My understanding regarding ceiling joists after reading the NY code book is this. Please correct me if I'm wrong. Ceiling joists must be used as rafter ties at a maximum distance of every 4 ft , if there is no structural ridge beam to support the rafters.

Not enough information to tell. A lot depends on span, loads, roof slopes and many other details.

quote:

Originally posted by Robert A.:
My understanding regarding ceiling joists after reading the NY code book is this. Please correct me if I'm wrong. Ceiling joists must be used as rafter ties at a maximum distance of every 4 ft , if there is no structural ridge beam to support the rafters.

Not true.

Section 803.3.1 of the PA UCC 2006 International Residential Code requires that when a ceiling is framed as a 'cathedral ceiling' and hence, there are no ceiling joists or rafter ties to resist lateral loads placed on the bearing walls, that the ridge must be designed as a girder or beam and the rafters hung from this beam.

Otherwise, ceiling joists or rafter ties must be installed for each rafter unless the roof assembly is engineered by a design professional.

The section you are referring to regarding 4 foot on center spacing only applies to collar ties when they are required to resist wind uplift in certain conditions.

Collar ties are not rafter ties and perform different functions.

Rafter ties/ceiling joists are designed to resist lateral loads that would push the top of bearing walls apart.

Collar ties and ridge straps are designed to keep the rafters from separating at the ridge beam under very low presurre conditions that can occur with extremely high wind conditions.

Bottom line: You need rafter ties or ceiling joists at each rafter or you need to have the roof-ceiling assembly engineered.

homebild

PA Certified Residential Building Code Inspector

This message has been edited. Last edited by: homebild,

I hear you homebild. Just playing devil's advocate here...When the c.joists run perpendicular to rafters, code states cross ties at a minimum every 4 ft. So, what would be the difference if c.joists run parralel to rafters? Why are they needed on every rafter then?

I re-read the code and it says "rafter ties shall be spaced no more than 4 ft". It does not mention collar ties.