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RESIDENTIAL: Rafters Not Meeting At Ridg

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Bill,

You have had several replies regarding offset rafters at the ridge plate and 
they have probably adequately addressed the problem (and it is a problem).  
Summarizing:

The free body diagram of the rafter should show only a horizontal force at 
the ridge plate and a vertical and a horizontal force at the wall support.  
Whatever the rafter bears against at the ridge has to be able to resist this 
horizontal force.  I have seen ridge plates that are 1 X's and 2 X's.  
Needless to say, the 1 X's are inadequate to resist any horizontal force 
except bearing and, therefore, must have another rafter on the opposite 
side.  I would not quibble with a 1/2 rafter thickness offset, but IMO 
anything more is excessive.

Either collar ties, or ceiling joist tension ties are needed to take the 
horizontal force at the wall.  Rafters offset at the ridge plate would 
require a skewed ceiling joist tension tie or collar tie.

With regard to the ripple effect in the roofing, I have seen something in 
NRCA's (National Roofing Contractors Association) magazine, "Professional 
Roofing," about the cause of rippling of the roof, but cannot recall 
exactly what it is.  You might be able to find some information on NRCA's web 
site, www.nrca.org.  You didn't state it, but the roof slope should be 4:12 
minimum for a shingle roof, unless you install (IIRC) at least a 2-ply 
membrane roof underneath the shingles.

As far as attic ventilation goes, the more the better as far as wood is 
concerned.  The effects of high temperatures is cumulative.  At one time, it 
was felt that the effects of temperatures below 150 deg. F was reversible, 
and above 150 deg. F was cumulative.  But now, as someone mentioned, there is 
now some indication that cumulative effects occur at temperatures below 150 
deg.  Note that the base temperature at which strength of wood is determined 
is 68 deg. F, and that the strength decreases as temperature increases above 
that value, and increases as temperature decreases below that value.  While I 
have never taken a thermometer in the attic with me here in Tucson, I would 
compare the heat in the attic to the temperature in a closed car (and I did 
at one time have a thermometer in my car, but threw it out when it pegged at 
140 deg.).

The HADD (Homeowners Against Defective Dwellings) web site has a lot of 
horror stories about well-known national builders.  Just because a builder is 
big and national doesn't mean that he/she builds good houses.

HTH

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Bill Polhemus wrote:

. > I am doing a forensic investigation for a "tract home" built by a 
. > well-known homebuilder in the Houston area. One of the more surprising 
. > things I found was that the rafters do not meet (are not collinear) at the
. > ridge plate but rather are staggered along the length of the plate. The 
. > spacing isn't even regular, but random.

. > According to UBC '97 2320.12.3 "Rafters shall be framed directly opposite
. > each other at the ridge." Obviously we see a code violation here, but I'm
. > interested in the implications of this. My assumption is that even though
. > the ridge plate might be able to withstand the lateral thrust of the 
. > rafter, the roof is considerably less stiff as a lateral load resisting
. > element, which could have some problems.

. > Can someone make any other observations about potential problems?

. > Also, secondarily the contractor apparently "forgot" to install a ridge 
. > vent in the roof. There are only two turbine vents in the entire roof 
. > (this for a house with approx. 1,500 sq. ft. of footprint area). Since I'm
. > seeing a "rippling effect" in some of the roof decking, I'm assuming that
. > there's "bad stuff" happening with excess heat in the attic space.

. > What are the implications of improper venting of the attic space? Any
. > thoughts on this?

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