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[Nick Gromicko]

Larry writes

The diaphram of the ceiling material nailed to the ceiling joists (no matter what direction they run) and attached to the top plate acts as the rafter ties in the short distance needed.

So if the attic has no flooring, just the ceiling drywall underneath substitues as rafter ties when the joists run perpendicular to the rafters? Seems wack if you ask me.

[Robert J. OConnor]

Originally Posted by gromicko

If collar ties don't help prevent rafter spread and only serve the purpose of holding down the rafters to the ridge board, then why are they required when the joists (rafter ties) are perpendicular (not parallel) to the rafters (no rafter ties)? If they don't help prevent rafter spread, why is their installation required when rafter ties are absent?

Collar ties (up higher near the ridge), gusset ridge plates, or metal ridge straps (in addition to the typical rafter/plate toenails) are needed no matter what direction the ceiling joists span to keep the top of the rafters from coming apart in higher winds. Many local framers consider use of both metal ridge straps and 2x4/2x6 collar ties good practice (in addition to the rafter ties).

Sometimes if the ceiling joists are framed perpendicular to the rafters, and a structural ridge beam isn't used, the wood collar ties will go into tension. But the usual wood collar ties closer to the ridge with a few 10d nails at the connections just doesn't have enough strength ... and the connections start splitting and/or pulling apart which allows rafter spread and ridge sagging.

I have seen it happen on many houses I have done evaluations on which only had typical collar ties higher up near the ridge. Usually requires additional larger and more closely spaced tie members with heavier bolted connections, or installation of a structural ridge beam to fix.

That is also why IRC 803.2.1 requires ceiling joists or rafter ties (continuous subflooring or wood/metal ties) ... parallel to the rafters at the wall top plate level, and attached to the rafter heel ... with fairly heavy connections per IRC Tables R602.3.1 & R802.5.1.9 ... or a structural ridge beam or special rafter tie design by a PE/RA is required.

JMO & 2-nickels ...

[Robert J. OConnor]

Originally Posted by gromicko

1/2 the sides of a hip roof are made of rafters that don't run parallel to the joists. Are rafter ties and/or collar ties required if the floor joists don't switch direction at the gables?

Requires a posted ridge where the hip rafters frame into the ridge (required per IRC 802.3) ... or a special design is needed for the first few rafter ties near the hip ridge, as they will be more heavily loaded.

JMO & 2-nickels ...

P.S. I agree the typical sheetrock ceilings don't make an effective roof rafter tie.

[Larry D. Kage]

Originally Posted by gromicko

Larry writes

So if the attic has no flooring, just the ceiling drywall underneath substitues as rafter ties when the joists run perpendicular to the rafters? Seems wack if you ask me.

Certainly not ideal but that is how the drywalled ceiling is acting.

A better feature in your design example would be 2x stock nailed perpendicular atop the ceiling joists parallel to and below the hip common rafters and connected to the exterior wall top plate. A diagonal brace or anchor point would be needed also to prevent the ceiling joists from moving all at once.

[Nick Gromicko]

Thanks guys. Very useful posts!

[Rick K. Kie]

I just ran into this same issue on a gable roof, the load bearing walls run parallel to the rafters and the ceiling joists run perpidicular to the rafters. The rafters are 2x6 with an approximate 14' span. Some of the rafters are cracked and some have stress cracking the roof is extremly wavy on one side.
Is it in our best intersest to explain to the customers the two different ways to remedy the problum or just refer them to a contractor or engineer?

[Joe Farsetta]

You point out the defect/concern, and refer them to a PE