The idea of suspending the middle of the ceiling joists will certainly
fix the ceiling sag, but at a price. The rafter/joist "truss" now
carries an additional load - that of the ceiling DL + LL. At this point,
the rafter-to-joist connections may now be overstressed, trading one
problem for another. Proceed with due caution. When you deviate from
"conventional framing", even a little, you may be on soggy ground.
Otherwise your analyses appear reasonable.
On Wed, 6 Jun 2001 23:17:49 -0700 "Structuralist" <dennis.wish(--nospam--at)gte.net>
> Steve and Russ,
> Thanks for your comments. Let me put a few things into some
> here. Actually what was stated in the first post was that it
> appeared that
> the ridge droped "a few inches" which would suggest three or more.
> I failed to state that upon inspection I did not verify any
> deflection in the ridge, but noted that the ridge was not "straight"
> the garage.
> I think that I let this go a bit too far and failed to correct some
> clearly. The two inch deflection figure came from one of the
> e-mails where
> I did some load calcs on the rafter ties - 2x6 DF #1 using current
> 97 UBC
> criteria. I considered a 6.0 psf dead load on the 2x6's which were
> long and spaced at 24-inch on center. Neglecting live load, the dead
> deflection was nearly 2-inches.
> Now, what I am leaning to believe that nothing actually happened
> along the
> ridge. One of the things I think we all missed asking was "how much
> was there between the bottom ends of the rafters which were flush
> framed to
> the ridgeboard?" Surely, if the ridgeline dropped a few inches the
> between the cut at the original slope and cut at the end of the
> rafter after
> a three inch drop would show some space or nail withdrawl from the
> into the ridgeboard. However, when looking at the picture where the
> occurs, there is no space at all. The fit is tight to the ridgeboard
> there is no indication that a change in slope caused the rafter to
> around the top corner crushing the face grain of the 2x ridgeboard.
> In other
> words, a consistant tight fit suggests no drop in the ridge.
> The picture also shows that the inside edge of the wood where the
> occurs is the same color as the face and outter edges of the
> This is possible as there is no ventilation in this part of the
> (another reason to believe that the ceiling was added later) and
> sunlight could not reach the wood to discolor it. However, I would
> inclined to say that if the ceiling and insulation was installed
> some years
> later, the ridge would have been subject to some outside light if
> diffused through an open garage. There is a patina to the wood which
> probably occured in the first couple of years of the home. The crack
> has the
> same patina as the exposed surfaces which makes me believe the crack
> been there since the home was constructed.
> All of this aside, there is a problem and that is the ceiling. I've
> the fix by hanging the ceiling joists (aka rafter ties) from a new
> Parallam to replace a site built double 2x8 beam spaning 22'-6" and
> a 12'-6" tributary ceiling load. The double 2x8 probably kept the
> ceiling from failing completely, but it did nothing to control the
> As per the collar ties, I am going to install them, but I will
> install them
> flush to the top of the new Parallam and use the collar ties to
> brace the compression edge of the beam at 4' on center. This will
> give me
> more than 60% more capacity in bending to the beam should the owner
> to throw some boxes up in the attic.
> FWIW - I calculated the beam for no lateral bracing to see how it
> the bending stress and at 22-feet unsupported, the load used reached
> 90% of the beams bending capacity. With the compression edge
> supported at 4'
> the beam had reached nearer to 40% of it bending limit.
> Finally, as to the ridge deflection the most reasonable explanation
> is that
> the 22-foot long board may have been installed crown side down.
> the flush, tight fit of the rafters, it would be a workmanship
> rather than one of defect or failure.
> I thank you all again. If I have convinced one of you I think that I
> have done my job on this one. I certainly convinced myself having to
> this to my peers. Shows the powers of a peer-2-peer services such as
> time -
> a tremendous tool.
> Thanks again everyone!
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