From: Roger Turk <73527.1356(--nospam--at)compuserve.com>
Date: Wed, 6 Jun 2001 09:27:16 -0400
Bear in mind that I am writing this quickly before I am fully awake.
Frankly, I think that you are grasping at straws for an explanation as to why
the garage roof sagged "a couple of inches" and that the reason for this is
It seems that everything that has been done prior to your involvement has
been a "band-aid" approach. Is what you are recommending a continuation of
As several people have pointed out, the ridge plate is non-structural and the
crack in it is not relevant. The plate could have been a 1 X as well as a
2 X. A lot of carpenters splice ridge plates where a rafter connects so
that there is not a long unsupported tail and a lot of these splices are
hard to see. Slope of grain in lumber can be as much as 1:6, so the crack
parallel to the grain is not an "undetected defect" if the grain meets the
As for your explanation of the deflecting ceiling joists "pushing" the
rafters up, don't forget that the thrust from the rafters is putting tension
in the ceiling joists and trying to keep the ceiling joists from deflecting,
like an archery bow keeps the bowstring straight and returns it to straight
after being deflected.
A quick calculation shows that a 2" drop in the ridge would result in a 1/2"
outward push on each of the exterior walls.
Why is there insulation in the attic of the garage? Around here, getting
insulation in the garage attic is like pulling hen's teeth.
Even without insulation in the attic, it would be very difficult to inspect
the condition of the rafter/ceiling joist/wall connection.
I think that the truth of the matter is that at this time, none of us has any
idea of what might have happened.
The house is in escrow --- it is not yet sold and I am sure that the buyers
(your clients?) are awaiting your report to negotiate an appropriate purchase
price. And, they would be relying on your report and solution to take
complete care of the problem. With what is not known, I don't know if you
could provide that kind of assurance.
I would be tempted to report that it cannot be determined what caused the sag
in the roof and I would probably recommend that in order to provide a
structurally adequate roof, that the existing roof be removed and replace
with a new structure. Engineering wise, it is a lot easier than futzing
around trying to jack up and patch a sagging roof system. Construction cost
wise, it is probably less expensive than the uncertain repairs and
engineering fee would be.
A. Roger Turk, P.E.(Structural)
Dennis Wish wrote:
. > Nels,
. > I thought I pretty well got to the root of that one. One assumption that
. > was made was that the roof ridge dropped where the ridge board cracked.
. > Since you and most of the others would agree that it is unlikely that a
. > cracked ridgeboard would have caused this type of damage if the existing
. > rafter ties were properly connected then the problem would have to be
. > elsewhere.
. > So let's look at the basic construction of the "carpenter truss":
. > 2x8 RR @ 24" o.c. slope 3:12
. > 2x12 Ridge Board
. > 2x6 Rafter Ties @ 24" o.c. - 25-feet long (continuous)
. > The first obvious fact is that a 25'-0" long 2x6 based upon current stress
. > values (assuming DF #1 grade) will deflect approximately 2-inches from
. > just the addition of a 1/2" gypsum ceiling and R-11 batt insulation using
. > a less than conservative 6.0-psf. This assumes no live load application
. > (who knows what the last owner stored in that attic space).
. > The two inch deflection is enough to substantiate the crack in the ceiling
. > at mid-span. Although the builder tried to brace the ceiling with a
. > built-up 2-2x8 beam 22'-6" long, the chances are that it was very little
. > help since it was so deficient in depth to help control bending much
. > better than the 2x6's acting alone.
. > So now we have the rafter ties deflecting causing the rafter tails to pull
. > inward. Am I wrong to conclude that it is possible, by the squeezing the
. > ends of the rafters together, one might cause the ridge to rise? While the
. > initial problem assumed that the ridge dropped because of a broken
. > ridgeboard, the ridge actually rose, except where the broken board
. > occurred, because of the weight of a ceiling and insulation installed on
. > Rafter Ties which deterred from their actual purpose?
. > The double 2x8 beam was not stiff enough to compensate for the defection
. > of the ties. Had it been, the ridge may never have changed.
. > I have decided to write this up as my professional opinion and to suggest
. > that in the process of installation of the new beam to check the
. > connection of the rafter ties to the roof rafters to insure that the 3-16d
. > nails are in place and there is sufficient connection to the double top
. > plate to eliminate the possibility that the rafter tails cut lose and
. > trust out under the weight of the roof.
. > I did inspect the exterior of the building and could see no cracking at
. > the corner where the stucco covering the eave meets the wall. In fact the
. > exterior finish had not been upgraded (painted or repaired) for at least a
. > few years and there are not cracks. The photos that I took show the ridge
. > and the eave. It takes a lot to see the deflection in the roof ridge and
. > it is highly likely that the deflection is more likely caused by
. > workmanship and not any rise or fall that is noticeable. I think what
. > draws more attention is the crack in the ceiling at mid-span of the rafter
. > ties and the noticeable ceiling deflection than at the roof. I've replaced
. > many a compo roof with flat tiles and have never seen a perfectly flat
. > roof - there is always some imperfection caused by crowning of the
. > rafters and I would think that even though this is the original roof, the
. > same high's and low's might be related to rafter crowning.
. > Does anyone think I'm on the right track here???>)
. > Thanks for all the help from everyone.
. > Dennis
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