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rdc 12-28-2010 02:56 PM

ceiling joist structural question
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As the picture shows, laying flat across the top of my ceiling joists is a 2x4 that is nailed to each joist. Then, toe-nailed to this 2x4 is a 2x6 on edge. It is not nailed to the joists, only to the 2x4. Each separate section of my home has this construction. My question: What is the purpose of this 2x6, and can I get rid of it without causing problems? It is in the way of running some rafter ties.
TIA, Roy

troubleseeker 12-29-2010 09:49 PM

This is known as a strongback and is typical for it to installed across ceiling joists. The vertical leg is generally nailed into the side of the flat plate, not toe nailed. It is used to help take a little of the bounce out of the ceiling joists, which are almost always mininum size for their span, and help stabilize them from twisting, both primary causes of nail pops.

I do not understand about them being in the way of installing rafter ties.

rdc 12-29-2010 10:44 PM

Thanks, troubleseeker! The rafter ties are being retrofitted and have to be threaded underneath an existing central air conditioner that sits on a slightly raised platform. There's enough room to go across the top of the flat 2x4 but the additional 6 inches to go over the vertical leg is too much. If I cut some notches in the vertical leg, will that significantly affect the stability of the joists?

CplDevilDog 12-30-2010 03:23 AM

Are rafter ties the same as collar ties? Horizontal framing running from one rafter to the opposite rafter?

Is the AC unit preventing them from being installed higher? They don't need to be installed that low.

rdc 12-30-2010 10:18 AM

Yes to the horizontal framing. My research says: Rafter ties are installed in the lower 1/3 of the rafter (and as low as possible to maximize their effectiveness) while collar ties are installed in the upper 1/3. Rafter ties prevent spreading while collar ties help to hold the roof together in high winds. I need rafter ties because I do not have ceiling joists that span the two sides of the roof - there are joists in the middle that run parallel to the roof beam. I have some spreading occurring along the roof beam.

The AC is preventing installing them in the lower 1/3 except for a small area beneath the unit. I can either lift the AC or cut some notches in the vertical leg of the strongback.

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