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WOOD: Criteria Selection of Joists

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Not sure how to make a succinct title for this post, so away we go:

Continuing the discussion with my meeting with my builder friend the other day, one of his biggest pet-peeves was "you aren't turning the ceiling joists the shortest way."

He looks at each room and just turns the joist whichever way yields the shortest span.

I took pains to indicate to him the requirement of the Code to have a continuous tie:


IBC 2000 R802.3.1 Ceiling joist and rafter connections. Ceiling joists and rafters shall be nailed to each other in accordance [the accompanying Tables] and the assembly shall be nailed to the top wall plate... Ceiling joists shall be continuous or securely joined where they meet over interior partitions and nailed to adjacent rafters to provide a continous tie across the building when such joists are parallel to the rafters.

Where ceiling joists are not parallel to rafters, subflooring or metal straps attached to the ends of the rafters shall be installed in a manner to provide a continous tie, or rafters shall be tied to 1-inch by 4-inch (nominal) minimum-size crossties...

Rafter ties shall be spaced not more than 4 feet on center.


Rafters tying the ends of the rafters together across the building form what Dennis Wish calls a "Carpenter's Truss".

Now, obviously we can use the rafters running the shorter direction if we like but we must provide subflooring or metal straps or other Code-approved means to carry the forces across the building. Or we can use Collar ties (what the Code calls "crossties"). The problem I have with crossties/collar-ties is that it "complicates" what is happening in the rafters. To me one cannot ignore the fact that you now have induced flexure in the rafters and therefore the design check of the rafter becomes a non-trivial thing. In short, I just don't like crossties/collar ties for that reason.

And if I come back and tell him that he's got to use "huge" rafters, he's gonna give me the same old song-and-dance about how I "overengineer" everything and "it costs me money, I can't compete" and "the inspector passes it all the time."

Now, in the opinion of those here who care: What is your opinion as to the relative cost of going with larger ceiling joists running parallel to the rafters where possible and installing subfloor as needed where they run perpendicular, versus LOTS of subfloor because we're running joists higgledy-piggledy to get the "shortest spans"?

And how about the whole "collar tie" thing? How do YOU feel about its effect on the rafter design?

Thanks for your input. I know most of you are bored by these residential discussions, but I appreciate those who do pipe up.

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