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Use of Collar Ties in Light Frame Wood Construction

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I have a client with a home in escrow. There is a cracked ridge board in the
garage and it appears that the ridge has dropped a few inches. The framing
is rafter tied by the 2x6 ceiling joists to the 2x8 roof rafters, the ridge
board is a 2x12 DF. The crack in the ridge board is parallel to grain which
runs diagonally through this section of the board - an obvious defect in
material not found during original construction.

I intended to jack up the roof and install 1x6 collar ties at 1/3 the
distance down from the peak of the roof. The attic is accessible to install
the collar ties and all other existing framing looks pretty good.

The 2000 IBC lists the use of 3-10d nails to secure the 1x to the rafters.
However, this 2000 IBC is not used in California and collar tie nailing is
not indicated on the 97 UBC Nailing Table. The only reference to collar ties
I could find was in Appendix Chapter 23 - Prescriptive Masonry Construction
in High-Wind Areas where Section 2112.10.2 states: "Gabled and sloped roof
members not supported by the ridge shall be tied by ceiling joists or
equivalent lateral ties located as close to where the roof member bears on
the wall as is practically possible, at not more than 48 inches on center.
Collar ties shall not be used for these lateral ties."

Now I am at a loss. In my opinion (as this was a common form of construction
for many years) collar ties are ideal to resolve this situation but this
condition requires them to act as lateral support. The existing ceiling ties
obviously help, but are not restraining all of the outward thrust of the
rafter tails since the ridge board cracked. Installing a Ridge beam seems an
inappropriate solution here as it would redistribute the roof load and
require the possible addition of two foundations at each end - requiring
that we cut through one slab and underpin the exposed slab edge.

Any suggestions or information that would help me justify the use of collar

If I still use collar ties (assuming the building department accepts the fix
as I suspect they will) I intended to install a 1x6 on each side and face
nail each end to the rafters with (3) 10d nails - spacing the collar ties
every other bay or 48" on center.

Dennis S. Wish, PE
Structural Engineering Consultant
(208) 361-5447 E-Fax

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