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RESIDENTIAL: Rafters Not Meeting At Ridg[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: RESIDENTIAL: Rafters Not Meeting At Ridg
- From: Roger Turk <73527.1356(--nospam--at)compuserve.com>
- Date: Fri, 19 Oct 2001 19:19:03 -0400
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? ******* ****** ******* ******** ******* ******* ******* *** * Read list FAQ at: http://www.seaint.org/list_FAQ.asp * * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to: * * http://www.seaint.org/sealist1.asp * * Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you * send to the list is public domain and may be re-posted * without your permission. Make sure you visit our web * site at: http://www.seaint.org ******* ****** ****** ****** ******* ****** ****** ********
- Steel angle with pointed load
- From: ken kirkland
- Steel angle with pointed load
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