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.
> -----Original Message-----
> From: Nels Roselund, SE [mailto:njineer(--nospam--at)att.net]
> Sent: Tuesday, June 05, 2001 8:42 PM
> To: seaint(--nospam--at)seaint.org
> Subject: Re: Use of Collar Ties in Light Frame Wo
> Steve's questions bring up some very important issues. What else
> is damaged
> beside the ridge? There must be more to discover concerning the damaged
> roof than you have mentioned so far. I didn't notice that you had figured
> out why the damage occurred unless it was related to a faulty
> ridge member.
> It seems unlikely that a faulty ridge member would be the cause
> -- the ridge
> member has very little work to do in a carpenter truss type roof system --
> it provides something to nail the upper ends of the rafters to.
> The ceiling joists are in the most effective position to restrain the
> rafters from spreading -- if the ceiling joists couldn't do the job, the
> collar ties at a higher position are not likely to provide a great
> improvement. I would want to find out whether or not the ceiling joists
> were nailed directly to the rafters, or only to the top plate,
> and how many
> nails were used in those connections.
> Nels Roselund
> Structural Engineer
> South San Gabriel, CA
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