. The steel was fabricated in an inverted "V" shape with the angle the same pitch as the roof.I though it was an innovative solution.As far as ridge board versus ridge beam; yes, a ridge beam is structural and when properly sized would allow for an open cathedral ceiling.A ridge board is not structural and would require additional methods to support the roof.
Simple solution to what?
"Simple solution to what"?Nothing is simple. I looked back and didn't see a mention of "simple" in my reply, so I'm not certain how to reply to your question.I was writing that the I though that the "V" steel plates were an "innovative solution" on how to carry roof loads to the walls without using a ridge beam or ties.Mongo
Edited 3/10/2008 5:06 pm ET by Mongo
"I know this is pretty basic and yet I appreciate hearing different ways people explain it to give me a more comprehensive understanding of the options and trade-offs."
It may seem pretty basic, but in my experience (and judging from some of the discussion on this board) a lot of builders have a pretty sketchy idea of how simple pitched roofs work.
You may have heard about flitch beams. They are a continuous piece of flat steel that is sandwiched between framing lumber and bolted. These were more common before engineered beams were available. They have the strength of a steel beam but are easy to attach other lumber to. In an open cathedral ceiling, you need a ridge beam. The beam is supported on the end walls on posts and can carry the load of the roof. This prevents the weight of the roof from forcing the exterior walls out. If you don't use a structural beam, you will need another method to keep the walls from pushing out. Rafter ties/joists and collar ties are an option. In medieval times, they used flying buttresses.
read this thread to get some more background.
Matt-That link takes me to a short thread about SDs, I'd like to read more about the subject (though the original response was pretty clear), if you have a 'better' link... ?Thanks,
David
Sorry... Thry this link