Rafter Tie Spacing

[PT99]

Rafter Ties
What is the usual spacing for rafter ties in a typical wood frame house cathedral ceiling with collar ties at 4 ft o.c. in the upper third.

 

[EngineeringEric]

If you have collar ties at 48", then I'd say the usual spacing of said ties is 48"?

Are you referring to a tie at the bottom of the rafter (like a ceiling joist)? or a tie at the upper portion? i've seen numerous nomenclatures so not sure if we are talking about the same thing.

But, i believe residential code minimum for the upper one is 48" in the IRC and the lower one is as needed, but usually every one to every other is the system can handle it. without knowing spans, pitches, sizes, etc it isn't really possible to give a one-size-fits all

 

[PT99]

I am referring to omitting ceiling joists and installing rafter ties just above, and also installing collar ties below the ridge board 1/3 below the ridge board, at 48 inches o.c. for the collar ties.

 

[BAretired]

Just above what? Make a sketch.

BA

 

[Dvdt18]

There is no usual spacing as it depends on your rafter design and the lateral thrust. If your using the rafter ties in conjunction with ceiling ties then the rafter ties are being utilized to reduce the span of the rafters and the ceiling ties to resist lateral thrust. If your omitting ceiling joists as you stated than the rafters and rafters should be designed with a raised rafter tie. The rafter design should be calculated to account for the moment from the raised rater ties as well as the rafter to rafter tie connections (hence the use of the rafter span adjustment factors indicated in IRC Section R802.5.). Recently I've designed them at 48 inches on center but had to account for a greater lateral thrust than providing them at 24 inches on centre (each rafter in my situation).

IRC Section R802.5.1 should shed some light on the requirements. Here's a link

https://codes.iccsafe.org/content/IRC2015/chapter-8-roof-ceiling-construction