Help this master plumber understand – if you will. We’re on a job site where the carpenter referred to exposed beams as “collar ties”. It’s a very nice home – sort of Frank Lloyd Wright-esq in its design – except for the vaulted ceiling with its ribbon skylight that runs along the roof-line where you’d typically expect to see a ridge-vent on the exterior – instead of glass.
Given that the roof-rafters are not called collars, where does this term collar-tie come from that represents the horizontal bar in a capital “A” of an open and plastered-to-the-peak (except for the long glassed skylight) roof truss design come from and why is it called a collar tie?
Replies
Collar tie is located in the upper 1/3 of the rafter runs and helps prevent the roof from hinging in high wind loads, they do nothing for resisting the outward force of the rafters on the walls.
Rafter ties located in the lower 1/3 in lieu of ceiling joists allowing a higher ceiling than the wall plates and resiste the outward force of the rafters on the walls.
neither of which you should cut with your chain saw while installing the vent. :)
I'm not a structural engineer, but I question the accuracy of your first statement that collar ties in the upper third do nothing to prevent spreading. Consider two flat pieces of roof with a connection along the ridge that can rotate (hinge) and no ceiling joists; put it on the ground and it is going to flatten out competely. Stand it back up with an angle to the roof pieces, tie some ropes connecting the rafters together at any point above the bottom, and set it back down. If the ropes are strong enough, the roof will retain its pitch (except for some bending of the rafters); the closer to the bottom the ropes are tied, the smaller the load will be on the ropes. So it's better to have the collar ties near the bottom to reduce the stress on them and their connections to the rafters. But I have seen one roof with an open ceiling that used plywood gussets at the peak in place of a ridge beam to take care of the spreading.
And none of this has anything to do with wind-load. If it is uplift that you are talking about, that should be taken care of with hurricane clips between the rafters and the top plate of the walls.
Edited 11/27/2007 1:22 pm ET by brod
In theory you're correct, but it won't pass a framing inspection in the real world unless there's something to couteract the outward vectors on the walls such as rafter ties or ceiling joists or some other engineered method like a structural ridge beam.
Search the archives here for collar tie and you'll have a whole lot more to read.
What you say sounds great but has little basis in recognized standards. The southern building code ( where they see a lot more uplift situations from hurricanes and tornadoes requires the use of collar ties or alternative hardware to resist hinging open at the ridge. The hurricane ties you speak of are for another purpose altogether - to prevent the eave from separating from the wall.It is true that collars in the upper third of the space may contribute somewhat to preventing wall spread, but the amount is so negligible that they are not recognized as such in the codes and need special engineering studies and stamps on plans to win approval. Those can include analysis of diaphragm action, wall buttressing, ridge beams, etc, none of necessarily would handle all the load stresses alone, but might in combination do the job.
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You wear your collar up high on your body, above those wide maanly shoulders where they narrow down to support your ridge cap
So does your house
;)
Seriously, the verb to collar is to restrain or to contain in modern useeage
The Verb meaning "to capture" is attested from 1613.
http://www.etymonline.com/index.php?term=collar
And that is what aa collar tie does, it keeps the rafters collared in a high wind
Welcome to the
Taunton University of Knowledge FHB Campus at Breaktime.
where ...
Excellence is its own reward!
Thanks gents! Who'd of thought they hold the roof from opening up in windy weather instead of helping to keep the bottom from spreading apart.
Vent? We don't need no steenkin vents! Our sort-of-new PA Uninformed Code eliminated the need for lots of the venting and has more loopholes than the US tax laws.
No chain saws here :), but I've been on a few job sites where chain saws were used to hack in HVAC. If I'd been the builder.......
First we check codes, then we check with the GC & in today's world of lumber products, we often check with the mfgrs to get the proper info on where to cut and what the max dim can be for holes vs. square or rectangular openings. Then we cut!
That's also why we specialize in hydronic radiant heating and mini-split units for the A/C. I can transport the same heating/cooling BTUs through 1/2" tubing as I can via an 8 x 18 duct. Quieter, cleaner & healthier too.